Antenna topics

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Antennas (or aerials) are an essential element of any radio link, whether it is for a high power transmitter like those used for broadcasting, or low power ones like those used wireless technologies such as WLAN or remote control and sensing applications. Apart from the power levels, antennas are used across the whole radio spectrum, from ELF right up to the microwave bands. Whatever the power, and the frequency, the basic theory remains the same, although the practical approach has to change.

Antenna Basics

Electromagnetic waves and basic antenna operation

Polarization

Antenna feed impedance - including radiation resistance, loss resistance and efficiency

Resonance and bandwidth

Directivity and gainFeeders

The ideal position for an antenna is rarely in the optimum position for the transmitting or receiving equipment. As a result a form of transmission line or feeder is required to transfer the signals and power to and from the antenna.

Coaxial feeder

Balanced feederWaveguide

Waveguide data

The dipole antenna

The dipole antenna is one of the most basic forms of antenna available. It is widely used on its own, and it is also used as the "driven" element in many other types of antenna.

The dipole antenna

Folded dipole

The vertical antenna

Vertical antennas are widely used in many areas, and are particularly widely used for mobile applications because they radiate all around them in the horizontal plane. This means that they do not need redirecting as the mobile station moves. The quarter wave vertical is the simplest, but there are many other designs that provide improved performance and gain.

Quarter wave vertical

Five eighths wavelength vertical

J pole vertical antenna Directional antennas

There is a good variety of different types of directive antenna that can be used. Although the yagi antenna is the most popular, it is by no means the only one, and other designs and approaches are more applicable in many instances.

Yagi

Log periodic beam antenna

Parabolic reflector

Horn antenna

Wideband antennas

Most antennas are only able to operate over a narrow bandwidth. There are some techniques that enable the bandwidth of an antenna to be increased considerably, and also some designs that are able to operate over very wide bandwidths.

Discone

Log periodic beam antennaLoop antennas

Where directivity and small size are required, loop antennas may often provide an answer. Although different types of loop have slightly different properties, they are able to provide a good antenna solution in many circumstances.

Loop antenna overview

Ferrite rod antennaApplications

Antennas can be used in many applications from reception of terrestrial and satellite television to point to point radio, short wave radio and much more.

Satellite antennas for satellite television and other satellite applications

Electromagnetic waves and antenna basics

Radio signals are a form of electromagnetic wave. They are the same type of radiation as light, ultra-violet and infra red rays, differing from them in their wavelength and frequency. Electromagnetic waves have both electric and magnetic components that are inseparable. The planes of these fields are at right angles to one another and to the direction of motion of the wave.

An electromagnetic wave

The electric field results from the voltage changes occurring in the antenna which is radiating the signal, and the magnetic changes result from the current flow. It is also found that the lines of force in the electric field run along the same axis as the antenna, but spreading out as they move away from it. This electric field is measured in terms of the change of potential over a given distance, e.g. volts per meter, and this is known as the field strength. Similarly when an antenna receives a signal the magnetic changes cause a current flow, and the electric field changes cause the voltage changes on the antenna.

There are a number of properties of a wave. The first is its wavelength. This is the distance between a point on one wave to the identical point on the next. One of the most obvious points to choose is the peak as this can be easily identified although any point is acceptable.

Wavelength of an electromagnetic wave

The wavelength of an electromagnetic wave

The second property of the electromagnetic wave is its frequency. This is the number of times a particular point on the wave moves up and down in a given time (normally a second). The unit of frequency is the Hertz and it is equal to one cycle per second. This unit is named after the German scientist who discovered radio waves. The frequencies used in radio are usually very high. Accordingly the prefixes kilo, Mega, and Giga are often seen. 1 kHz is 1000 Hz, 1 MHz is a million Hertz, and 1 GHz is a thousand million Hertz i.e. 1000 MHz. Originally the unit of frequency was not given a name and cycles per second (c/s) were used. Some older books may show these units together with their prefixes: kc/s; Mc/s etc. for higher frequencies.

The third major property of the wave is its velocity. Radio waves travel at the same speed as light. For most practical purposes the speed is taken to be 300 000 000 meters per second although a more exact value is 299 792 500 meters per second.

Frequency to Wavelength Conversion

Although wavelength was used as a measure for signals, frequencies are used exclusively today. It is very easy to relate the frequency and wavelength as they are linked by the speed of light as shown:

Lambda = c / f

Where lambda = the wavelength in meters

f = frequency in Hertz

c = speed of radio waves (light) taken as 300 000 000 meters per second for all practical purposes.

Field measurements

It is also interesting to note that close to the antenna there is also an inductive field the same as that in a transformer. This is not part of the electromagnetic wave, but it can distort measurements close to the antenna. It can also mean that transmitting antennas are more likely to cause interference when they are close to other antennas or wiring that might have the signal induced into it. For receiving antennas they are more susceptible to interference if they are close to house wiring and the like. Fortunately this inductive field falls away fairly rapidly and it is barely detectable at distances beyond about two or three wavelengths from the antenna.

Antenna polarization

Polarization is an important factor for antennas. Both antennas and electromagnetic waves are said to have a polarization. For the electromagnetic wave it is effectively the plane in which the electric vibrates. This is important when looking at antennas because they are sensitive to polarization, and generally only receive or transmit a signal with a particular polarization. For most antennas it is very easy to determine the polarization. It is simply in the same plane as the elements of the antenna. So a vertical antenna (i.e. one with vertical elements) will receive vertically polarized signals best and similarly a horizontal antenna will receive horizontally polarized signals.

An electromagnetic wave

It is important to match the polarization of the antenna to that of the incoming signal. In this way the maximum signal is obtained. If the antenna polarization does not match that of the signal there is a corresponding decrease in the level of the signal. It is reduced by a factor of cosine of the angle between the polarization of the antenna and the signal.

Accordingly the polarization of the antennas located in free space is very important, and obviously they should be in exactly the same plane to provide the optimum signal. If they were at right angles to one another (i.e. cross-polarized) then in theory no signal would be received.

For terrestrial applications it is found that once a signal has been transmitted then its polarization will remain broadly the same. However reflections from objects in the path can change the polarization. As the received signal is the sum of the direct signal plus a number of reflected signals the overall polarization of the signal can change slightly although it remains broadly the same.

Polarization categories

Vertical and horizontal are the simplest forms of polarization and they both fall into a category known as linear polarization. However it is also possible to use circular polarization. This has a number of benefits for areas such as satellite applications where it helps overcome the effects of propagation anomalies, ground reflections and the effects of the spin that occur on many satellites. Circular polarization is a little more difficult to visualize than linear polarization. However it can be imagined by visualizing a signal propagating from an antenna that is rotating. The tip of the electric field vector will then be seen to trace out a helix or corkscrew as it travels away from the antenna. Circular polarization can be seen to be either right or left handed dependent upon the direction of rotation as seen from the transmitter.

Another form of polarization is known as elliptical polarization. It occurs when there is a mix of linear and circular polarization. This can be visualized as before by the tip of the electric field vector tracing out an elliptically shaped corkscrew.

However it is possible for linearly polarized antennas to receive circularly polarized signals and vice versa. The strength will be equal whether the linearly polarized antenna is mounted vertically, horizontally or in any other plane but directed towards the arriving signal. There will be some degradation because the signal level will be 3 dB less than if a circularly polarized antenna of the same sense was used. The same situation exists when a circularly polarized antenna receives a linearly polarized signal.

Applications

Different types of polarization are used in different applications to enable their advantages to be used. Linear polarization is by far the most widely used. Vertical polarization is often used for mobile or point to point applications. This is because many vertical antennas have an omni-directional radiation pattern and it means that the antennas do not have to be re-orientated as positions are changed if for example a moving vehicle. For other applications the polarization is often determined by antenna considerations. Some large multi-element antenna arrays can be mounted in a horizontal plane more easily than in the vertical plane. This is because the antenna elements are at right angles to the vertical tower of pole on which they are mounted and therefore by using an antenna with horizontal elements there is less physical and electrical interference between the two. This determines the standard polarization in many cases.

In some applications there are performance differences between horizontal and vertical polarization. For example medium wave broadcast stations generally use vertical polarization because ground wave propagation over the earth is considerably better using vertical polarization, whereas horizontal polarization shows a marginal improvement for long distance communications using the ionosphere. Circular polarization is sometimes used for satellite communications as there are some advantages in terms of propagation and in overcoming the fading caused if the satellite is changing its orientation.

Antenna feed impedance

- including radiation resistance, loss resistance and efficiency

When a signal source is applied to an antenna at its feed point, it is found that it presents a load impedance to the source. This is a complex impedance being made up from resistance, capacitance and inductance. In order to ensure the optimum efficiency of the transfer it is necessary to match the antenna to the load, and this requires some understanding of the operation of the antenna in this respect.

The feed impedance of the antenna results from a number of factors including the size and shape of the antenna, the frequency of operation and its environment. The impedance seen is normally complex, i.e. consisting of resistive elements as well as reactive ones. The resistive elements are made up from two constituents, namely the "loss resistance" and secondly the "radiation resistance."

Loss resistance

The loss resistance arises from the actual resistance of the elements in the antenna, and power dissipated in this manner is lost as heat. Although it may appear that the "DC" resistance is low, at higher frequencies the skin effect is in evidence and only the surface areas of the conductor are used. As a result the effective resistance is higher than would be measured at DC. It is proportional to the circumference of the conductor and to the square root of the frequency.

The resistance can become particularly significant in high current sections of an antenna where the effective resistance is low. Accordingly to reduce the effect of the loss resistance it is necessary to ensure the use of very low resistance conductors.

Radiation resistance

The other resistive element of the impedance is the "radiation resistance". This can be thought of as virtual resistor. It arises from the fact that power is "dissipated" when it is radiated. The aim is to "dissipate" as much power in this way as possible. It varies from one type of antenna to another, and from one design to another. It is dependent upon a variety of factors. However a typical half wave dipole operating in free space has a radiation resistance of around 73 Ohms.

Reactive elements

There are also reactive elements to the feed impedance. These arise from the fact that the antenna elements act as tuned circuits that possess inductance and capacitance. At resonance where most antennas are operated the inductance and capacitance cancel one another out to leave only the resistance of the combined radiation resistance and loss resistance. However either side of resonance the feed impedance quickly becomes either inductive (if operated below the resonant frequency) or capacitive (if operated above the resonant frequency).

Efficiency

It is naturally important to ensure that the proportion of the power dissipated in the loss resistance is as low as possible, leaving the highest proportion to be dissipated in the radiation resistance as a radiated signal. The proportion of the power dissipated in the radiation resistance divided by the power applied to the antenna is the efficiency.

A variety of means can be employed to ensure that the efficiency remains as high as possible. These include the use of optimum materials for the conductors to ensure low values of resistance, large circumference conductors to ensure large surface area to overcome the skin effect, and not using designs where very high currents and low feed impedance values are present. Other constraints may require that not all these requirements can be met, but by using engineering judgement it is normally possible to obtain a suitable compromise.

Antenna resonance and bandwidth

Two major factors associated with radio antennas are their resonant or centre operating frequency and the bandwidth over which they can operate. They naturally are very important feature of the operation of the antenna and as such they are mentioned in specifications for particular antennas and are a particularly important facet. Whether the antenna is used for broadcasting, WLAN, cellular telecommunications, PMR or any other application, the performance of the antenna is paramount, and the resonant frequency and bandwidth are of great importance.

Antenna resonance

An antenna is a form of tuned circuit consisting of inductance and capacitance, and as a result it has a resonant frequency at the frequency where the capacitive and inductive reactances cancel each other out. At this point the antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

Impedance of an antenna with frequency

The capacitance and inductance of an antenna are determined by the physical properties of the antenna and its environment. The major feature of the antenna is its dimensions. It is found that the larger the antenna or more strictly the antenna elements, the lower the resonant frequency. For example antennas for UHF terrestrial television have relatively small elements, whilst those for VHF broadcast sound FM have larger elements indicating a lower frequency. Antennas for short wave applications are larger still.

Bandwidth

Most antennas are operated around the resonant point. This means that there is only a limited bandwidth over which it can operate efficiently. Outside this the levels of reactance rise to levels that may be too high for satisfactory operation. Other characteristics of the antenna may also be impaired away from the centre operating frequency.

The bandwidth is particularly important where transmitters are concerned as damage may occur to the transmitter if the antenna is operated outside its operating range and the transmitter is not adequately protected. In addition to this the signal radiated by the antenna may be less for a number of reasons.

For receiving purposes the performance of the antenna is less critical in some respects. It can be operated outside its normal bandwidth without any fear of damage to the set. Even a random length of wire will pick up signals, and it may be possible to receive several distant stations. However for the best reception it is necessary to ensure that the performance of the antenna is optimum.

Impedance bandwidth

One major feature of an antenna that does change with frequency is its impedance. This in turn can cause the amount of reflected power to increase. If the antenna is used for transmitting it may be that beyond a given level of reflected power damage may be caused to either the transmitter or the feeder, and this is quite likely to be a factor which limits the operating bandwidth of an antenna. Today most transmitters have some form of SWR protection circuit that prevents damage by reducing the output power to an acceptable level as the levels of reflected power increase. This in turn means that the efficiency of the station is reduced outside a given bandwidth. As far as receiving is concerned the impedance changes of the antenna are not as critical as they will mean that the signal transfer from the antenna itself to the feeder is reduced and in turn the efficiency will fall. For amateur operation the frequencies below which a maximum SWR figure of 1.5:1 is produced is often taken as the acceptable bandwidth.

In order to increase the bandwidth of an antenna there are a number of measures that can be taken. One is the use of thicker conductors. Another is the actual type of antenna used. For example a folded dipole which is described fully in Chapter 3 has a wider bandwidth than a non-folded one. In fact looking at a standard television antenna it is possible to see both of these features included.

Radiation pattern

Another feature of an antenna that changes with frequency is its radiation pattern. In the case of a beam it is particularly noticeable. In particular the front to back ratio will fall off rapidly outside a given bandwidth, and so will the gain. In an antenna such as a Yagi this is caused by a reduction in the currents in the parasitic elements as the frequency of operation is moved away from resonance. For beam antennas such as the Yagi the radiation pattern bandwidth is defined as the frequency range over which the gain of the main lobe is within 1 dB of its maximum.

For many beam antennas, especially high gain ones it will be found that the impedance bandwidth is wider than the radiation pattern bandwidth, although the two parameters are inter-related in many respects.

Antenna directivity and gain

Antennas (aerials) do not radiate equally in all directions. It is found that all realizable radio antennas radiate more in some directions than others. The actual pattern is dependent upon the type of antenna, its size, the environment and a variety of other factors. This directional pattern can be used to ensure that the power radiated is radiated in the desired directions.

It is normal to refer to the directional patterns and gain in terms of the transmitted signal. It is often easier to visualize the antenna is terms of its radiated power, however the antenna performs in an exactly equivalent manner for reception, having identical figures and specifications.

In order to visualize the way in which an antenna radiates a diagram known as a polar diagram is used. This is normally a two dimensional plot around an antenna showing the intensity of the radiation at each point for a particular plane. Normally the scale that is used is logarithmic so that the differences can be conveniently seen on the plot. Although the radiation pattern of the antenna varies in three dimensions, it is normal to make a plot in a particular plane, normally either horizontal or vertical as these are the two that are most used, and it simplifies the measurements and presentation. An example for a simple dipole antenna is shown below.

Polar diagram of a half wave dipole in free space

Antennas are often categorized by the type of polar diagram they exhibit. For example an omni-directional antenna is one which radiates equally (or approximately equally) in all directions in the plane of interest. An antenna that radiates equally in all directions in all planes is called an isotropic antenna. As already mentioned it is not possible to produce one of these in reality, but it is useful as a theoretical reference for some measurements. Other antennas exhibit highly directional patterns and these may be utilized in a number of applications. The Yagi antenna is an example of a directive antenna and possibly it is most widely used for television reception.

Polar diagram for a yagi antenna

There are a number of key features that can be seen from this polar diagram. The first is that there is a main beam or lobe and a number of minor lobes. It is often useful to define the beam-width of an antenna. This is taken to be angle between the two points where the power falls to half its maximum level, and as a result it is sometimes called the half power beam-width.

Antenna gain

An antenna radiates a given amount of power. This is the power dissipated in the radiation resistance of the antenna. An isotropic radiator will distribute this equally in all directions. For an antenna with a directional pattern, less power will be radiated in some directions and more in others. The fact that more power is radiated in given directions implies that it can be considered to have a gain.

The gain can be defined as a ratio of the signal transmitted in the "maximum" direction to that of a standard or reference antenna. This may sometimes be called the "forward gain". The figure that is obtained is then normally expressed in decibels (dB). In theory the standard antenna could be almost anything but two types are generally used. The most common type is a simple dipole as it is easily available and it is the basis of many other types of antenna. In this case the gain is often expressed as dBd i.e. gain expressed in decibels over a dipole. However a dipole does not radiated equally in all directions in all planes and so an isotropic source is sometimes used. In this case the gain may be specified in dBi i.e. gain in decibels over an isotropic source. The main drawback with using an isotropic source as a reference is that it is not possible to realize them in practice and so that figures using it can only be theoretical. However it is possible to relate the two gains as a dipole has a gain of 2.1 dB over an isotropic source i.e. 2.1 dBi. In other words, figures expressed as gain over an isotropic source will be 2.1 dB higher than those relative to a dipole. When choosing an antenna and looking at the gain specifications, be sure to check whether the gain is relative to a dipole or an isotropic source.

Apart from the forward gain of an antenna another parameter which is important is the front to back ratio. This is expressed in decibels and as the name implies it is the ratio of the maximum signal in the forward direction to the signal in the opposite direction. This figure is normally expressed in decibels. It is found that the design of an antenna can be adjusted to give either maximum forward gain of the optimum front to back ratio as the two do not normally coincide exactly. For most VHF and UHF operation the design is normally optimized for the optimum forward gain as this gives the maximum radiated signal in the required direction.

Gain / beam-width balance

It may appear that maximizing the gain of an antenna will optimize its performance in a system. This may not always be the case. By the very nature of gain and beam-width, increasing the gain will result in a reduction in the beam-width. This will make setting the direction of the antenna more critical. This may be quite acceptable in many applications, but not in others. This balance should be considered when designing and setting up a radio link.

Coaxial feeder or cable (coax)

- used to feed antenna systems

The most common type of antenna feeder used today is undoubtedly coaxial feeder or coax. Coax offers advantages of convenience of use while being able to provide a good level of performance. As a result coaxial antenna feeder is universally used for domestic feeder applications for TV and Hi-Fi antenna systems. Additionally professionals make very widespread use of coax I their antenna systems, although for some applications other types of feeder such as open wire feeder, or waveguides may be used.

Basics

Coaxial feeder consists of two concentric conductors. The centre conductor is almost universally made of copper. Sometimes it may be a single conductor whilst at other times it may consist of several strands.

The outer conductor is normally made from a copper braid. This enables the cable to be flexible which would not be the case if the outer conductor was solid, although in some varieties made for particular applications it is. To improve the screening double or even triple screened cables are sometimes used. Normally this is accomplished by placing one braid directly over another although in some instances a copper foil or tape outer may be used. By using additional layers of screening, the levels of stray pick-up and radiation are considerably reduced. The loss is marginally lower.

Between the two conductors there is an insulating dielectric. This holds the two conductors apart and in an ideal world would not introduce any loss, although it is one of the chief causes of loss in reality. This dielectric may be solid or as in the case of many low loss cables it may be semi-airspaced because it is the dielectric that introduces most of the loss. This may be in the form of long "tubes" in the dielectric, or a "foam" construction where air forms a major part of the material.

Finally there is a final cover or outer sheath. This serves little electrical function, but can prevent earth loops forming. It also gives a vital protection needed to prevent dirt and moisture attacking the cable.

Cross section though coaxial cable

How it works

A coaxial cable carries current in both the inner and the outer conductors. These current are equal and opposite and as a result all the fields are confined within the cable and it neither radiates nor picks up signals.

This means that the cable operates by propagating an electromagnetic wave inside the cable. As there are no fields outside the cable it is not affected by nearby objects. Accordingly it is ideal for applications where the cable has to be routed through or around buildings or close to many other objects. This is a particular advantage of coaxial feeder when compared with other forms of feeder such as two wire (open wire, or twin) feeder.

Characteristic impedance

All feeders posses characteristic impedance. For coaxial cable there are two main standards that have been adopted over the years, namely 75 and 50 ohms.

75 ohm cable is used almost exclusively for domestic TV and VHF FM applications. However for commercial, amateur and CB applications 50 ohms has been taken as the standard. The reason for the choice of these two standards is largely historical but arises from the fact that 75 ohm coax gives the minimum weight for a given loss, while 50 ohm coax gives the minimum loss for a given weight.

These two standards are used for the vast majority of coax cable which is produced but it is still possible to obtain other impedances for specialist applications. Higher values are often used for computer installations, but other values including 25, 95 and 125 ohms are available. 25 ohm miniature cable is extensively used in magnetic core broadband transformers. These values and more are available through specialist coax cable suppliers.

Impedance determination

The impedance of the coax is chiefly governed by the diameters of the inner and outer conductors. On top of this the dielectric constant of the material between the conductors has a bearing. The relationship needed to calculate the impedance is given simply by the formula:

D = Inner diameter of the outer conductor

d = Diameter of the inner conductor

Capacitance and inductance

The capacitance of a line varies with the spacing of the conductors, the dielectric constant, and as a result the impedance of the line. The lower the impedance, the higher the capacitance for a given length, because the conductor spacing is decreased. The capacitance also increases with increasing dielectric constant, as in the case of an ordinary capacitor.

It is also often necessary to know the inductance of a line as well.

Practical aspects

The loss introduced by a feeder is a critical element of its operation. While the specification for a given type of cable will state the loss it introduces, further losses can be introduced if it is installed badly. Any moisture entering the cable will produce a considerable increase. If any moisture passes into the dielectric material spacing the inner and outer conductors, this will impair the performance of the dielectric, and increase the level of loss. Moisture will also cause the outer braid to oxidize, and reduce the conductivity between the small conductors making up the braid.

It is therefore very important to seal the end of the cable if it is to be used externally, and ensure that no moisture enters. It is also necessary to ensure that the outer sheath of the cable remains intact and is not damaged during installation or further use.

On some occasions it is necessary to bury coaxial cable. Ideally normal cable should not be buried directly as this relies purely on the outer sheath for protection and it is not designed for these conditions. Instead it can be run through buried conduit manufactured for carrying buried cables. This has the advantage that it is easy to replace. Alternatively a form of coax known as "bury direct" can be used.

All cables have a bend radius. In order to prevent damage they should not be bent into curves tighter than this. If coax is bent beyond its limit then damage to the inner construction of the cable may result. In turn this can lead to much higher levels of loss.

Balanced antenna feeder

- including open wire, two-wire, twin, and ribbon feeders

Balanced feeder is a form of feeder that can be used for feeding balanced antennas (i.e. antennas that do not have one connection taken to ground). It is mainly used on frequencies below 30 MHz can offer the advantage of very low levels of loss. The feeder or transmission line is also referred to by other names including twin, two wire, open wire, and sometimes even ribbon feeder. These names often depend upon the type of construction of the particular form.

It is used less than coaxial feeder or coax, although it is able to offer some significant advantages over coax in some applications.

Basics

A balanced or twin feeder consists of two parallel conductors unlike coax that consists of two concentric conductors.. The currents flowing in both wires run in opposite directions but are equal in magnitude. As a result the fields from them cancel out and no power is radiated or picked up. To ensure efficient operation the spacing of the conductors is normally kept to within about 0.01 wavelengths.

The feeder exists in a variety of forms. Essentially it is just two wires that are closely spaced in terms of the radio frequency of operation. In practical terms manufactured feeder is available and it consist of two wires contained within a plastic sheath that is also used as a spacer between them to keep the spacing, and hence the impedance constant. Another form commonly called open wire feeder simply consists of two wires kept apart by spacers that are present at regular intervals along the feeder. It has an appearance a little akin to a rope ladder.

Twin feeder a form of balanced feeder

Impedance

Like coaxial cable, the impedance of twin feeder is governed by the dimensions of the conductors, their spacing and the dielectric constant of the material between them. The impedance can be calculated from the formula given below.

Where

D is the distance between the two conductors

d is the outer diameter of the conductors

Epsilon is the dielectric constant of the material between the two conductors

Types

This type of feeder can take a variety of forms. An "open wire" feeder can be made by having two wires running parallel to one another. Spacers are used every fifteen to thirty centimeters to maintain the wire spacing. Usually these are made from plastic or other insulating material. Typically this feeder may have an impedance of around 600 ohms, although it is very dependent upon the wire, and the spacing used.

The feeder may also be bought as flat 300 ohm ribbon feeder consisting of two wires spaced with a clear plastic. This is the most common form and is the type that is used for manufacturing temporary VHF FM antennas. If used outside this type absorbs water into the plastic dielectric. Not only does this significantly increase the loss on damp days, but the moisture absorbed causes the wire to oxidize which in turn leads to increased losses over the longer term.

The feeder can also be bought with a black plastic dielectric with oval holes spaced at intervals in spacing. This type gives far better performance than the clear plastic varieties which absorb water if used outside.

Waveguide

- a basic introduction to the waveguide and the theory behind their operation

Waveguides are used in a variety of applications to carry radio frequency energy from one pint to another. In their broadest terms a waveguide is defined as a system of material that is designed to confine electromagnetic waves in a direction defined by its physical boundaries. This definition gives a very broad view of waveguides, and indeed waveguide theory is used in a number of applications to provide waveguide applications in a number of areas.

Typically a waveguide is thought if as a transmission line comprising a hollow conducting tube, which may be rectangular or circular within which electromagnetic waves are propagated. Unlike coaxial cable, there is no centre conductor within the waveguide. Signals propagate within the confines of the metallic walls that act as boundaries

Rectangular waveguide

Waveguides will only carry or propagate signals above a certain frequency, known as the cut-off frequency. Below this the waveguide is not able to carry the signals. The cut-off frequency of the waveguide depends upon its dimensions. In view of the mechanical constraints this means that waveguides are only sued for microwave frequencies. Although it is theoretically possible to build waveguides for lower frequencies the size would not make them viable to contain within normal dimensions and their cost would be prohibitive.

Connecting signals to a waveguide

A signal can be entered into the waveguide in a number of ways. The most straightforward is to use what is known as a launcher. This is basically a small probe which penetrates a small distance into the centre of the waveguide itself as shown. Often this probe may be the centre conductor of the coaxial cable connected to the waveguide. The probe is orientated so that it is parallel to the lines of the electric field which is to be set up in the waveguide. An alternative method is to have a loop which is connected to the wall of the waveguide. This encompasses the magnetic field lines and sets up the electromagnetic wave in this way. However for most applications it is more convenient to use the open circuit probe. These launchers can be used for transmitting signals into the waveguide as well as receiving them from the waveguide.

Waveguide launcher

Waveguide parameters

- data for waveguides in terms of their frequency range, material, attenuation, dimensions etc

Waveguides used for the transmission of radio frequency energy come in a variety of sizes. The size or more correctly the dimensions of the waveguide determine the properties, including parameters such as the cut-off frequency and so forth. Accordingly waveguides come in a variety of standard sizes.

The figures given below are for rigid rectangular waveguides, as these are the most common form of waveguide used.

Waveguide parametersRigid Rectangular WaveguidesWG DesignFreq range*Cut-off *Theoretical attn dB / 30mMaterialBand Dimensions (mm)

WG00 0.32 - 0.49 0.256 0.051 - 0.031 Alum B 584 x 292

WG0 0.35 - 0.53 0.281 0.054 - 0.034 Alum B,C 533 x 267

WG1 0.41 - 0.625 0.328 0.056 - 0.038 Alum B,C 457 x 229

WG2 0.49 - 0.75 0.393 0.069 - 0.050 Alum C 381 x 191

WG3 0.64 - 0.96 0.513 0.128 - 0.075 Alum C 292 x 146

WG4 0.75 - 1.12 0.605 0.137 - 0.095 Alum C,D 248 x 124

WG5 0.96 - 1.45 0.766 0.201 - 0.136 Alum D 196 x 98

WG6 1.12 - 1.70 0.908 0.317 - 0.212 Brass D 165 x 83

WG6 1.12 - 1.70 0.908 0.269 - 0.178 Alum D 165 x 83

WG7 1.45 - 2.20 1.157 D,E 131 x 65

WG8 1.70 - 2.60 1.372 0.588 - 0.385 Brass E 109 x 55

WG8 1.70 - 2.60 1.372 0.501 - 0.330 Alum E 109 x 55

WG9A 2.20 - 3.30 1.736 0.877 - 0.572 Brass E,F 86 x 43

WG9A 2.20 - 3.30 1.736 0.751 - 0.492 Alum E,F 86 x 43

WG10 2.60 - 3.95 2.078 1.102 - 0.752 Brass E,F 72 x 34

WG10 2.60 - 3.95 2.078 0.940 - 0.641 Alum E,F 72 x 34

WG11A 3.30 - 4.90 2.577 F,G 59 x 29

WG12 3.95 x 5.85 3.152 2.08 - 1.44 Brass F,G 48 x 22

WG12 3.95 x 5.85 3.152 1.77 - 1.12 Alum F,G 48 x 22

WG13 4.90 - 7.05 3.711 G,H 40 x 20

WG14 5.85 - 8.20 4.301 2.87 - 2.30 Brass H 35 x 16

WG14 5.85 - 8.20 4.301 2.45 - 1.94 Alum H 35 x 16

WG15 7.05 - 10.0 5.26 4.12 - 3.21 Brass I 29 x 13

WG15 7.05 - 10.0 5.26 3.50 - 2.74 Alum I 29 x 13

* frequency in GHz and for TE10 modeAlum = Aluminum

The dipole antenna

The dipole antenna or dipole aerial is one of the most important and commonly used types of antenna. It is widely used on its own, and it is also incorporated into many other antenna designs where it forms the radiating or driven element for the antenna.

Basic facts

As the name suggests the dipole antenna consists of two terminals or "poles" into which radio frequency current flows. This current and the associated voltage causes and electromagnetic or radio signal to be radiated. Being more specific, a dipole is generally taken to be an antenna that consists of a resonant length of conductor cut to enable it to be connected to the feeder. For resonance the conductor is an odd number of half wavelengths long. In most cases a single half wavelength is used, although three, five, . wavelength antennas are equally valid.

The basic half wave dipole antenna

The current distribution along a dipole is roughly sinusoidal. It falls to zero at the end and is at a maximum in the middle. Conversely the voltage is low at the middle and rises to a maximum at the ends. It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum. This provides a low impedance feed point which is convenient to handle. High voltage feed points are far less convenient and more difficult to use.

When multiple half wavelength dipoles are used, they are similarly normally fed in the centre. Here again the voltage is at a minimum and the current at a maximum. Theoretically any of the current maximum nodes could be used.

Three half wavelength wave dipole antennasFeed impedance

The feed impedance of a dipole antenna is dependent upon a variety of factors including the length, the feed position, the environment and the like. A half wave centre fed dipole antenna in free space has an impedance 73.13 ohms making it ideal to feed with 75 ohm feeder.

The feed impedance of a dipole can be changed by a variety of factors, the proximity of other objects having a marked effect. The ground has a major effect. If the dipole antenna forms the radiating element for a more complicated antenna, then elements of the antenna will have an effect. Often the effect is to lower the impedance, and when used in some antennas the feed impedance of the dipole element may fall to ten ohms or less, and methods need to be used to ensure a good match is maintained with the feeder.

Polar diagram

The polar diagram of a half wave dipole antenna that the direction of maximum sensitivity or radiation is at right angles to the axis of the antenna. The radiation falls to zero along the axis of the antenna as might be expected.

Polar diagram of a half wave dipole in free space

If the length of the dipole antenna is changed then the radiation pattern is altered. As the length of the antenna is extended it can be seen that the familiar figure of eight pattern changes to give main lobes and a few side lobes. The main lobes move progressively towards the axis of the antenna as the length increases.

Antenna length

The length of a dipole is the main determining factor for the operating frequency of the dipole antenna. Although the antenna may be an electrical half wavelength, or multiple of half wavelengths, it is not exactly the same length as the wavelength for a signal traveling in free space. There are a number of reasons for this and it means that an antenna will be slightly shorter than the length calculated for a wave traveling in free space.

For a half wave dipole the length for a wave traveling in free space is calculated and this is multiplied by a factor "A". Typically it is between 0.96 and 0.98 and is mainly dependent upon the ratio of the length of the antenna to the thickness of the wire or tube used as the element. Its value can be approximated from the graph:

Multiplication factor "A" used for calculating the length of a dipole

In order to calculate the length of a half wave dipole the simple formulae given below can be used:

Length (meters) = 150 x A / frequency in MHz

Length (inches) = 5905 x A / frequency in MHz

Using these formulae it is possible to calculate the length of a half wave dipole. Even though calculated lengths are normally quite repeatable it is always best to make any prototype antenna slightly longer than the calculations might indicate. This needs to be done because changes in the thickness of wire being used etc may alter the length slightly and it is better to make it slightly too long than too short so that it can be trimmed so that it resonates on the right frequency. It is best to trim the antenna length in small steps because the wire or tube cannot be replaced very easily once it has been removed.

The folded dipole antenna

- providing a higher impedance and greater bandwidth

The standard dipole is widely used in its basic form. However under a number of circumstances a modification of the basic dipole, known as a folded dipole provides a number of advantages that can be used to advantage.

In its basic form a dipole consists of a single wire or conductor cut in the middle to accommodate the feeder. It is found that the feed impedance is altered by the proximity of other objects, especially other parasitic elements that may be used in other forms of antenna. This can cause problems with matching and because resistance losses in the antenna system can start to become significant.

Additionally many antennas have to be able to operate over large bandwidths and a standard dipole may be unable to fulfill this requirement adequately.

The basic folded half wave dipole

A variation of the dipole, known as a folded dipole provides a solution to these problems, offering a wider bandwidth and a considerable increase in feed impedance. The folded dipole is formed by taking a standard dipole and then taking a second conductor and joining the two ends. In this way a complete loop is made as shown. If the conductors in the main dipole and the second or "fold" conductor are the same diameter, then it is found that there is a fourfold increase in the feed impedance. In free space, this gives a feed impedance of around 300 ohms. Additionally the antenna has a wider bandwidth.

Impedance increase

In a standard dipole the currents flowing along the conductors are in phase and as a result there is no cancellation of the fields and radiation occurs. When the second conductor is added, this can be considered as an extension to the standard dipole with the ends folded back to meet each other. As a result the currents in the new section flow in the same direction as those in the original dipole. The currents along both the half-waves are therefore in phase and the antenna will radiate with the same radiation patterns etc as a simple half-wave dipole.

The impedance increase can be deduced from the fact that the power supplied to a folded dipole is evenly shared between the two sections which make up the antenna. This means that when compared to a standard dipole the current in each conductor is reduced to a half. As the same power is applied, the impedance has to be raised by a factor of four to retain balance in the equation Watts = I^2 x R.

Applications

Folded dipoles are sometimes used on their-own, but they must be fed with a high impedance feeder, typically 300 ohms. However they find more uses when a dipole is incorporated in another antenna with other elements nearby. This has the effect of reducing the dipole impedance. To ensure that it can be fed conveniently, a folded dipole may be used to raise the impedance again to a suitable value.

Quarter wave vertical antenna

- including the ground plane antenna

Vertical antennas are widely used at all frequencies from MF up to VHF and beyond. They exist in a variety of forms including the quarter wave vertical and ground plane antennas. They possess many advantages and are widely used for medium wave broadcasting as well as for mobile applications in areas including private mobile radio.

The reason for this widespread use is the omni-directional radiation pattern that they give in the horizontal plane. This means that the antennas do not have to be re-orientated to keep the signals constant as the car moves it position.

Single element vertical antennas posses an omni-directional radiation pattern (in the horizontal plane). This means that the antennas do not have to be re-orientated when used in mobile applications as the vehicle moves. This is obviously an essential requirement.

A further advantage is that much of the radiation is at right angles to the antenna element, and as a result it travels close to the earth's surface where the receiving stations are located. Radiation directed upwards is wasted in many instances as VHF transmissions are normally not reflected by the ionosphere.

For medium wave broadcast stations a particular advantage is that the radiation is vertically polarized. It is found that the vertically polarized transmissions propagate further via the ground wave that these transmissions use.

Basic element

Like the name suggests the antenna consists of a quarter wavelength vertical element. The antenna is what is termed "un-balanced" having one connection to the vertical element and using an earth connection or simulated earth connection to provide an image for the other connection.

The voltage and current waveforms show that at the end the voltage rises to a maximum whereas the current falls to a minimum. Then at the base of the antenna at the feed point, the voltage is at a minimum and the current is at its maximum. This gives the antenna a low feed impedance. Typically this is around 20 ohms.

A quarter wave vertical

The ground is obviously an important part of the antenna. Many MF and HF installations use a ground connection for this. These ground systems need to be very effective fort he antenna to perform satisfactorily. They must obviously have a very low resistance, and often utilize large "mats" of radials extending out from the base of the antenna to ensure excellent RF performance.

For VHF and UHF installations, height is obviously important and antennas need to be raised to ensure they are above the nearby obstructions. Also for mobile installations it is clearly not possible to use a true earth connection. In these cases a simulated earth is used. For mobile applications this consists of the body of the vehicle. The antenna mounting will normally enable a suitable connection to be made to the vehicle body, sometimes using a capacitive connection. However it is necessary to ensure that the vehicle body is metal, and not plastic in the vicinity of the antenna mounting.

For fixed stations a set of radials simulating a ground plane is used. In theory the ground plane should extend out to infinity, but in practice a number of radials a quarter wave-length long is used. Typically for many VHF applications four radials is sufficient.

A radial system used with a quarter wave vertical

If the radials are bent downwards from the horizontal, then the feed impedance will be raised. A 50 ohm match is achieved when the angle between the ground plane rods and the horizontal is 42 degrees. Another solution is to include an impedance matching element in the antenna. Normally this is in the form of a tapped coil that can be conveniently housed in the base of the antenna.

Folded element

In view of the low impedance presented to the feeder by the antenna, methods must be found of presenting a good match and some have already been outlined. Another is to use a folded element. In the same way that a folded dipole increases the feed impedance of the antenna, so a folded vertical element can be used. If the diameter of both sections is the same, then an increase by a ratio of 4:1 is achieved. This would bring the impedance to 80 ohms and will provide an acceptable match to 75 ohm feeder. By using a smaller diameter grounded element the feed impedance can be reduced so that a good match to 50 ohm coax can be achieved.

Summary

The quarter wave vertical antenna is widely used in view of its simplicity and convenience. To improve on its performance other types of vertical are available. It is also possible to use further verticals and feed them with different phases to provide gain to the overall antenna system.

Five eighths wavelength vertical antenna

- providing gain by adding length

Vertical antennas find widespread use in applications where an "all round" radiation pattern is required. In these applications it is necessary to keep the maximum amount of radiation parallel to the earth. It is in applications such as these that the five eighths wavelength vertical antenna has become widely used.

Development

The most straightforward vertical antenna is the quarter wavelength version. However it is found that by extending the length of the vertical element, the amount of power radiated at a low angle is increased. If a half wave dipole is extended in length the radiation at right angles to the antenna starts to increase before finally splitting into several lobes. The maximum level of radiation at right angles to the antenna is achieved when the dipole is about 1.2 times the wavelength.

Gain

When used as a vertical radiator against a ground plane this translates to a length of 5/8 wavelength. It is found that a five eighths vertical has a gain of close to 4 dB. To achieve this gain the antenna must be constructed of the right materials so that losses are reduced to the absolute minimum and the overall performance is maintained, otherwise much of the advantage of using the additional length will be lost.

Matching

For most applications, it is necessary to ensure that the antenna provides a good match to 50 ohm coaxial cable. It is found that a 3/4 wavelength vertical element provides a good match, and therefore the solution to the 5/8 wavelength antenna is to make it appear as a 5/8 radiator but have the electrical length of a 3/4 element. This is achieved by placing a small loading coil at the base of the antenna to increase its electrical length.

Mechanical considerations

Five eighths wavelength vertical antennas are often used on automobiles. Accordingly one of the main constraints is to ensure that the coil at the base of the antenna is being kept rigid and does not bend as the antenna flexes with the movement of the car. If there is too much flexing then the match to the feeder will change and the operation will be impaired.

The J or J Pole Antenna

- the J pole antenna is a vertical antenna that does not require radials

The J or J pole antenna has found favor in many applications. The J antenna has a number of advantages over the standard vertical antennas such as the quarter wavelength vertical antenna and the five eights wavelength antenna. Unlike the other vertical antennas just mentioned, the J pole antenna does not require radials for its operation. In applications where radials may appear unsightly or where they may not be suitable for other reasons, the J pole antenna provides a useful alternative. Additionally its length means that the J pole antenna also provides some gain over a normal quarter wavelength vertical. These two attributes make the j pole antenna the ideal type for many applications. As a result the J Pole antenna is finding many applications, many of which are at VHF and above. Here it forms a compact self contained antenna that can fit in many locations and can give a high level of performance without a large visual impact.

Although the fact that the J antenna does not have any radials may make it appear that it will not work, it is a well established design. It is a form of antenna known as a Zepp or Zeppelin antenna that found favor in the 1930s as an HF antenna. This antenna gained its name from the fact that it was used on the Zeppelin airships. It consists of a half wave radiating element which is end fed using a quarter wave stub of open wire or 300 Ohm balanced feeder used to match the impedance to the normal 50 Ohm coaxial feeder.

The development of the J or J Pole antenna

The diagram shows the development of the J pole antenna and its operation. This shows the antenna radiating element which is a half wavelength. Being end fed this presents a high impedance to the feeder and this is matched using a half wave matching stub. In the first form of the antenna, the radiating element is fed from the source, with the other leg of the stub providing a passive balance. It can also be seen that it is possible to feed the antenna using the other arm of the stub.

The development of the J or J Pole antenna

The final implementation of the J pole antenna uses the stub to provide a good match to 50 Ohm cable. The feed point is moved up or down the stub to provide the best match, and adjustment can be made once the antenna is in position if required. In this way any spurious changes resulting from the position, etc can be removed.

The J pole antenna is quite easy to construct and gives good results. The main disadvantage is that it can be a little more difficult to adjust than some other forms. The reason for this is that impedance matching has to be accomplished by altering the trimming length of the stub.

The length of the half wave radiating stub for the j pole antenna can be determined using the same formula as used in calculating the length of a half wave dipole. The physical length of the balanced feeder will depend on the velocity factor of the feeder in use. For open wire feeder the velocity factor is nearly unity and the length will be very close to that of the free space quarter wavelength. If 300 twin feeder is used then the length required will be shorter because its velocity factor is about 0.85.

The Yagi antenna

The Yagi or Yagi-Uda radio antenna or aerial is one of the most successful designs of directive radio antenna in use today. It is used in a wide variety of applications where an antenna with gain and directivity is required. It has become particularly popular for television receiving applications. Here most households that use a television have a Yagi antenna directed towards the broadcast transmitter to give sufficient signal to provide a high quality picture.

The full name for the antenna is the Yagi-Uda antenna. It was derives it name from its two Japanese inventors Yagi and his student Uda. The antenna itself was first outlined in a paper that Yagi himself presented in 1928. Since then its use has grown rapidly to the stage where today a television antenna is synonymous with an antenna having a central boom with lots of elements attached.

The antenna

The Yagi has a dipole as the main radiating or driven element. Further "parasitic" elements are added which are not directly connected to the driven element. Instead they pick up power from the dipole and re-radiate it such a manner that it affects the properties of the antenna as a whole.

Basic concept of a Yagi antenna

The parasitic elements operate by re-radiating their signals in a slightly different phase to that of the driven element. In this way the signal is reinforced in some directions and cancelled out in others. It is found that the amplitude and phase of the current that is induced in the parasitic elements is dependent upon their length and the spacing between them and the dipole or driven element.

Using a parasitic element it is not possible to have complete control over both the amplitude and phase of the currents in all the elements. This means that it is not possible to obtain complete cancellation in one direction. Nevertheless it is still possible to obtain a high degree of reinforcement in one direction and have a high level of gain, and also have a high degree of cancellation in another to provide a good front to back ratio.

To obtain the required phase shift an element can be made either inductive or capacitive. If the parasitic element is made inductive it is found that the induced currents are in such a phase that they reflect the power away from the parasitic element. This causes the antenna to radiate more power away from it. An element that does this is called a reflector. It can be made inductive by tuning it below resonance. This can be done by physically adding some inductance to the element in the form of a coil, or more commonly by making it longer than the resonant length. Generally it is made about 5% longer than the driven element.

If the parasitic element is made capacitive it will be found that the induced currents are in such a phase that they direct the power radiated by the whole antenna in the direction of the parasitic element. An element which does this is called a director. It can be made capacitive tuning it above resonance. This can be done by physically adding some capacitance to the element in the form of a capacitor, or more commonly by making it about 5% shorter than the driven element.

It is found that the addition of further directors increases the directivity of the antenna, increasing the gain and reducing the beam-width. The addition of further reflectors makes no noticeable difference.

The antenna exhibits a directional pattern consisting of a main forward lobe and a number of spurious side lobes. The main one of these is the reverse lobe caused by radiation in the direction of the reflector. The antenna can be optimized to either reduce this or produce the maximum level of forward gain. Unfortunately the two do not coincide exactly and a compromise on the performance has to be made depending upon the application.

Polar diagram of the Yagi antenna

Gain

The gain of a Yagi antenna is governed mainly by the number of elements in the antennas. However the spacing between the elements also has an effect. As the overall performance of the antenna has so many inter-related variables, many early designs were not able to realize their full performance. Today computer programs are used to optimize designs before they are even manufactured and as a result the performance of antennas has been improved somewhat.

Feed impedance

It is possible to vary the feed impedance of a Yagi over a wide range. Although the impedance of the dipole itself would be 73 ohms in free space, this is altered considerably by the proximity of the parasitic elements. The spacing, their length and a variety of other factors all affect the feed impedance presented by the dipole to the feeder. In fact altering the element spacing has a greater effect on the impedance than it does the gain, and accordingly setting the required spacing can be used as one design technique to fine tune the required feed impedance. Nevertheless the proximity of the parasitic elements usually reduces the impedance below the 50 ohm level normally required. It is found that for element spacing distances less than 0.2 wavelengths the impedance falls rapidly away.

To overcome this, a variety of techniques can be sued. One is to use a folded dipole for the driven element. This provides an increase in impedance of around four times dependent upon the ratio of the thicknesses of the basic dipole conductor and the "fold" conductor. Other techniques involve using gamma matches, delta matches, baluns and the like. Delta matches can be very convenient. They involve "fanning out" the connection to the driven element. This method has the advantage that the driven element does not need to be broken to apply the feed as shown. As this is really applicable to a balanced feeder, a balun is required if coaxial cable is to be used.

A gamma match is another alternative that is often used. The outer or braid of the coax feeder is connected directly to the centre of the driven element. This can be done because the RF voltage at the centre is zero at this point. The inner conductor of the feeder carrying the RF current is taken out along the driven element. The inductance of the arm is then tuned out by the variable capacitor. When adjusting the antenna design, both the variable capacitor and the point at which the arm contacts the driven element are adjusted. Once a value has been ascertained for the variable capacitor, its value can be measured and a fixed component inserted if required.

The log periodic antenna

One of the major drawbacks with many antennas is that they have a relatively small bandwidth. This is particularly true of the Yagi beam antenna. One design named the log periodic is able to provide directivity and gain while being able to operate over a wide bandwidth.

The log periodic antenna is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain. It is sometimes used on the HF portion of the spectrum where operation is required on a number of frequencies to enable communication to be maintained. It is also used at VHF and UHF for a variety of applications, including some uses as a television antenna.

Capabilities

The log periodic antenna was originally designed at the University of Illinois in the USA in 1955.

The antenna is directional and is normally capable of operating over a frequency range of about 2:1. It has many similarities to the more familiar Yagi because it exhibits forward gain and has a significant front to back ratio. In addition to this the radiation pattern stays broadly the same over the whole of the operating band as do parameters like the radiation resistance and the standing wave ratio. However it offers less gain for its size than does the more conventional Yagi.

Basics

The log periodic antenna can exist in a number of forms. The most common is the log periodic dipole array (LPDA). It basically consists of a number of dipole elements. These diminish in size from the back towards the front. The main beam of the antenna coming from the smaller front. The element at the back of the array where the elements are the largest is a half wavelength at the lowest frequency of operation. The element spacings also decrease towards the front of the array where the smallest elements are located. In operation, as the frequency changes there is a smooth transition along the array of the elements that form the active region. To ensure that the phasing of the different elements is correct, the feed phase is reversed from one element to the next.

A log periodic dipole array

It is possible to explain the operation of a log periodic array in straightforward terms. The feeder polarity is reversed between successive elements. Take the condition when the antenna is approximately in the middle of its operating range. When the signal meets the first few elements it will be found that they are spaced quite close together in terms of the operating wavelength. This means that the fields from these elements will cancel one another out as the feeder sense is reversed between the elements. Then as the signal progresses down the antenna a point is reached where the feeder reversal and the distance between the elements gives a total phase shift of about 360 degrees. At this point the effect which is seen is that of two phased dipoles. The region in which this occurs is called the active region of the antenna. Although the example of only two dipoles is given, in reality the active region can consist of more elements. The actual number depends upon the angle [Greek letter alpha] and a design constant.

The elements outside the active region receive little direct power. Despite this it is found that the larger elements are resonant below the operational frequency and appear inductive. Those in front resonate above the operational frequency and are capacitive. These are exactly the same criteria that are found in the Yagi. Accordingly the element immediately behind the active region acts as a reflector and those in front act as directors. This means that the direction of maximum radiation is towards the feed point.

Feed arrangements

The log periodic dipole antenna presents a number of difficulties if it is to be fed properly. The feed impedance is dependent upon a number of factors. However it is possible to control this by altering the spacing, and hence the impedance for the feeder that connects each of the dipole elements together. Despite this the impedance varies with frequency, but this can be overcome to a large extent by making the longer elements out of a larger diameter rod. Even so the final feed impedance does not normally match to 50 ohms on its own. It is normal for a further form of impedance matching to be required. This may be in the form of a stub or even a transformer. The actual method employed will depend to a large degree on the application of the antenna and its frequency range.

Overview

The log periodic antenna is a particularly useful design when modest levels of gain are required, combined with wideband operation. A typical antenna will provide between 4 and 6 dB gain over a bandwidth of 2:1 while retaining an SWR level of better than 1.3:1. With this level of performance it is ideal for many applications, although a log periodic antenna will be much larger than a Yagi that will produce equivalent gain. However the Yagi is unable to operate over such a wide bandwidth.

The parabolic reflector antenna

- also widely termed the dish antenna

The parabolic reflector or "dish" antenna has been used far more widely in recent years with advent of satellite television (TV). However this antenna finds uses in many radio and wireless applications at frequencies usually above about 1GHz where very high levels of antenna gain are required along with narrow beam-widths. In many professional applications these parabolic reflectors are used for satellite as well as for radio astronomy and it is used in many microwave links, often being seen on radio relay towers and mobile phone antenna masts. In all these applications very high levels of gain are required to receive the incoming signals that are often at a very low level. For transmitting they are able to concentrate the available radiated power into a narrow beam-width, ensuring all the available power is radiated in the required direction.

The Goldstone parabolic reflector antenna

Image courtesy NASA

Basics

The antenna consists of a radiating system that is used to illuminate a reflector that is curved in the form of a paraboloid. This shape enables a very accurate beam to be obtained. The antenna exists in two basic forms. These are termed the focal feed reflector where source of radiation is placed at the focal point of the parabola and this is used to illuminate the reflector.

An alternative form of feeding the antenna is known as a Cassegrain reflector system. Here the radiation is fed through the centre of the reflector towards a hyperbolic reflector which reflects the radiation back onto the parabolic reflector. In this way it is possible to control the radiation more accurately.

Diagram of a focal feed parabolic reflector antenna

The gain is a function of the diameter of the reflecting surface, the surface accuracy, and the quality of the illumination from the radiator. Despite these factors it is possible to estimate the gain of the antenna which can be deduced from the following formula:

G = 10 log10 k (pi D)^2 / lambda^2

Where

G is the gain over an isotropic source

k is the efficiency factor which is generally about 50%

D is the diameter of the parabolic reflector in meters

Lambda is the wavelength of the signal in meters

From this it can be seen that very large gains can be achieved if sufficiently large reflectors are used. However when the antenna has a very large gain, the beam-width is also very small and the antenna requires very careful control over its position. In professional systems electrical servo systems are used to provide very precise positioning.

To provide the optimum illumination of the reflecting surface, the level of illumination should be greater in the centre than at the sides. It can be shown that the optimum situation occurs when the centre is around 10 to 11 dB greater than the illumination at the edge. Lower levels of edge illumination result in lower levels of side lobes.

The reflecting surface antenna forms a major part of the whole system. In many respects it is not as critical as may be thought at first. Often a wire mesh may be used. Provided that the pitch of the mesh is small compared to a wavelength it will be seen as a continuous surface by the radio signals. If a mesh is used then the wind resistance will be reduced, and this provides significant advantages.

Focal feed system

The antenna consists of a radiating element which may be a simple dipole or a waveguide horn antenna. This is placed at the focal point of the parabolic reflecting surface. The energy from the radiating element is arranged so that it illuminates the reflecting surface. Once the energy is reflected it leaves the antenna system in a narrow beam. As a result considerable levels of gain can be achieved.

Achieving this is not always easy because it is dependent upon the radiator that is used. For lower frequencies a dipole element is often employed whereas at higher frequencies a circular waveguide may be used. In fact the circular waveguide provides one of the optimum sources of illumination.

Cassegrain feed system

The Cassegrain feed system, although requiring a second reflecting surface has the advantage that the overall length of the antenna between the two reflectors is shorter than the length between the radiating element and the parabolic reflector. This is because there is a reflection in the focusing of the signal which shortens the physical length. This can be an advantage in some systems.

Diagram of a focal feed parabolic reflector antenna with a Cassegrain feed

Summary

For most domestic systems a small reflector combined with a focal point feed are used, providing the simplest and most economical form of construction. This is the form that is most widely sued for satellite television applications. These antennas may not always look exactly like the traditional full dish antenna. For mechanical and production reasons the feed is often offset from the centre and a portion of the paraboloid used, again offset from the centre. This provides mechanical advantage. Nevertheless the principles are exactly the same.

Horn antenna

- an overview of the horn antenna used in microwave applications

The horn antenna is used in the transmission and reception of microwave signals, and the antenna is normally used in conjunction with waveguide feeds. The horn antenna gains its name from its appearance. The waveguide can be considered to open out or to be flared, launching the signal towards the receiving antenna.

Horn antennas are often used as gain standards, and as feeds for parabolic or 'dish' antennas, as well as being used as antennas in their own right. One particular use of horn antennas themselves is for short range radar systems, such as those used for automotive speed enforcement.

When used as part of a parabolic reflector, the horn is orientated towards the reflector surface, and is able to give a reasonably even illumination of the surface without allowing radiation to miss the reflector. In this way it is able to maximize the efficiency of the overall antenna. The use of the horn antenna also minimizes the spurious responses of the parabolic reflector antenna to signals that are not in the main lobe.

Horn antenna

Basic concept

The horn antenna may be considered as an RF transformer or impedance match between the waveguide feeder and free space which has an impedance of 377 ohms. By having a tapered or having a flared end to the waveguide the horn antenna is formed and this enables the impedance to be matched. Although the waveguide will radiate without a horn antenna, this provides a far more efficient match.

In addition to the improved match provided by the horn antenna, it also helps suppress signals traveling via unwanted modes in the waveguide from being radiated.

However the main advantage of the horn antenna is that it provides a significant level of directivity and gain. For greater levels of gain the horn antenna should have a large aperture. Also to achieve the maximum gain for a given aperture size, the taper should be long so that the phase of the wave-front is as nearly constant as possible across the aperture. However there comes a point where to provide even small increases in gain, the increase in length becomes too large to make it sensible. Thus gain levels are a balance between aperture size and length. However gain levels for a horn antenna may be up to 20 dB in some instances.

Horn antenna types

There are two basic types of horn antenna: pyramid and conical. The pyramid ones, as the name suggests are rectangular whereas the corrugated ones are usually circular. The corrugated horn provides a pattern that is nearly symmetrical, with the E and H plane beam-widths being nearly the same. Additionally it is possible to control the side lobes better with a conical or corrugated horn antenna.

The discone antenna

- for wide band applications

The discone antenna is widely used where an omni-directional wide band or bandwidth antenna is needed. It finds many uses, particularly for all type of radio scanning and monitoring applications from the commercial or military monitoring services to the home scanner enthusiast for frequencies above 30 MHz.

Overview

The discone antenna receives its name from its distinctive shape. The antenna consists of a top "disc" formulated from a number of elements arranged in a disc at the top, and further elements pointing downwards in the shape of a cone. Although the antenna could be made as a full disc and a cone, this would considerably increases its weight and wind loading, which would not be advisable from mechanical considerations.

This type of antenna can operate over frequency ranges of up to 10:1 dependent upon the particular design, and it also offers a relatively low angle of radiation (and reception). This makes it ideal for VHF / UHF applications as its greatest sensitivity is parallel or almost parallel to the Earth. However towards the top of its frequency range it is found that the angle of radiation increases somewhat.

Although it is widely used for receiving applications, it is less commonly used for transmitting. There are several reasons for this. Although it offers a wide bandwidth, it is not optimized for a particular band of frequencies and is less efficient than many other designs that are available. Additionally the wideband with of the antenna means that spurious signals can be radiated more easily and the level of reflected power will vary over the operating range and may rise above acceptable limits in some areas.

Physical aspects

The antenna consists of three main components: the insulator, the cone elements and the disc elements.

Of the antenna components the insulator size governs a number of factors of the performance of the antenna. It is made from insulating material and acts to hold the disc and cone elements in place, keeping them a fixed distance apart. In fact this distance is one of the factors that determine the overall frequency range of the antenna.

Secondly, the cone elements should be a quarter wave-length at the minimum operating frequency. This can be calculated from the formula A = 75000 / frequency (MHz) millimeters where A is the length of the cone elements.

Thirdly the disc elements should be made to have an overall length of 0.7 of a quarter wave-length. This can be calculated from the formula B = 52550 / frequency (MHz) millimeters. The diameter of the top of the cone is mainly dependent upon the diameter of the coaxial cable being used. This determines the upper frequency limit of the antenna. The smaller the diameter the higher the frequency. For many designs operating in the VHF / UHF region of the radio spectrum it is around 15 millimeters. The spacing between the cone and the disc should be about a quarter of the inner diameter of the cone, i.e. around three of four millimeters.

Operation

The way in which the discone operates is relatively complicated, but it can be envisaged in a simplified manner. The disc and cone elements sufficiently simulate an electrically complete disc and cone from which the energy is radiated. As a result the greater the number of elements, the better the simulation, although in reality there is a balance between performance, cost and wind resistance. Often around six elements are used, but the number is not critical.

In operation energy from the feeder meets the antenna and spreads over the surface of the cone from the apex towards the base until the vertical distance between the point on the cone and the disc is a quarter wavelength. In this way it is possible for the energy to be radiated or received efficiently.

The antenna radiates and receives energy that is vertically polarized, and the radiation pattern is omni-directional in the horizontal plane. The antenna radiates most of the energy at a low angle which it maintains over the most of the operating range. Typically there is little change over a range of 5:1 and above this a slight increase in the angle.

With the feed point at the top of the antenna the current maximum point is also at the top. It is also found that below the minimum frequency the antenna presents a very bad mismatch to the feeder. However once the frequency rises above this point then a reasonable match to 50 ohm coax is maintained over virtually the whole of the band.

The log periodic antenna

One of the major drawbacks with many antennas is that they have a relatively small bandwidth. This is particularly true of the Yagi beam antenna. One design named the log periodic is able to provide directivity and gain while being able to operate over a wide bandwidth.

The log periodic antenna is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain. It is sometimes used on the HF portion of the spectrum where operation is required on a number of frequencies to enable communication to be maintained. It is also used at VHF and UHF for a variety of applications, including some uses as a television antenna.

Capabilities

The log periodic antenna was originally designed at the University of Illinois in the USA in 1955.

The antenna is directional and is normally capable of operating over a frequency range of about 2:1. It has many similarities to the more familiar Yagi because it exhibits forward gain and has a significant front to back ratio. In addition to this the radiation pattern stays broadly the same over the whole of the operating band as do parameters like the radiation resistance and the standing wave ratio. However it offers less gain for its size than does the more conventional Yagi.

Basics

The log periodic antenna can exist in a number of forms. The most common is the log periodic dipole array (LPDA). It basically consists of a number of dipole elements. These diminish in size from the back towards the front. The main beam of the antenna coming from the smaller front. The element at the back of the array where the elements are the largest is a half wavelength at the lowest frequency of operation. The element spacings also decrease towards the front of the array where the smallest elements are located. In operation, as the frequency changes there is a smooth transition along the array of the elements that form the active region. To ensure that the phasing of the different elements is correct, the feed phase is reversed from one element to the next.

A log periodic dipole array

It is possible to explain the operation of a log periodic array in straightforward terms. The feeder polarity is reversed between successive elements. Take the condition when the antenna is approximately in the middle of its operating range. When the signal meets the first few elements it will be found that they are spaced quite close together in terms of the operating wavelength. This means that the fields from these elements will cancel one another out as the feeder sense is reversed between the elements. Then as the signal progresses down the antenna a point is reached where the feeder reversal and the distance between the elements gives a total phase shift of about 360 degrees. At this point the effect which is seen is that of two phased dipoles. The region in which this occurs is called the active region of the antenna. Although the example of only two dipoles is given, in reality the active region can consist of more elements. The actual number depends upon the angle [Greek letter alpha] and a design constant.

The elements outside the active region receive little direct power. Despite this it is found that the larger elements are resonant below the operational frequency and appear inductive. Those in front resonate above the operational frequency and are capacitive. These are exactly the same criteria that are found in the Yagi. Accordingly the element immediately behind the active region acts as a reflector and those in front act as directors. This means that the direction of maximum radiation is towards the feed point.

Feed arrangements

The log periodic dipole antenna presents a number of difficulties if it is to be fed properly. The feed impedance is dependent upon a number of factors. However it is possible to control this by altering the spacing, and hence the impedance for the feeder that connects each of the dipole elements together. Despite this the impedance varies with frequency, but this can be overcome to a large extent by making the longer elements out of a larger diameter rod. Even so the final feed impedance does not normally match to 50 ohms on its own. It is normal for a further form of impedance matching to be required. This may be in the form of a stub or even a transformer. The actual method employed will depend to a large degree on the application of the antenna and its frequency range.,/p>

Overview

The log periodic antenna is a particularly useful design when modest levels of gain are required, combined with wideband operation. A typical antenna will provide between 4 and 6 dB gain over a bandwidth of 2:1 while retaining an SWR level of better than 1.3:1. With this level of performance it is ideal for many applications, although a log periodic antenna will be much larger than a Yagi that will produce equivalent gain. However the Yagi is unable to operate over such a wide bandwidth.

Loop antenna

- an overview of the basics of the different types of loop antennas.

Loop antennas, or more correctly, closed loop antennas are widely used in many applications, often providing advantages over other types of antenna. Loop antennas can be placed into two categories, namely small loops and large loops. The terms refer to their size when compared to a wavelength of the frequency in use.

Small loop antennas

Small loop antennas can be likened to coils, as they have the same current distribution as ordinary 'circuit' coils, having the same phase and amplitude through the whole coil. To achieve this, the total length of the conductor used in the loop antenna must be no more than about 0.1 wavelengths long. Any longer than this and the current phase and amplitude will start to vary over the length of the conductor and some of the properties start to change.

Small loop antennas may also be split into those that use a single turn, and those that have a multi-turn loop, as in the case of a coil. One common form of multi-turn small loop antenna is the popular ferrite rod antenna that is used in many domestic portable radios and is also starting to be used in applications such as RFID devices. Another form of this antenna was the frame antenna or aerial found in many domestic radio sets of the 1940s and 1950s. Here a multi-turn coil about 30 centimeters or more square was built into the set to act as the antenna.

Multi-turn loop antennas are nor normally used for transmitting because the losses are high and the level of heat dissipated can give rise to rapid temperature increases. Instead single turn loop antennas may be used if a loop antenna is needed. These antennas have a number of advantages and disadvantages.

The main advantages of loop antennas are their size and directivity. Often a single turn small loop antenna is much smaller than a wavelength by its definition. They are also quite directive, and this can be used to direct the radiated power in the required direction. Both these advantages can be very useful in many applications. They find uses for transmitting and receiving, particularly on the MF and HF or short wave bands. Here they provide very compact antennas for applications such as amateur radio and shipping, etc. as well as receiving antennas for MF or medium wave receivers.

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