Upload
nisha-kumari
View
220
Download
0
Embed Size (px)
Citation preview
8/12/2019 ANTENNA for Communication Systems-2008cep
1/231
ANTENNAS FOR MODERN
COMMUNICATION SYSTEMSBy
BI NAY K. SARKAR.
I SRO CHAI R PROFESSOR
( Lecture prepared for delivering at the Summer School on Recent Trend in AntennaTechnology to be held at KIIT , Bhubaneswar on Aug.05,2008 )
Department of Electrical and Electronic Communication Engineering
Indian Institute of Technology, KharagpurKharagpur-721302
8/12/2019 ANTENNA for Communication Systems-2008cep
2/231
1 INTRODUCTION:
Antenna:A usually metallic device (as a rod or wire) for radiating or receiving radio waves.
An antenna can be any conductive structure that can carry an electrical current. If it carries a time
varying electrical current, it will radiate an electromagnetic wave, may be not efficiently or in a
desirable manner but it will radiate.
Antennas are the connecting linkbetween RF signals in an electrical circuit, such as between a
PCB and an electromagnetic wave propagating in the transmission media between the transmitter
and the receiver of a wirelesslink.
In the transmitter, the antenna transforms the electrical signal into an electromagnetic wave byexciting either an electrical or a magnetic field in its immediate surroundings, the near field.
Antennas that excite an electrical field are referred to as electrical antennas; antennas exciting a
magnetic field are called magnetic antennas.
The oscillating electrical or magnetic field generates an electromagnetic wave that propagates
with the velocity of light c. The speed of light in free space cois 300000 km/s. If the wave travels
in a dielectric medium with the relative dielectric constant r, the speed of light is reduced to:
Co/(r)1/2
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=link&x=&y=http://www.rfdesignline.com/http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=wireless&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=wireless&x=&y=http://www.rfdesignline.com/http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=link&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
3/231
Concept of Radiation:
A typical radiating system will compose of a source, a transmission line and antenna as shown
below
8/12/2019 ANTENNA for Communication Systems-2008cep
4/231
If the antenna system is not properly designed, the transmission line could act as an energy
storage device. Zg
RL
RA
Antenna
Transmission line
Source
Losses
LineAntenna
VSWR
Minimization Losses
Transmission line matching
Reduce loss resistance, RLof antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
5/231
Two Wires:
8/12/2019 ANTENNA for Communication Systems-2008cep
6/231
Applying voltage across two-conductor transmission line creates an electric field between
the conductors.
Electric field is associated with it electric lines of force which are tangent to the electricfield at each point and their strength is proportional to the electric field intensity.
Electric line of forceacts on free electron in the conductorforce them to displace
movement of charge creates electric currentthat in turn creates magnetic field intensity
associated with magnetic field intensitymagnetic lines of force which are tangent to
magnetic field.
8/12/2019 ANTENNA for Communication Systems-2008cep
7/231
T- period: Lines of force created
between arms of a small center fed
dipole in time when charge reached
maximum value & lines have
traveled .
During next original lines travel another (total
) and charge density on conductors begin to
diminish.
This can be accomplished by introducing opposite
charges which at the end of first half of the periodhave neutralized the charge on the conductors.
Lines of force created by opposite charges shown
by dashed line and travel by distance.
4
2
4
8/12/2019 ANTENNA for Communication Systems-2008cep
8/231
2. Types of Antennas
Various antennas used in radar and Communication systems:
1 ) Dipole
2 ) Monopole
3 ) Sleeve Monopole
4 ) Loop
5 ) Helix6 ) Yagi- Uda.
7 ) Log-Periodic
8 ) Horn
9 ) Slotted waveguide
10 ) Microstrip antennas.
11 ) Parabolic reflector12 ) Cassegrain reflector
13 ) Array
14) Special Antennas
15) MEMS Antennas
8/12/2019 ANTENNA for Communication Systems-2008cep
9/231
Dipole Antenna
Half -Wave Dipole
Half -Wave Dipole Antenna.The half-wave dipole antenna (Figure ) is the basis of many other antennas and is also
used as a reference antenna for the measurement of antenna gain and radiated power
density.
At the frequency of resonance, i.e., at the frequency at which the length of the dipole
equals a half-wavelength, we have a minimum voltageand a maximum current at the
terminations in the center of the antenna, so the impedance is minimal.
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=voltage&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=voltage&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
10/231
Therefore, we can compare the half-wave dipole antenna with a series RLC resonant circuit
. For a lossless half-wave dipole antenna, the series resistance of the equivalent resonant
circuit equals the radiation resistance, generally between 60 and 73 , depending on the
ratio of its length to the diameter.
The bandwidth of the resonant circuit (or the antenna) is determined by the L-to-C ratio. A
wire with a larger diameter means a larger capacitance and a smaller inductance, which
gives a larger bandwidthfor a given series resistance. That's why antennas made for
measurement purposes have a particularly large wire diameter.
As opposed to the (only hypothetical) isotropic radiator, real antennas such as the half-wave
dipole have a more or less distinct directional radiation characteristic.
The radiation pattern of an antenna is the normalized polar plot of the radiated powerdensity, measured at a constant distance from the antenna in a horizontal or vertical plane.
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=bandwidth&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=bandwidth&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
11/231
Figure below shows the radiation pattern of a half-wave dipole antenna.
Radiation Pattern of a Half-Wave Dipole Antenna
Since the dipole is symmetrical around its axis, the three-dimensional radiation pattern rotatesaround the wire axis.
8/12/2019 ANTENNA for Communication Systems-2008cep
12/231
The isotropic gain of a half-wave dipole antenna is 2.15 dB. Therefore, in the direction
perpendicular to the wire axis, the radiated power density is 2.15 dB larger than that of the
isotropic radiator.
There is no radiation in the wire axis. The half-wave dipole produces linear polarization with the
electrical field vector in line with, or in other words parallel to, the wire axis.
Because the half-wave dipole is often used as a reference antenna for measurements, sometimesthe gain of an antenna is referenced to the radiated power density of a half-wave dipole instead of
an isotropic radiator.
Also the effective radiated power (ERP), which is the power delivered to an ideal dipole thatgives the same radiation density as the device under test, is used instead of the EIRP. The
relations Gdipole = Gisotropic - 2.15 dB and ERP= EIRP - 2.15 dB can be applied.
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=ERP&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=ERP&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
13/231
The half-wave dipole needs a differential feed because both of its terminations have the same
impedance to ground. This can be convenient if the transmitter outputor the receiver inputhave
differential ports.
A balun will be used along with the half-wave dipole in case of single-ended transmitters or
receivers, or if an antenna switch is used.
For external ready-manufactured half-wave dipoles, the balun is visually built-in to the antenna
and provides a single-ended interface.
The half-wave dipole is an electrical antenna. This means that it is easily detuned by materials
with a dielectric constant larger than 1 within its reactive near field.
If, for instance, the housing of a device is in the reactive near field, the housing has to be present
when the antenna is matched.
The human body has a large dielectric constant of approximately 75. As a result, if an electrical
antenna is worn on the body or held in the hand, it can be strongly detuned.
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=output&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=input&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=input&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=output&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
14/231
If the antenna is built as two traces on a PCB, the dielectric constant of the PCB material has to
be considered. The electrical field in the reactive near field region spreads out partially into the
PCB material, partially into the surrounding air. This gives an effective dielectric constant effbetween that of the air and the PCB material :
Where h is the thickness of the PCB material, w is the trace width of the dipole arms. Therequired length of the half-wave dipole is then:
Underneath the dipole and within the reactive near field, no ground plane is allowed.
8/12/2019 ANTENNA for Communication Systems-2008cep
15/231
A
8/12/2019 ANTENNA for Communication Systems-2008cep
16/231
Monopole Antennas
Quarter-Wave Monopole
In many cases, the half-wave dipole is just too large. Also, the needed differential feed is often a
disadvantage. If we replace one branch of the dipole antenna by an infinitely large ground plane,
due to the effect of mirroring, the radiation pattern above the ground plane remains unaffected.
This new structure is called a monopole antenna.
Building Up the Quarter-Wave Monopole.
Because all the radiated power is now concentrated in the half-space above the ground plane, thegain of the monopole is 3 dB larger than the gain of the dipole.
Often a large ground plane is not feasible. The Marconi antenna replaces the (not realizable)
infinitely large ground plane by several open-ended /4-Stubs, called the counterpoise.
A f h d i l b i h l k lik b di l Wh
8/12/2019 ANTENNA for Communication Systems-2008cep
17/231
A further reduction to only one stub gives a structure that looks like a bent dipole antenna. When
designing a monopole antenna, the radiator should go as long as possible perpendicular to the
ground stub or the ground plane. Bends close to the feeding point reduce the radiation resistance
and the efficiency of the antenna.
The ideal quarter-wave monopole has a linear polarization with the vector of the electrical field in
the wire axis. If the ground plane becomes unsymmetrical, the direction of polarization will be
tilted towards the larger part of the ground plane, but still remains linear.
The radiation resistance of an ideal quarter-wave monopole is half of that of a dipole; dependingon the ratio of length to diameter of the radiator between 30 and 36.5 .
Like the half-wave dipole, the quarter-wave monopole is an electrical antenna. It is influenced by
the dielectric constant of the material in the reactive near field.
The same formulas for the effective dielectric constant and the required length as for the half-
wave dipole hold for the quarter-wave monopole.
T bl 1 i h l h f h lf di l d l i f d
8/12/2019 ANTENNA for Communication Systems-2008cep
18/231
Table 1gives the length of half-wave dipoles and quarter-wave monopoles in free space and on a
PCB for commonly used short-range frequencies. For the PCB antennas, a PCB thickness of h =
1.5 mm and a trace width of w = 1 mm has been assumed; the PCB material is FR4 with r= 4.2.
This gives an effective dielectric constant of r= 2.97.
It has to be mentioned that parasitic components, such as capacitance to ground, inductance
introduced by bends in the antenna as well as the influence of the package, alter the antenna
impedance.
F l t th d l i ti ll th t l th t
8/12/2019 ANTENNA for Communication Systems-2008cep
19/231
For monopole antennas, the ground plane is sometimes smaller than a quarter-wave length or not
perpendicular to the radiator. In practice, the exact length of the dipole or the monopole has
therefore to be determined by measuring the feed impedance with a vector network analyzer.
Sometimes the available space limits the length of an antenna. The antenna is made as long as the
geometry permits, which can be smaller than one quarter wavelength. A monopole shorter than a
quarter-wave length can be considered as a quarter-wave monopole, which is used at a frequency
lower than the frequency of resonance.
According to the equivalent schematic of an antenna , the input impedance at the frequency of
operation will then be a series connection of a resistor and a capacitor.
The series capacitance can be resonated out by a series inductor. A monopole antenna shorter than
l/4 with a series inductor is also referred to as a loaded stub antenna.
8/12/2019 ANTENNA for Communication Systems-2008cep
20/231
The radiation resistance of a loaded stub decreases with decreasing length.
The smaller radiation resistance and the larger L-to-C ratio increase the quality factor and make
the bandwidth smaller than for a quarter-wave monopole. Approximations for the radiation
resistance of a monopole antenna are:
At the frequency of operation (i.e., resonance), the impedance of the short stub will be that of a
small resistor (radiation resistance plus loss resistance) with a series capacitor.
F th S ith Ch t i Fi th t t hi t 50 b hi d b
8/12/2019 ANTENNA for Communication Systems-2008cep
21/231
From the Smith Chart in Figure we can see that matching to a 50 source can be achieved by a
series inductor and a parallel capacitor.
Matching of a Short Loaded Stub Antenna
Figure below shows an example of a loaded stub PCB antenna with matching components.
Loaded Stub PCB Antenna With Matchin Com onents
The series ind ctor and the parallel capacitor transform the antenna impedance to 50 the input
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
22/231
The series inductor and the parallel capacitortransform the antenna impedance to 50 , the input
impedance of the filter (FIL1).
For dipole or monopole antennas, the component values for the series inductor and the parallel
capacitor (CP) have to be determined by measuring the feed impedance at point A in Figure 10
(with LS = 0 resistor and CPleft unpopulated) with a vector network analyzer.
Once this is determined, we can use a Smith Chart to assist in matching the antenna to 50 usingLSand CP.
Derivatives of the monopole are the inverted-L and inverted-F antennas as shown in Figure
below.
Inverted-L Antenna and Inverted-F Antenna
In the inverted-L antenna, the monopole does not run perpendicularly to the ground plane over its
whole length but is bent parallel to the ground plane after some distance. This helps to save
space, but decreases the radiation resistance because the radiator comes closer to the ground
plane. An additional matching circuit is needed to match the low-feed impedance to the usual
transmission line impedance of 50 .
If we proceed from the feed point of the inverted L antenna to the end we notice that the voltage
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
23/231
If we proceed from the feed point of the inverted-L antenna to the end, we notice that the voltage
increases (while the current decreases) from a maximum voltage value at the feeding point to
almost zero at the end.
This means, that the antenna impedance has its minimum if we feed the
antenna as shown in Figure a) and increases if we move the feeding point towards the end.
The inverted-F antenna in Figure bis an inverted-L antenna with a feeding tap that gives larger
antenna impedance.
If the antenna is tapped at the right location, no additional matching circuit is required.
The structure of inverted-F antennas and, in particular, the location of the tap, is usually
determined by electromagnetic simulations.
8/12/2019 ANTENNA for Communication Systems-2008cep
24/231
Monopole Discone Antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
25/231
Widebandth10:1 range.
Omnidirectiional in horizontal plane.
Vertically polarized.
Gain is similar to a dipole. Z approaches 50 ohms.
Application: RX scanner antenna for VHF and UHF.
Can also be used for TX.
PCB Monopole Antenna Module
8/12/2019 ANTENNA for Communication Systems-2008cep
26/231
PCB Monopole Antenna Module
Figure shows the layout of the PCB monopole antenna. In order to save PCB space the
monopole has been bent by 90 degree. In the PCB layout, the monopole was made longer than
calculated according to Table 1. This should give some room for possible manufacturing
tolerances.
Layout of the PCB Monopole Antenna.
Using a vector network analyzer, we measured the antenna impedance on the upper pad of L1
and cut back the monopole until real antenna impedance was achieved.
The antenna impedance in resonance is 35 5 which is within the theoretical value range of 30
http://rfdesignline.com/howto/lowpowerrf/showArticle.jhtml?articleId=197006085&pgno=2http://rfdesignline.com/howto/lowpowerrf/showArticle.jhtml?articleId=197006085&pgno=28/12/2019 ANTENNA for Communication Systems-2008cep
27/231
The antenna impedance in resonance is 35.5 , which is within the theoretical value range of 30
to 36.5 . The mismatch loss to 50 is as low as 0.13 dB in this case. No further matching
components have been used; inductor L1 was replaced by a 0 resistor. C2 was left unpopulated.
The radiation characteristic of the antenna module was measured in an anechoic chamber with
the test module upright (see Figure ) and flat (see Figure ) on the turntable.
The outer boundary of the radiation patterns given in this report correspond to an effective
radiated power (dipole related) of ERP= + 10 dBm; the scale is 20 dB/division.
25. Vertical Radiation Pattern o the Stub Module U ri ht .
The radiation pattern is almost angle-independent the maximum ERP is + 6 5 dBm
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=ERP&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=ERP&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
28/231
The radiation pattern is almost angle-independent, the maximum ERP is + 6.5 dBm,
corresponding to EIRP = 6.5 dBm + 2.15 dB = +8.65 dBm. The TRF4903 delivers +8 dBm of
output power. The maximum antenna gain is therefore +0.65 dB.
As expected, the horizontal radiation pattern has a more pronounced radiation characteristic:
. Horizontal Radiation Pattern of the Stub Module (Flat).
The maximum ERP is +10.85 dBm, corresponding to EIRP = +13 dBm. With +8-dBm transmit
power, this gives an antenna gain of 5 dB.
Electrical antennas are sensitive to detuning by dielectric material in their reactive near field.
Figure shows the horizontal radiation pattern of the same stub module attached to the arm of atest erson.
8/12/2019 ANTENNA for Communication Systems-2008cep
29/231
Horizontal Radiation Pattern of the Stub Module Close to the Human Body.
The maximum ERP is -4.4 dBm, compared to +10.85 dBm measured on the free stub module.
The loss due to detuning and absorption is as large as 15.25 dB in the direction of maximum
radiation.
8/12/2019 ANTENNA for Communication Systems-2008cep
30/231
8/12/2019 ANTENNA for Communication Systems-2008cep
31/231
3. Sleeve Monopole antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
32/231
3. Sleeve Monopole antenna
4. Loop antennas:
8/12/2019 ANTENNA for Communication Systems-2008cep
33/231
4. Loop antennas:
Small Loop Antennas
8/12/2019 ANTENNA for Communication Systems-2008cep
34/231
S oop e s
Small Loop Antenna With Differential Feed
The loop antenna shown in Figure has a differential feed. Often a ground plane is made part ofthe loop, giving a single-ended feed as shown in Figure .
Single-Ended Loop Antenna
The small arrows indicate the current flow through the loop. On the ground plane, the current is
mainly concentrated on the surface. The electrical behavior of the structure in Figure is thereforesimilar to that of the loo with differential feed shown in Fi ure .
8/12/2019 ANTENNA for Communication Systems-2008cep
35/231
The following considerations on small loop antennas are based on [4] and assume that the current
is constant over the loop. This means that the circumference must be smaller than one tenth of a
wavelength.
Although this is rarely the case, the given formulas describe the principal behavior and can beused as a starting point for the loop antenna design.
If the current is constant over the loop, we can consider the loop as a radiating inductor with
inductance L, where L is the inductance of the wire or PCB trace. Together with the capacitor C,
this inductance L builds a resonant circuit. Often a resistor Ratt is added to reduce the quality
factor of the antenna and to make it less sensitive to tolerances. Of course, this resistor dissipatesenergy and reduces the antenna's efficiency.
The following calculations hold for circular loops with the radius a for square loops with the side
length a. A rectangular antenna with the sides a1 and a2 will be approximated by an equivalent
square with the side length:
The length (circumference) of the wire building the loop will be called U, where U = 2a for a
circular loop, or 4a for a square loop.
For the calculation of the inductance, the wire radius b, where b is 1/2 the diameter of the actual
8/12/2019 ANTENNA for Communication Systems-2008cep
36/231
Figure belowshows the equivalent schematic of a small loop antenna.
, ,
wire used to fabricate the antenna, is needed. In the frequent case where a loop antenna is
realized by a trace on a PCB, b = 0.35.d + 0.24.w can be used, where d is the thickness of the
copper layer and w is the trace width.
Equivalent Schematics of the Small Loop Antenna
The radiation resistance of loop antennas is small, typically smaller than 1 . The loss resistance
8/12/2019 ANTENNA for Communication Systems-2008cep
37/231
p , yp y
Rloss describes the ohmic losses in the loop conductor and the losses in the capacitor.
Usually, the losses in the capacitor cannot be neglected. Interestingly, the thickness of the copper
foil is not needed for the calculation of the loss resistance because due to the skin effect, the
current is confined on the conductor surface.
Together with the loop inductance L, which is the inductance of the wire, the capacitor C builds a
series resonant circuit.
In practice, the L-to-C ratio of this resonant circuit is large giving a high quality factor (Q). This
would make the antenna sensitive to tolerances. That's why often an attenuating resistor Ratt isadded to reduce the Q.
To describe the influence of Ratt on the loop antenna, we make a parallel to series conversion and
use the equivalent series resistance Ratt_trans. The resistance value of Ratt_trans is determined
by the acceptable tolerance of the capacitor and the geometry of the loop.
The maximum usable quality factor is calculated from the capacitance tolerances C/C:
The series transformed attenuation resistance then will be:
8/12/2019 ANTENNA for Communication Systems-2008cep
38/231
This gives the efficiency of the loop antenna:
In most cases, the radiation resistance is much smaller than the loss resistance and the
transformed attenuation resistance, giving a poor efficiency. In this case, the approximation:
can be used. Rr is determined by the loop area, which is a2 for circular loops, a2for square loops,
and a1a2for rectangular loops.
Figure in next slideshows the efficiency of small circular loop antennas versus their diameter
for an assumed tolerance of 5 percent. The trace width has been assumed as 1 mm, the copper
thickness as 50 um; but both values have only a minor influence on the efficiency, which is
mainly determined by the attenuation resistance Ratt. As expected, the efficiency increases with
increasing diameter.
8/12/2019 ANTENNA for Communication Systems-2008cep
39/231
In both cases, matching to 50 will be difficult. That's why the loop antenna is often tapped,
8/12/2019 ANTENNA for Communication Systems-2008cep
40/231
giving an impedance in between the too small and the too large values. Figure belowshows an
example:
Example of a Tapped PCB Loop Antenna.
A series feed (in the lower right corner) would give a small impedance. A parallel feed (directly at
the capacitors) would give a much too large impedance.
The tap provides an impedance close to 50 in this example. The loop capacitor has been split
into two series capacitors C1 and C2. This makes it possible to realize capacitance values. R1 is
the damping resistor which de-Qs the circuit, thus increasing the bandwidth and subsequently
Unfortunately, there are no easy formulas that describe the tapped structure and give the right
8/12/2019 ANTENNA for Communication Systems-2008cep
41/231
location for the tap.
The line from the antenna termination to the tap is not a transmission line and will disturb the
field in the antenna. Therefore, we have to find out the optimal structure by electromagnetic
simulations.
Often a trial and error procedure is used as an alternative.
For example, using a vector network analyzer, we determine the capacitance value that gives the
best return loss and the largest resistance value that gives the required bandwidth.
The loop antenna gives a linear polarization with the vector of the electric field oscillating in the
plane built by the loop.
In contrast to all of the antennas discussed so far, loop antennas are magnetic antennas.
This means that they are not detuned by the dielectric constant of the material in their reactivenear field. That's why loop antennas are often used for body-worn or hand-held equipment.
Loop Antenna Module
8/12/2019 ANTENNA for Communication Systems-2008cep
42/231
Figure belowhas the layout of the loop antenna module. The loop capacitoris a series
connection of the two capacitors C40 and C44; this allows realizing small capacitance values in
fine steps. R11 is the attenuation resistor; L5 and C46 should help to improve the matching to 50
experimentally, through an iterative approach.
PCB Loop Antenna
The loop width is 25 mm, the height 11.5 mm, the trace width 1.5 mm with a copper thickness of
50 m.
According to the formulas in Figure 15, the inductance is L = 40.9 nH and the radiation
resistance Rr = 0.22 .
The calculated capacitance needed for resonance at 915 MHz is 0.74 pF.
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
43/231
Usual tolerance values for low cost capacitors are around five percent. For the small capacitance
needed here, the effective tolerance would be larger because the parasitic capacitance of the
damping resistor, the PCB pads, and even the solder material contributes to the total capacitance
uncertainty. We assumed a total tolerance of 20 percent, which gives a maximum quality factor:
The impedance of the loop inductance is ZL = 2.x 915 MHz x 40.9 nH = 235 . The damping
resistance must therefore be not larger than Ratt = 10.5 x 235 = 2.47 k.
In the test module, a 2.2-k resistor was chosen, giving Q = 2.2 k/235 = 9.36. The
transformed series attenuation resistance is then Ratt_trans = ZL/Q = 235 / 9.36 = 25.1 .
Compared to that, we can neglect the loss resistance of the copper trace on the PCB.
The theoretical antenna efficiency is then:
The loop antenna has been tapped to increase the antenna impedance. The position of the tap was
8/12/2019 ANTENNA for Communication Systems-2008cep
44/231
determined empirically by electromagnetic simulations.
The antenna impedance was measured on the assembled PCB on the upper pad of C51; L5 was
replaced by a 0- resistor at first.
We varied the loop capacitors C40 and C44 to bring the loop into resonance. A series connection
of 0.8 pF and 0.5 pF gives the reflection coefficient of 0.22.ej35, corresponding to a mismatch
loss of 0.2 dB. No further matching was required, so C46 was left unpopulated.
The capacitance of the series connection of the 0.5-pF and 0.8-pF capacitors is 0.3 pF and thussmaller than the calculated value of 0.74 pF. One explanation is that the parasitic capacitance of
the resistor also contributes to the loop capacitance. Also, the parasitic inductance of the
capacitors themselves makes their capacitance look larger at high frequencies than their nominal
value.
Figure next slidehas the horizontal radiation pattern of the loop antenna module arranged flat ona turntable.
8/12/2019 ANTENNA for Communication Systems-2008cep
45/231
Radiation Pattern of the Loop Antenna Module.
The maximum ERP is +4.15 dBm, corresponding to EIRP = +6.3 dBm. This gives an antenna
gain of -1.7 dB, much more than calculated. The reason is that the circumference of the loop isnot negligible with respect to the wavelength. All calculations were made under the assumption
that the circumference is smaller than one tenth of the wavelength. The geometrical
circumference of the loop antenna in the test module is 73 mm, the wavelength in free space for
915 MHz is 327 mm. Assuming the effective dielectric constant of a trace on FR4 material of
2.97 , the wavelength on the PCB is:
8/12/2019 ANTENNA for Communication Systems-2008cep
46/231
The circumference is therefore 73 mm / 190 mm = 0.38 times the wavelength. The loop does not
act like a purely magnetic antenna any more; it will also excite the electrical field in its reactive
near field region. As a result, the behavior is in between that of a small loop and an electrical
antenna.
For a loop antenna, a smaller influence of dielectric material in the reactive near field on the
tuning and the radiation characteristic is expected. Figure below shows the radiation pattern of
the loop module attached to the forearm of a test person.
Radiation Pattern of the Loop
Antenna Module Close to the HumanBody.
The ERP is -9.75 dBm, corresponding to EIRP = -7.6
dBm. Given the transmitter power of +8 dBm, the
antenna gain is -15.6 dB, almost 14 dB worse than infree space.
Note that the gain is still larger than the theoretical
value of -20.6 dB. This again comes from the large
dimensions which make the antenna more similar to
an electrical radiator.
In order to achieve greater independence from the
influences of the surrounding material, the size of the
loop antenna must be made smaller. This reduces the
radiation resistance and L-to-C ratio and decreases
the efficiency.
8/12/2019 ANTENNA for Communication Systems-2008cep
47/231
8/12/2019 ANTENNA for Communication Systems-2008cep
48/231
Small loop antennas are insensitive to varying dielectric conditions in their reactive near field.
Th b d l i f bl d h d h ld d i b h h l ffi i
8/12/2019 ANTENNA for Communication Systems-2008cep
49/231
They can be a good solution for portable and hand-held devices but have a much lower efficiency
than electrical antennas.
Only antennas with a circumference smaller than one tenth of a wavelength can be considered as
purely magnetic antennas.
Larger loops have a higher gain but also a higher sensitivity to the environmental conditions.
Size matters: Always keep in mind that Chu's and Wheeler's limit determines the product of the
bandwidth and the efficiency for a given antenna dimension.
An extremely small antenna cannot be efficient and tolerance-insensitive at the same time
5. Helix.
8/12/2019 ANTENNA for Communication Systems-2008cep
50/231
8/12/2019 ANTENNA for Communication Systems-2008cep
51/231
Transversal Mode Helical Antenna
Another option to reduce the size of a monopole is to coil it up into a helix as shown in Figure bel
Helix Antenna on a Ground Plane
When the coil circumference and the spacing between adjacent turns are comparable to the wavele
the antenna radiates a circular polarized beam in the axis of the helix. These antennas are called axmode helicals.
In small short-range applications, the helix diameter and the spacing between turns are much small
than a wavelength. So, the result is a normal mode helical antenna.
The radiation pattern of a normal mode helix is similar to that of a monopole; the maximum
di ti di l t th h li i
8/12/2019 ANTENNA for Communication Systems-2008cep
52/231
radiation occurs perpendicular to the helix axis.
Due to the shape and the size of the ground plane, radiation patterns of practical antennas can
show deviations from this idealized form.
The radiation from a normal mode helix is elliptically polarized. Usually the component having
the electrical field vector parallel to the antenna axis is stronger than the component which is
parallel to the ground plane.
The exact calculation of transversal mode helical antennas is not as simple as for dipole and
monopole antennas.
Usually they are designed empirically: start with a wire that is half a wavelength long, wind it up
into a helix, and measure the antenna impedance using a vector network analyzer. Then, cut it
back until nearly real input impedance at the frequency of operation is obtained.
Real input impedance means that the antenna is in resonance. Fine-tuning of the frequency of
i ibl b i t t hi th h li
8/12/2019 ANTENNA for Communication Systems-2008cep
53/231
resonance is possible by compressing or stretching the helix.
Even if the antenna is in resonance, it will not be matched to 50 yet.
The input impedance will be the sum of the radiation and loss resistances, usually smaller than 50
. For the design of the needed additional matching circuit, we can use the Smith Chart as
described above.
Chu's and Wheeler's [1,2] limit on the bandwidth for a given dimension also holds for helix
antennas.
A small transversal mode helix, therefore, has tight bandwidth and is sensitive to tolerances of thematching components.
8/12/2019 ANTENNA for Communication Systems-2008cep
54/231
As opposed to the monopole, underneath and around the antenna there is a ground plane. The
white dots in the previous Figure are vias connecting the top side ground with the internal
8/12/2019 ANTENNA for Communication Systems-2008cep
55/231
white dots in the previous Figure are vias connecting the top side ground with the internal
ground layer of the PCB.
The antenna impedance was measured on the upper pad of C7. Without matching components
and with L1 replaced by a 0- resistor, the measured reflection coefficient was = 0.91.e-143.
L1 = 5.6 nH and C2 = 16 pF give an acceptable reflection coefficient of = 0.35.e147. Figure
29has the radiation pattern. The test module was placed flat on the turntable.
Horizontal Radiation Pattern of the Helix Module
The maximum ERP is -10.6 dBm, corresponding to EIRP = -8.45 dBm. With +8-dBm transmit
power follows a gain as low as 16 45 dB
8/12/2019 ANTENNA for Communication Systems-2008cep
56/231
power follows a gain as low as -16.45 dB.
The small dimensions of the antenna lead to a small radiation resistance and a strong effect of
losses in the PCB and the matching components. Also, the size of the PCB ground plane is not
large compared to the wavelength. That's why the shape and the size of the ground plane have an
effect on the radiation pattern.
Figure below shows the radiation pattern under the same conditions but with the helix module
attached to the forearm of a test person.
The PCB ground plane was between the helix and the arm; the forearm was held in a horizontalposition.Horizontal Radiation Pattern of the
Helix Module Close to the Human Body
Despite the lower efficiency in the
upper right corner (which comes from
the absorption by the chest of the test
person), the radiation pattern is identical
to that of the free helix module.
The PCB ground plane acts as a shield
and makes the radiation patternindependent on the tissue underneath it.
6. Yagi - Uda
8/12/2019 ANTENNA for Communication Systems-2008cep
57/231
Array antennas can be used to increase directivity.
Parasitic array does not require a direct connection to each element by a feed network.
The parasite elements acquire their excitation from near field coupling by the driven
element.
A Yagi-Uda antenna is a linear array of parallel dipoles.
The basic Yagi unit consists of three elements:
1. Driver or driven element
2. Reflector
3. Director
8/12/2019 ANTENNA for Communication Systems-2008cep
58/231
Develops an endfire radiation pattern.
Optimum spacing for gain of a reflector and driven element is 0.15 to 0.25 wavelengths
Director to director spacings are 0.2 to 0.35 wavelengths apart.
Reflector length is typically 0.5 wavelengths or 1.05 that of the driven element.
The driven element is calculated at resonance without the presence of parasitic
elements.
The directors are usually 10 to 20% shorter than at resonance.
8/12/2019 ANTENNA for Communication Systems-2008cep
59/231
Gain is related to boom length and number of directors.
Max directivity of a 3 element Yagi is 9 dBi or 7dBd.
Addition of directors up to 5 or 6 provides significant increase in gain. Addition of more
directors has much less impact on gain.
Increasing N from 3 to 4 results in 1 dB increase.
Adding a director to go from 9 to 10 presents a 0.2 dB gain improvement.
Adding more reflectors has minimal impact on gain however does impact on feedpoint Zand the backlobe.
8/12/2019 ANTENNA for Communication Systems-2008cep
60/231
8/12/2019 ANTENNA for Communication Systems-2008cep
61/231
Metal booms can be implemented because voltage is at zero midway through the
element.
Other factors that effect resonant lengths:
1. A comparatively large boom will require parasitic elements to increase their
length.
2. Length to diameter ratio of the elements.
8/12/2019 ANTENNA for Communication Systems-2008cep
62/231
8/12/2019 ANTENNA for Communication Systems-2008cep
63/231
8/12/2019 ANTENNA for Communication Systems-2008cep
64/231
8/12/2019 ANTENNA for Communication Systems-2008cep
65/231
7. Log-Periodic
8/12/2019 ANTENNA for Communication Systems-2008cep
66/231
8/12/2019 ANTENNA for Communication Systems-2008cep
67/231
lengthselementrespectiveisLwhere
L
L
L
L
L
L......
4
3
3
2
2
1
.
.sin
.....
1
4
3
3
2
2
1
shortestisD
themgcloangleofapexand
elementsbetweenspacingsrepresentsDwhere
D
D
D
D
D
D
2tan
2 1
1 DL
Alpha is the angle of the apex of tapered elements and is typically 30 degrees.
. :
8/12/2019 ANTENNA for Communication Systems-2008cep
68/231
Horn is widely used as a feed element for
large radius astronomy dish
satellite tracking dish communication dish
feed for reflector and lenses
common element for phased arrays
universal standard for calibration
Horn is
simple in construction
easy to excite
versatile
large gain good overall performance
Types of Horn
8/12/2019 ANTENNA for Communication Systems-2008cep
69/231
yp
1. E-plane sectorial horn
2. H-plane sectorial horn
3. Pyramidal
4. Conical
5. Corrugated
6. Aperature matched
7. Multimode
8. Dielectric loaded
8/12/2019 ANTENNA for Communication Systems-2008cep
70/231
2
P E H
0 1 12
D D D32ab
1 4G ( a b ).2
Conical hornSupport any polarization .
8/12/2019 ANTENNA for Communication Systems-2008cep
71/231
Corrugated horn
9. Parabolic antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
72/231
i) Parabolic cylinder
Feed: linear dipole linear array or slotted waveguide.
ii) Parabolic (parabola of revolution).
Feed: pyramidal or conical horn. F/d ratio, spill over.
8/12/2019 ANTENNA for Communication Systems-2008cep
73/231
10. Cassegrain reflector
To improve the performance of large ground based microwave reflector antennas for satellitetracking and communication, it has been proposed that a two reflector system be utilized.
This is known as cassegrain dual reflector system. The use of second reflector, a hyperboloid
known as sub reflector give an additional degree of freedom for achieving good performance.
Cassegrain antenna is usually attractive for applications that requires gain of 40 dB or greater.
8/12/2019 ANTENNA for Communication Systems-2008cep
74/231
8/12/2019 ANTENNA for Communication Systems-2008cep
75/231
11. Slotted waveguide antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
76/231
is widely used as radar antenna because its simple construction and compact shape .
8/12/2019 ANTENNA for Communication Systems-2008cep
77/231
8/12/2019 ANTENNA for Communication Systems-2008cep
78/231
12.Microstrip Antennas
8/12/2019 ANTENNA for Communication Systems-2008cep
79/231
-
Basic Characteristics:
Microstrip antennas consist of very thin (t
8/12/2019 ANTENNA for Communication Systems-2008cep
80/231
8/12/2019 ANTENNA for Communication Systems-2008cep
81/231
MICROSTRIP LINE:
8/12/2019 ANTENNA for Communication Systems-2008cep
82/231
In a microstrip line most of the eletric field lines are concentrated underneath the
microstrip.
Because all fields do not exist between microstrip and ground plane we have a
different dielectric constant than that of the substrate. It is less, depending on
geometry.
The electric field underneath the microstrip line is uniform across the line. It is
possible to excite an undesired tranverse resonant mode if the frequency or line
width increases. It now behaves like a resonator consuming power.
A standing wave develops across its width as it acts as a resonator. The electric fieldis at a maximum at both edges and goes to zero in the centre.
Microstrip discontinuities can be used to advantage.
8/12/2019 ANTENNA for Communication Systems-2008cep
83/231
Abrupt truncation of microstrip lines develop fringing fields storing energy and
acting like a capacitor because changes in electric field distribution are greater than
that for magnetic field distribution.
The line is electrically longer than its physical length due to capacitance.
For a microstrip patch the width is much larger than that of the line the fringingfields also radiate.
An equivalent circuit for a microstrip patch illustrates a parallel combination of
conductance and capacitance at each edge.
Radiation from the patch is linearly polarized with the E field lying in the same
direction as path length.
8/12/2019 ANTENNA for Communication Systems-2008cep
84/231
Endfire radiation can also be accomplished by judicious mode selection.For
8/12/2019 ANTENNA for Communication Systems-2008cep
85/231
Endfire radiation can also be accomplished by judicious mode selection.For
rectangular patch, usually
The strip (patch) and the ground plane are separated by a dielectric sheet (substrate)
The substrates that are most desirable for antenna performance are thick
substrates whose dielectric constant is lower because they provide better efficiency,larger bandwidth, loosely bound fields for radiation into space but at the expense of
larger element size.
8/12/2019 ANTENNA for Communication Systems-2008cep
86/231
tconsdielectricrelative
wavelengthspacefreeiswhere
re
o
re
o
tan
2/112
1115.0
/5.0
W
H
L
rreffr
reo
13. Arrays.- beam shaping scanning, large gain
8/12/2019 ANTENNA for Communication Systems-2008cep
87/231
8/12/2019 ANTENNA for Communication Systems-2008cep
88/231
The total field of the array is determined by the vector addition of the fields radiated by the
individual elements.
In an array of identical elements, there are five controls that can be used to shape the overall
pattern of the antenna.
( i) the geometrical configuration of the overall array (linear, circular, rectangular, etc)
(ii) relative displacement between the elements
(iii) excitation amplitude of the individual elements
(iv) excitation phase of the individual elements
(v) the relative pattern of the individual element
8/12/2019 ANTENNA for Communication Systems-2008cep
89/231
MICROMACHINED ANTENNAS
8/12/2019 ANTENNA for Communication Systems-2008cep
90/231
Size of the antenna gets smaller as the frequency increases demandinghigher precision manufacturing which can be overcome by micro
machining technique
Incorporation of micro machined actuators within the antenna itself tofacilitate special features to the antenna such as beam shaping and
reconfigurability
Microstrip antennas
8/12/2019 ANTENNA for Communication Systems-2008cep
91/231
8/12/2019 ANTENNA for Communication Systems-2008cep
92/231
8/12/2019 ANTENNA for Communication Systems-2008cep
93/231
Circuit operation demand higher dielectric constant substrate whereasantenna operation requires low dielectric constant substrate.
8/12/2019 ANTENNA for Communication Systems-2008cep
94/231
Effective dielectric constant is reduced by drilling small holes in the
antenna substrate
8/12/2019 ANTENNA for Communication Systems-2008cep
95/231
8/12/2019 ANTENNA for Communication Systems-2008cep
96/231
8/12/2019 ANTENNA for Communication Systems-2008cep
97/231
Antenna performance can be improved by removing materials of the
substrate below the antenna .
Improve antenna characteristics achieved by forming trenches below
the radiating edges (instead of forming cavity below the patch) so that
the conductor of the patch was overhanging.
8/12/2019 ANTENNA for Communication Systems-2008cep
98/231
40% improved in BW and marked improvement inradiation pattern achieved at 13.8GHz
8/12/2019 ANTENNA for Communication Systems-2008cep
99/231
Mutual coupling between antenna array elements is reduced by makingcavities below the patch radiators. Surface waves which contribute to the
8/12/2019 ANTENNA for Communication Systems-2008cep
100/231
mutual coupling are excited on the substrate above a cut off frequency
determined by its thickness.
More than 2/3 of the power is lost as surface waves on a 200m thick
substrate.
Micromachived Reconfigurable Antennas
Antennas capable of adaptively changing their characteristics arecalled reconfigurable antennas.
Patch radiator of a micro machined antenna is rotated about a
fulcrum to get beam steering capabilities
8/12/2019 ANTENNA for Communication Systems-2008cep
101/231
The antenna is patterned on 100m fuzed quartz atop 500 m
ili b t t d d b i f t i i
8/12/2019 ANTENNA for Communication Systems-2008cep
102/231
silicon substrate, suspended by a pair of torsion springs.
The rotation is achieved by electrostatic forces activating fixed
electrode on the substrate.
MEMS actuators used to adjust the angles of the arms of a Ve
antennas operating at 17.5GHz
Reconfigurable multiband phasedarray antennas are receiving a lot
of attention lately due to the emergence of RF MEMS (microelectro
8/12/2019 ANTENNA for Communication Systems-2008cep
103/231
y g (
mechanical systems) switches [1-6].
A MEMS- switched reconfigurable multi band antenna is one that
can be dynamically reconfigured within a few microseconds to serve
different applications at drastically different frequency bands, such as
communications at L band (1-2 GHz) and synthetic aperture radar(SAR) at Xband (8-12.5 GHz).
They can be used both for ground and airborne moving target
indication (GMTI/AMTI) at these frequencies in order to detect moving
targets such as vehicles on the ground and low observables in the air.
8/12/2019 ANTENNA for Communication Systems-2008cep
104/231
The RF MEMS switch is attractive because it shows of achieving
execellent switching characteristics over an extremely wide band (DC40 GHz and upwards).
These switches can also be used to develop wideband phase shifters .
Although there is currently a tremendous amount of research in RF
MEMS devices, reliability and packaging of the switches continue to be
problematic.
The switches are also limited in their power handling capability.
Reconfigurable patch Module (RPM):
In the design and fabrication of a dual L/X band reconfigurable
8/12/2019 ANTENNA for Communication Systems-2008cep
105/231
In the design and fabrication of a dual L/X band reconfigurable
antenna, microstrip antenna elements were chosen due to their inherent
low profile, which is suitable for satellite and UAVapplications.
RT/ duroid 5880 material with a dielectric constant of 2.2 and a loss
tangent of 0.0009 at 10 GHz is usually used.
The material thickness must be chosen carefully, since it controls both
the bandwidth and array scanning performance.
The thicker the material, the more bandwidth, particularly at the low
frequency end.
However, if the substrate becomes too thick, surface waves are generated
and array scanning performance and efficiency are lost.
8/12/2019 ANTENNA for Communication Systems-2008cep
106/231
8/12/2019 ANTENNA for Communication Systems-2008cep
107/231
Figure 1 shows a picture of the 3x3RPM fabricated on 0.125 duroid.
The patches are 0.370square and separated by 0.590on center.
Interconnecting tabs are 0.050wide and 0.085long.
The reconfigurable antenna was actually fabricated as two separated
prototypes (OPEN and CLOSED configurations) for testing in thelaboratory.
8/12/2019 ANTENNA for Communication Systems-2008cep
108/231
Figure 1: Picture of 3x3 array prototypes fabricated on 0.125 thick substrate in (a)
OPEN configuration; and (b) CLOSEDconfiguration.
1.2% impedance bandwidth for the Lband configuration and greaterthan 7% bandwidth at Xband are achieved.
8/12/2019 ANTENNA for Communication Systems-2008cep
109/231
This bandwidth was limited primarily by the substrate thickness.
Computer simulation results for both the return loss and radiationpatterns agree well with the measurements.
Reconfigerable Antenna at C- band
In a number of radar applications it is desired to have
8/12/2019 ANTENNA for Communication Systems-2008cep
110/231
In a number of radar applications, it is desired to have
limited antenna beam scanning and also different beam
patterns (like pencil beam, fan beam, etc.)
Microelectromechanical system (MEMS) technology is made
it possible to realize such antenna with simple configuration.
In one configuration, the elements of a phased array antennaare switched using RF MEMS switches so that aperture area
and / or antenna elements sizes are changed.
In another configuration which uses V-antenna, the arms ofthe V-antenna is moved using MEMS actuators so that the
beam can be scanned and also the beam can be shaped.
In many applications in proximity fuse, there is a requirementof small light weight antenna with direction of beam along the
8/12/2019 ANTENNA for Communication Systems-2008cep
111/231
axis of the fuse.
If one get the facility of beam scanning, it provides good
system flexibility.
Usually, scanning beam requires either costly phase shifters orcumbersome mechanical arrangement.
This system eliminates the need of phase shifter and
cumbersome mechanical arrangement yet it provides beamscanning.
The overall system is light weight, simple and cheap.
The basic antenna is a Vee-antenna on silicon substrate, fabricated usingMEMS (micro- electromechanical system) technology and for beam
scanning electrostatic actuators is used as shown in fig 1
8/12/2019 ANTENNA for Communication Systems-2008cep
112/231
scanning electrostatic actuators is used as shown in fig.1..
When the angles between the arms of the antenna is changed, the beam
shape is altered .
When both the arms rotated together in one direction, the beam is
scanned.
8/12/2019 ANTENNA for Communication Systems-2008cep
113/231
Fig-1: MEMS Reconfigurable Vee- Antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
114/231
Fig2: Top view and details of a MEMS reconfigurable Vee-
antenna.
The system is capable of not only beam scanning, but also beamshaping. The characteristics of Vee-antenna depends on the included
angle 2 of V dipole and the length l of the arms The optimum included
8/12/2019 ANTENNA for Communication Systems-2008cep
115/231
angle,2of V dipole and the length, l of the arms. The optimum included
angles and maximum directivities are given by
3 2
0 2
149.3 603.4 809.5 443.6
for 0.5 1.52
13.39 78.27 169.77
for 1.5 3
0
2.94 1.15 for 0.5 3lD
By changing the angle, 2othe beam shape may be modified.
For scanning the beam, it requires the arms of V to be moved
8/12/2019 ANTENNA for Communication Systems-2008cep
116/231
simultaneously keeping the same included angle of V.
The arms of the V-antenna is movable through pulling or pushing byactuators.
One end of the antenna arm is held by rotational hinge locked on the
substrate which allows the arms to rotate with the hinge as the center of
the circle.The antenna arms are pulled or pushed by support bars connected to the
actuators with movable rotation hinges on both ends.
The movable rotation hinges translate the lateral movement of the
actuators to circular movement of the antenna arms which are electrically
separated from the support bar by dielectric material.
Fig.2 shows the system and Fig. 3 shows the performance at 3.00 GHz
reported (1) in the literature.
8/12/2019 ANTENNA for Communication Systems-2008cep
117/231
Fig 3 : E-plane co-pol and cross-pol patterns compared with the
theoretical co-pol patterns for the 3-GHZ model.
The actuations used for moving the arms of the antenna is usually ofelectrostatic type. Which consists of a capacitor arrangement, where one
of the plate is movable by the application of bias voltage This produces
8/12/2019 ANTENNA for Communication Systems-2008cep
118/231
of the plate is movable by the application of bias voltage. This produces
displacement.
However , when a voltage is applied across the system, one plate moves
towards the other (are plate is kept fixed), resulting in a net gap
d = d0-x
The capacitance with the plates in new position is
0
0
AC d
1
000
0 0 0 0 0
1
( ) ( ) ( ) ( ) ( )( ) 1
A A xC C
dd d x
Q t x t Q t Q t x t V t
C d C C d
The displacement x(t ) changes with the change of voltage V(t).
8/12/2019 ANTENNA for Communication Systems-2008cep
119/231
Specifications (Typical)
Frequency : 5.0 GHZ
Beamwidth : 3
Scanning angle : 45
Size : 5.5x 5.5 cm2
8/12/2019 ANTENNA for Communication Systems-2008cep
120/231
8/12/2019 ANTENNA for Communication Systems-2008cep
121/231
8/12/2019 ANTENNA for Communication Systems-2008cep
122/231
8/12/2019 ANTENNA for Communication Systems-2008cep
123/231
8/12/2019 ANTENNA for Communication Systems-2008cep
124/231
8/12/2019 ANTENNA for Communication Systems-2008cep
125/231
Multiple Frequency Antennas
8/12/2019 ANTENNA for Communication Systems-2008cep
126/231
8/12/2019 ANTENNA for Communication Systems-2008cep
127/231
8/12/2019 ANTENNA for Communication Systems-2008cep
128/231
8/12/2019 ANTENNA for Communication Systems-2008cep
129/231
8/12/2019 ANTENNA for Communication Systems-2008cep
130/231
3. FUNDAMENTAL PARAMETERS OF ANTENNASRadiation Pattern
Radiation Power density.
Radiation Intensity
8/12/2019 ANTENNA for Communication Systems-2008cep
131/231
y
Directivity
Gain
EfficiencyBeamwidth
Beam efficiency
Bandwidth
Polarization
Input impedance
Antenna equivalent areas.Antenna Radar Cross section
Antenna Temperature.
Radiation Pattern:
Defined as a mathematical functions or graphical representation of the radiation properties of
the antenna as a function of space coordinatesIn general, the pattern of an antenna is three- dimensional. Because it is impractical to measure a
three dimensional pattern, a no. of two dimensional patterns are measured.
ii) The minimum no. of two dimensional pattern is two and they are E
and H- plane patterns.
Two dimensional pattern is obtained by fixing one of the angle ( and ) while varying other.
8/12/2019 ANTENNA for Communication Systems-2008cep
132/231
Power Pattern: -A trace of the received power at a constant radius
Amplitude F ield Pattern: -A graph of the spatial variation of the electric (or magnetic)
field along a constant radius.
8/12/2019 ANTENNA for Communication Systems-2008cep
133/231
8/12/2019 ANTENNA for Communication Systems-2008cep
134/231
8/12/2019 ANTENNA for Communication Systems-2008cep
135/231
8/12/2019 ANTENNA for Communication Systems-2008cep
136/231
IsotropicRadiatorA hypothetical lossless antenna having equal radiation in all directions.Directional Antennahaving the property of radiating or receiving electromagnetic waves more
effectively in some directions than in others.
8/12/2019 ANTENNA for Communication Systems-2008cep
137/231
OmniDirectionalhaving an essentially nondirectional pattern in a given plane and a
directional pattern in any orthogonal plane.
Principal Patternfor linearly polarized antenna, performance is described in terms of principal
Eand H-plane patterns.
E- plane- the plane containing the electric field vector and the direction of maximum radiation.
H-Plane- the plane containing the magnetic field vector and the direction of maximum radiation.
Field region:-
There are three main components to the radiated electromagnetic fields surrounding an antenna.
Two near field regions and far field region.
In the reactive near field, reactive field components predominate over the radiated field. Thismeans that any variations in the electrical properties (for electrical antennas) or magnetic
properties (for magnetic antennas) have a strong influence on the antenna's impedance at the
antenna feed point. The distance from the antenna to the boundary of the reactive near field
8/12/2019 ANTENNA for Communication Systems-2008cep
138/231
region is commonly assumed as:
In the radiating near field, the radiated field predominates, and the antenna impedance is only
slightly influenced by the surrounding media in this region. But the dimensions of the antenna
cannot be neglected with respect to the distance from the antenna. This means that the angular
distribution of the radiation pattern is dependent on the distance. For measurements of the
radiation pattern, the distance from the antenna should be larger than the radiating near fieldboundary, otherwise the measured pattern will be different from that under real life conditions.
The diameter of the radiating near field is
with D as the largest dimension of the antenna.For distances larger than R2, the radiation pattern is independent of the distance, meaning we are
in the far field region. In a practical application, the distance between transmitter and receiver
antennas is usually in this region.
Radiating near fieldRadiating near field (fresnel region)
Far Field (Fraunhofer
i )Radiating near field
8/12/2019 ANTENNA for Communication Systems-2008cep
139/231
2
2
2DR
3
1 0.62 D
R (fresnel region)
region)Radiating near field
(fresnel region)
Radiating near field(fresnel region)
The field strengths of the near field components decreases rapidly with the increasing
distance from the antenna, one component being inversely related to distance squared and the
other distance cubed.
Far field-radiated field is TEM. Wave front is practically plane
22 /R D
8/12/2019 ANTENNA for Communication Systems-2008cep
140/231
Relativepower
dB
sinD
U
24 /R D R=
R=
R=
Calculated radiation patterns of a paraboloid antenna.
Radiation & Steradian:
One radian is defined as the plane angle with its vertex at the center of a circle of radius r that
is subtended by an arc length, r.
8/12/2019 ANTENNA for Communication Systems-2008cep
141/231
Steradian:
-as the solid angle with its vertex at the center of a sphere of radius, r that is subtended by a
spherical, surface area equal to that of a square of each side of length, r
Surface area = 4r2
The infinitesimal area dA on the surface of a sphere of radius, r is
dA = r2sindd(sr)
Therefore, the element of solid angle d of a sphere can be written as
( sr)2 sindA
d d d
r
Isotropic Radiator
The concept of the isotropic radiator is often used to describe radiated power and antenna gain.
The isotropic radiator is a hypothetical antenna, which radiates the supplied RF power equally in
ll di i h d i di f h i i di i h f h
8/12/2019 ANTENNA for Communication Systems-2008cep
142/231
all directions. The power density at a distance rfrom the isotropic radiator is therefore the
supplied power divided by the area of a sphere with the radius r.
1. Isotropic Radiator
If we measure the power density in some distance from a device under test, the effective isotropicradiated power (EIRP) is the power which we would have to supply to an isotropic radiator in
order to get the same power density in the same distance. The EIRP describes the power radiation
capability of a device (including its antenna).
8/12/2019 ANTENNA for Communication Systems-2008cep
143/231
From the EIRP, we can calculate the electrical field strength at a given distance from the radiator,which is specified in some government and regional regulations. The density of the radiated
power D (in W/m2) measured in the distance rfrom an isotropic radiator radiating the total
power EIRP is the radiated power divided by the surface area of the sphere with the radius r:
f we measure the power density in some distance from a device under test, the effective isotropicradiated power (EIRP) is the power which we would have to supply to an isotropic radiator in
order to get the same power density in the same distance. The EIRP describes the power radiation
capability of a device (including its antenna).
From the EIRP, we can calculate the electrical field strength at a given distance from the radiator,which is specified in some government and regional regulations. The density of the radiated
power D (in W/m2) measured in the distance rfrom an isotropic radiator radiating the total
power EIRP is the radiated power divided by the surface area of the sphere with the radius r:
8/12/2019 ANTENNA for Communication Systems-2008cep
144/231
The relationship between the electrical field strength and the power density is the same asbetween voltage and power in an electrical circuit.
With the impedance of free space Z0 = 377 = 120 , the rms value of the electrical field
strength is then:
This gives:
Or:
Taking the logarithm on both sides gives the EIRP value in dBm:
EIRP[dBm]=E[dBV/m]+20 logr[meters]-10log30-90 dB
8/12/2019 ANTENNA for Communication Systems-2008cep
145/231
In standard test setups, the electrical field strength is often measured at a distance of 3 m. In thiscase we can use the simple formula:
EIRP[dBm] = E[dBV/m] " 95.23 dB
As opposed to the hypothetical isotropic radiator, real antennas exhibit more or less distinct
directional radiation characteristics. The radiation pattern of an antenna is the normalized polar
plot of the radiated power density, measured at a constant distance from the antenna in a
horizontal or vertical plane.
The isotropic gain Giso of an antenna indicates how many times the power density of thedescribed antenna in the main direction of propagation is larger than the power density from an
isotropic radiator at the same distance.
Antenna gain does not imply an amplification of power; it comes only from the bundling of the
available radiated power in certain directions.
8/12/2019 ANTENNA for Communication Systems-2008cep
146/231
Radiation Power Density:The power associated with the electromagnetic wave is described by Poynting vector.
W H
8/12/2019 ANTENNA for Communication Systems-2008cep
147/231
W = instantaneous Poynting vector(W/m2)
= inst. Electric field intensity (V/m)H = inst. Magnetic field intensity (A/m)
W is power density. The total power crossing a closed surface can be obtained by integrating the
normal component of the Poynting vector over the entire surface.
Time average pointing vector (average power) can be written as.
Wav(x,y,z)=[w(x,y,x,t)]av = Re[EH*]. W/m2
*
* 2
. .
( , , , ) Re[ ( , , ) ]
( , , , ) Re[ ( , , ) ]
1Re( )
21 1
Re[ ]. Re[ ]2 2
s s
jwt
jwt
jwt jwt jwt
j wt
P W ds W n da
x y x t E x y z e
H x y x t H x y z e
Ee Ee E e
W H E H E He
s
*
P rad=Pav= . . da
1= Re[ ]
sWrad ds wav n
E H ds
8/12/2019 ANTENNA for Communication Systems-2008cep
148/231
Re[ ].2 s
E H ds
Radiation Intensity:
Power radiated from an antenna per unit solid angle
U - radiation intensity (W/solid angle)
W radradiation density (W/m2)
The radiation intensity is also related to the far zone electric field of an antenna by
The total power is obtained by integrating radiation intensity over the entire solid angle
of 4.
2 U r W rad
2 2
, ( , ,2
rU E r
2
0 0P rad = U d sinU d d
8/12/2019 ANTENNA for Communication Systems-2008cep
149/231
Where d= element of solid angle = sindd
2
y
x
For an isotropic source, U is independent of angles &
or Radiation intensity of an isotropic source is
0 0 0rad= 4P U d U d U
0
rad
4
PU
8/12/2019 ANTENNA for Communication Systems-2008cep
150/231
2
0
0 0
P rad= U , ( , )sin
,4
d B F d d
FD
8/12/2019 ANTENNA for Communication Systems-2008cep
151/231
2
0 0
0 2
0 0
2
0 0
, 4
, sin
, max4
, sin
4
=, sin , max
4 = A- beam solid angle
A
D
F d d
FD
F d d
F d d F
The beam solid angle A is defined as the solid angle through which all the power of
the antenna would flow if its radiation intensity is constant (and equal to the maximum
value of U) for all angles with A
Gain:
Absolute gain: Ration of the intensity, in a given direction, to the radiation intensity that would be
obtained if the power accepted by the antenna were radiated isotropically.
8/12/2019 ANTENNA for Communication Systems-2008cep
152/231
Relative gainthe ratio of the power gain in a given direction to the power gain of a reference
antenna in its reference direction.Reference antenna usually a dipole, horn or any other antenna. Whose gain can be calculated or
known.
4 ( )
U , = 4
Pin
Radiation Intensity
gain total input accepted power
0 0
4 ( , )
( )
P rad= lcd Pin
,G( , ) 4
rad
G( , ) ,
, max , max D
UG
Pin lossless isotropic source
Ulcd
P
lcdD
G G lcdD lcd
8/12/2019 ANTENNA for Communication Systems-2008cep
153/231
Antenna Efficiency
The radiation resistance is part of the impedance of the antenna at its feed point. Additionally, we
have the loss resistance Rloss which accounts for the power dissipated into heat as well as
reactive components L and C Figure 2 has an equivalent circuit that describes the antenna
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
154/231
reactive components L and C. Figure 2has an equivalent circuitthat describes the antenna
around its resonant frequency.
2. Antenna Equivalent Circuit
The inductor and the capacitorin the equivalent circuit build a series resonant circuit. The
antenna impedanceZis:
At the frequency of resonance,
the reactances of the capacitor and the inductor cancel each other out soonly the resistive part of
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
155/231
the reactances of the capacitor and the inductor cancel each other out, soonly the resistive part of
the antenna impedance remains.
The inductance L and the capacitance Cin the equivalent schematic are determined by the
antenna geometry. If we want to build an antenna for a given frequency, we have to find a
geometry that is resonant at the frequency of operation, such as a wire with a certain length.
At the frequency of resonance, the antenna inputimpedance equals Rr + Rloss. The antenna
efficiency hin resonance is the ratio of the radiated power to the total power accepted by theantenna from the generator:
At frequencies other than the resonant frequency, the antenna input impedance is either capacitiveor inductive. This phenomenon is why it is possible to tune an existing antenna by adding a series
capacitor or inductor.
The L-to-C ratio determines thebandwidthof the antenna for given radiation and loss resistances.
For the same resistance values, a larger L-to-C ratio means a higher quality factor Q and a
smaller bandwidth.
The values of L and C in the equivalent schematic depend on the antenna geometry.Often we candeduct intuitively how a variation of the geometry can influence L and C. The quality factor is
influenced by a contribution Qrad from the radiation resistance and Qloss from the loss
resistance. The overall Q of the antenna is:
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=C&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=input&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=bandwidth&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=bandwidth&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=input&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=C&x=&y=8/12/2019 ANTENNA for Communication Systems-2008cep
156/231
Chu [1] and Wheeler [2] gave the theoretical limit for the quality factor Q and the fractional
bandwidth of a lossless antenna as:
with aas the radius of the smallest circumscribing sphere surrounding the antenna.
The selectivity of the antenna can help to suppress unwanted out-of-band emissions; but not
always a small bandwidth is desirable. A small bandwidth means stringent requirements on thetolerances of the matching components and the antenna itself. For a given dimension of a small
antenna, we can only increase the bandwidth if we introduce intentional losses. The bandwidth of
an antenna with the efficiency is then:
-Antenna Efficiency takes into account the losses at the input terminal and within antenna
structure.
8/12/2019 ANTENNA for Communication Systems-2008cep
157/231
1. Reflection loss due to mismatch between transmission line and antenna
2. I2R losses (conduction and dielectric)
Voltage reflection coeff. at the input terminals of the antenna.
Zininput impedance of the antenna
Z0Characteristic impedance of the transmission line.
.
0
in 0
in 0
Z =
Z
r c dl l e e
Z
Z
2
01l lcd
lcd = lc ld = antenna radiation efficiency.
8/12/2019 ANTENNA for Communication Systems-2008cep
158/231
Half Power Beamwidth:
Important figure of merit. Often traded off between it and sidelobe level. Beamwidth
decreases as sidelobe level increases and viceversa. Resolution capability of two targets or
radiation sources depends on beamwidth.
Resolution capability of an antenna to distinguish two sources is equal to half the first nullbeamwidth (FNBW/2) which is usually used to approximate the half power beamwidth.
Beam efficiency: (BE)
2FNBW HPBW
1( )
( )
Power transmitted received within cone angleBE
Power transmitted received by the antenna
1 half angle of the cone within which the percentage of the total power is to be found.
12
0 0
2
( , )sinU d d
BE
8/12/2019 ANTENNA for Communication Systems-2008cep
159/231
0 0
( , )sinU d d
If 1 is chosen as the angle where first null or minimum occurs, then the beam efficiency will
indicate the amount of power in the major lobe compared to the total power.
A BE required for Radar, Radiometry and other applications.
Bandwidth:
The range of frequencies within which the performance of the antenna with respect to
some characteristics conforms to a specified standard.
Polarization
Polarization describes the trace that the tip of the electrical field vector builds during the
propagation of the wave. In the far field, we can consider the electromagnetic wave as a plane
wave.
8/12/2019 ANTENNA for Communication Systems-2008cep
160/231
wave.
In a plane electromagnetic wave, the electrical and the magnetic field vectors are orthogonal tothe direction of propagation and also orthogonal to each other. In the general case, the tip of the
electrical field vector moves along an elliptical helix, giving an elliptical polarization. The wave
is called right-hand polarized if the tip of the electrical field vector turns clockwise while
propagating; otherwise it is left-hand polarized.
If the two axis of the ellipse have the same magnitude, the polarization is called circular. If one of
the two axis of the ellipse becomes zero, we have linear polarization. Similarly, polarization is
vertical if the electrical field vector oscillates perpendicularly to ground, and it is horizontal if its
direction of oscillation is parallel to the ground plane.
A transmission system has the best performance (ideal case) when the polarization of the
transmitter and the receiver antenna are identical to each other. Circular polarization on one end
and linear polarization on the other gives 3-dB loss compared to the ideal case. If both antennas
are linearly polarized but 90 turned to each other, theoretically no power is received. The same
phenomenon happens if one antenna is right-hand circularly polarized and the other one is left-
hand circularl olarized In an indoor environment, reflections in the transmission path may change the
polarization, which makes the polarization of the received wave difficult to predict. If
one of the antennas is portable, we have to make sure that it works in any position.
Circular polarization at one end and linear polarization at the other end results in a
principal loss of 3 dB, but avoids the case of a total blackout, where no power is
8/12/2019 ANTENNA for Communication Systems-2008cep
161/231
p p , , p
received.
Input-Impedance:
The impedance presented by an antenna at its terminals or the ratio of voltage to current at a
pair of terminals or the ratio of appropriate components of the electric to magnetic fields at a
point.
Generator Radiated wave
8/12/2019 ANTENNA for Communication Systems-2008cep
162/231
( )g g g
g
t A g r L g A g
V V VIZ Z Z R R R j X X
8/12/2019 ANTENNA for Communication Systems-2008cep
163/231
1
2 22( )
g
g
r L g A g
V
IR R R j X X
The power delivered to the antenna for radiation is given by.
And that dissipated as heat by
The remaining power is dissipated as heat on the internal resistance Rg of the generator.
22
2 2
1
2 2
g rr g
r L g A g
V RP I Rr
R R R X X
22
2 2
1
2 2
g LL g L
r L g A g
V RP I R
R R R X X
2
2 22
g g
g
V RP
R R R X X
8/12/2019 ANTENNA for Communication Systems-2008cep
164/231
r L g A g R R R X X
Maximum power is delivered to the antenna when we have conjugate matching i.e
Rr+RL=Rg
XA = -Xg
For this case2
2
2
2
2
8 ( )
8 ( )
8
L
g rr
r L
g
Lr L
g
g
g
V RP
R R
RV
P R R
VP
R
Power supplied by the generator during conjugate matching.
2
01 1
2 4
g
S g g
r L
VP V I
R R
8/12/2019 ANTENNA for Communication Systems-2008cep
165/231
Half of the power dissipated as heat in the internal resistance of the generator and the other half isdelivered to the antenna. Of the power that is delivered to the antenna, part is radiated through the
mechanism provided by the radiation resistance and the other is dissipated as heat.
Antenna Vector Effective Length and Equivalent Areas:
Vector effective length: The effective length of an antenna whether it be linear or anaperture antenna, is a quantity that is used to determined the voltage induced on the open-circuit
terminals of the antenna when a wave impinges upon it.
` VocOpenCircuit voltage at antenna terminalsEiincident electric field
LeVector effective length
r L
i
oc eV = E . L
Antennas Equivalent Areas:
These are used to describe the power capturing characteristics of the antenna when a wave
impinges on it. It is defined as.
The ratio of the available power at the terminals of a receiving antenna to the power flux
density of a plane wave incident on the antenna from that direction, the wave being polarization
8/12/2019 ANTENNA for Communication Systems-2008cep
166/231
matched to the antenna.
Aeeffective aperture m2
PTPower delivered to the load (W)
WiPower density of incident wave (W/m2)
All the power that is intercepted, collected or captured by an antenna is not delivered to the load.
2
2T TTe
i i
I RPAW W
Under conjugate matching only half of the captured power is delivered to the load, the other half
is scattered and dissipated as heat.
To account for the scattered and dissipated power, scattering loss and capture equivalent areas are
defined.
Capture area = Effective area + Scattering area + loss area
Maximum effective areaAperture efficiency =
Physical area
1, for aperture antenna
>1, for wire antenna
8/12/2019 ANTENNA for Communication Systems-2008cep
167/231
8/12/2019 ANTENNA for Communication Systems-2008cep
168/231
8/12/2019 ANTENNA for Communication Systems-2008cep
169/231
8/12/2019 ANTENNA for Communication Systems-2008cep
170/231
L A
A
L A
Z Z
Z Z
8/12/2019 ANTENNA for Communication Systems-2008cep
171/231
s s s
A
t
I 1E (ZL)=E (o) - 1 I 2
tE
To separate out structural and antenna mode scattering terms, assume the antenna is loaded with
conjugatematched impedance (ZL=Z*A)
*s s * s
L A
t
*
*
*
I 1 E (Z )=E (Z ) -
I 2 2
t
A
L A
L A
ZAE
R
Z Z
Z Z
- electric field scattered by the antenna with conjugatematched load.s *
AE (Z )
* * *( ) 2s A m A A A mI Z I Z Z R I I*mscattering current when antenna is conjugatematched (ZL=Z*A)
*I
8/12/2019 ANTENNA for Communication Systems-2008cep
172/231
The total radar cross section of the antenna terminated with a load ZLcan be written as
* *s s tmL At
IE Z E Z E
I
2
1 s a j rA e
s-RCS due to structural term
a-RCS due to antenna mode term
r-relative phase between the structural and antenna mode terms
-Total RCS with antenna terminated with ZLIf the antenna short circuited (A=-1) then
short=sIf the antenna open circuited (A=-1), then
2
2s a j ropen e
If the antenna, matched (A=-0),then
2s a j r
matched e
8/12/2019 ANTENNA for Communication Systems-2008cep
173/231
8/12/2019 ANTENNA for Communication Systems-2008cep
174/231
4. ANTENNA MEASUREMENTS
The radiation characteristics of the antennas can be analyzed by methods like
GTD
Moment Method
Finite-Difference Time Domain (FDTD)
Fi i El