Basic Electronics 8
What is RF?
Basic RF systems
Basic RF test equipments
Basic Definition RF (Radio frequency) is a frequency or rate of oscillation within
the range of about 3 Hz to 300 GHz.
Since most of this range is beyond the vibration rate that most mechanicalsystems can respond to, RF usually refers to oscillations in electricalcircuits or electromagnetic radiation.
Electrical currents that oscillate at RF have special properties not sharedby direct current signals. One such property is the ease with which it can ionize air to create a
conductive path through air. Another special property is an electromagnetic force that drives the RF
current to the surface of conductors, known as the skin effect which isuseful when designing RF circuits.
Another property is the ability to appear to flow through paths thatcontain insulating material, like the dielectric insulator of a capacitor.
The degree of effect of these properties depends on the frequency of thesignals.
RF Terminology Hertz (Hz)
A term representing cycles/second (cps). The unit of measure is named after Heinrich Hertz, German physicist. For example, 1 Hz means that an event repeats once per second, 2 Hz is
twice per second, and so on.
Frequency The number of occurrences of a repeating event per unit time. For example:
* The frequency of the standard pitch A above middle C on a piano is usually defined as 440 Hz, (440 cps).
* In North America the frequency of the alternating current (AC) is 60 Hz.
* Visible light from deep red to violet has frequencies of 430 to 750 THz(Terahertz).
RF Terminology Wavelength ()
The wavelength is related to the frequency by the formula: wavelength = wave speed/ frequency.
* For example = c/f, where c is the speed of light (in meters/sec) and f is frequency (Hz).
Higher frequencies have shorter wavelengths. Lower frequencies have longer wavelengths.
Frequency Multipliers Hz (Hertz) Cycles/Second 1 Hz kHz (kilohertz) One Thousand Hertz 1,000 Hz MHz (megahertz) One Million Hertz 1,000,000 Hz GHz (gigahertz) One Billion Hertz 1,000,000,000 Hz THz (terahertz) One Trillion Hertz 1,000,000,000,000 Hz
RF Terminology Decibel (dB)
A logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level.
This reference level is represented by a suffix.
*For example: "dBm" indicates the reference quantity is one milliwatt
(one thousandth of a watt) and dBW indicates the reference quantity is one Watt.
Since it expresses a ratio of two quantities with the same unit, it is dimensionless.
It has a number of advantages, such as the ability to represent very large or small numbers.
*For example: LOG10(1,000,000) = 6; LOG10(0.000001) = -6
RF Terminology Decibel (dB)
It has the ability to carry out multiplication of ratios by simple addition and subtraction.* For example: Doubling output power is 3 dB while a quadrupling is 6 dB.
Therefore, if the antenna gain is doubled (3 dB) and the transmitter power is quadrupled (6 dB), the overall improvement is 3 + 6 = 9 dB.
Insertion Loss (I.L.) The transmission loss from input to output, measured in dB.
RF Terminology VSWR (Voltage Standing Wave Ratio)
SWR is simply the ratio of the resistance of the termination and thecharacteristic impedance of the line.* For example: A 75 ohm load will give an SWR of 1.5 when used to terminate
a 50 ohm cable since 75/50 = 1.5.
Return Loss (R.L.) The ratio of the power reflected back from the line to the power
transmitted into the line.
Reflective Loss The transmission loss due to the reflection of power at a discontinuity
: Applying a signal from a 50 ohm source into a 90 ohm receptor is animpedance mismatch.
RF Terminology Watt
It measures the rate of energy conversion. Using the units of ampere and volt, work (energy) is done at a rate of
one watt when one ampere flows through a potential difference of one volt.*For Example
: 1 watt = 1A x 1V; 120VAC applied to a 100W bulb draws 0.833 A
RF Terminology Bandwidth
In terms of RF, Bandwidth is the difference between the upper and lower cutoff frequencies.
Can refer to a filter, a communication channel, an antenna or a signal spectrum, and is typically measured in hertz.
Basic RF Systems from Space to
Basic RF Systems from Space to
Transponders A transponder is the combination receiver/transmitter component of the communication subsystem that receives, processes, and transmits radio signals.
The term is a contraction of the words transmitter and responder. Many spacecraft have multiple transponders for backup protection.
Transponder The transmitter and receiver are co-located in one unit and can be phase coherent with one another such that the transmit (downlink) carrier source is derived from and is phase coherent with the received (uplink) carrier from the ground station. This coherency allows precise estimations of orbit and speed from
measurements of Doppler offset and rate of the downlink frequency at the ground station.
The XPNDR receives and detects digital command signals that control and configure the spacecraft.
The XPNDR transmits telemetered data from the spacecraft data subsystem which can contain health status and science data.
The XPNDR demodulates the ranging signal contained in the uplink and re-modulates it onto the downlink. Thus, by measuring the return propagation time, the distance between
the ground station and the satellite can be calculated.
Short for transmitter-receiver, a device that both transmits and receives analog or digital signals.
In radio communications, a transceiver is a two-way radio that combines both a radio transmitter and a receiver that exchanges information in half-duplex mode.
The transmit frequency may be some ratio of the receive frequency but they are not phase coherent with one another.
Ranging and two way doppler cannot be performed when a spacecraft uses a transceiver as its radio, but one way dopplercan be performed.
Antennas Gain Antenna performance is primarily established by its gain. Gain must always be measured against a know reference. Unfortunately, there are many "so-called" gain references.Choosing the wrong reference could cost you up to 2.0 dB in
performance. Most commercial antenna suppliers specify gain in dBd
(gain over a half-wave dipole).
The half-wave dipole has a very predictable radiation pattern similar tothat of a donut. Another reference, especially at microwave frequencies, is dBi.
This term refers to gain over an isotropic radiator, a theoretical antennathat radiates equally well in all directions (such as the Sun).
If an antenna is specified in dBi gain, it will appear to haveapproximately 2.15 dB higher gain than dBd.
This may be a nifty way to impress the customer but it does not mean thatyou get more gain if dBi is the reference. Beware if the supplier only quotes antenna gain in just "dB".
What is the reference?For instance, some mobile antennas are specified in dB gain over a
quarter wave whip which is about 0.6 dB less gain than a dipole.
VSWR (voltage-standing-wave-ratio) represents the degree with which an antenna is "matched" to the system impedance.
Most modern antennas, receivers and transmitters are designed for peakperformance when operating into a 50 Ohm transmission line.
If the VSWR is too high, the transmitter power may be reduced as well as the strength of the received signal.
The typical commercial standard for maximum allowable VSWR across the entire bandwidth of a system is 1.5:1.
You should specify the maximum VSWR and the operating frequency bandwidth when specifying your antenna.
A VSWR of 2:1 or greater usually is considered unacceptable since it increases losses in the transmission line.
Incidentally, decreasing the VSWR below 1.5:1 will often be expensive and will have little noticeable improvement.
Radiation Patterns A graphical depiction of the relative field strength transmitted from or
received by the antenna. Antenna radiation patterns are taken at one frequency, one polarization,
and one plane cut.
plots in the plane of the axis is the azimuth or "E-plane"
plots in the plane perpendicular to the axis is the elevation or "H-plane The patterns are usually presented in polar or rectilinear form with a dB
strength scale. Patterns are normalized to the maximum graph value, 0 dB, and a
directivity is given for the antenna.
This means that if the side lobe level from the radiation pattern were down -13 dB, and the directivity of the antenna was 4 dB, then the sidelobe gain would be -9 dB. Three types of plotting scales are in common usage; linear, linear
logarithmic and modified logarithmic.
Another popular antenna specification is the "front-to-back" (F/B) ratio. It is defined as the difference in dB between the maximum gain or front
of the antenna (usually 0 degrees) and a point exactly 180 degrees behind the f