Antenna and transmission line measurements and troubleshooting: measuring SWR, effects of high SWR, causes of feed line failures; Basic coaxial cable characteristics; Use of dummy loads when testing
This group covers measuring and troubleshooting antennas and feed lines, how to test a transmitter without radiating, and what to know about coaxial cable.
Dummy loads
A dummy load lets you test or adjust a transmitter without sending a signal over the air. A typical RF dummy load is a 50-ohm non-inductive resistor mounted on a heat sink, which turns transmitter power into heat. It needs no power supply or relay.
SWR basics
SWR (standing wave ratio) describes how well the antenna impedance matches the feed line.
1:1 indicates a perfect match, with no reflected power.
Higher ratios mean a worse mismatch, so an SWR of 4:1 indicates an impedance mismatch.
SWR is a ratio, not decibels of gain or loss.
Most solid-state transmitters automatically reduce output power as SWR rises past a certain level, to protect the output transistors. Reducing power does not lower SWR, and it is not an FCC rule requirement.
Measuring instruments
A directional wattmeter measures forward and reflected power, and so can be used to determine SWR. SWR meters work on the same principle.
An antenna analyzer shows whether an antenna is resonant at the desired operating frequency, by showing SWR and impedance across frequencies.
Voltmeters and ohmmeters, a VTVM, a Q meter and a frequency counter do not tell you whether an antenna is resonant.
Feed line losses and failures
Power lost in a feed line is converted into heat. It isn’t radiated as harmonics.
The main cause of coaxial cable failure is moisture contamination, which corrodes conductors and degrades the dielectric. The outer jacket needs to resist ultraviolet light, because UV damages the jacket and lets water into the cable.
Foam versus solid dielectric
Foam-dielectric coax has less loss per foot than solid-dielectric cable. Foam cable is not better at resisting moisture and doesn’t give a better 50-ohm match.
Key facts to memorize
Dummy load: 50-ohm non-inductive resistor on a heat sink.
Perfect match: 1:1 SWR.
4:1 SWR means a mismatch.
Transmitters fold back power at high SWR to protect output transistors.
Directional wattmeter: determines SWR.
Antenna analyzer: checks resonance.
Feed line loss becomes heat.
Moisture is the top cause of coax failure; UV resistance protects the jacket.
Foam dielectric: less loss per foot.
Memory hooks
Dummy load = no signal on the air, only heat.
Water and sunlight kill coax: seal connectors and use UV-resistant jacket.
Questions in this group (11): answers shown
T7C01
What is the primary purpose of a dummy load?
A. To prevent transmitting signals over the air when making tests ✓ Correct answer
B. To prevent over-modulation of a transmitter
C. To improve the efficiency of an antenna
D. To improve the signal-to-noise ratio of a receiver
A dummy load absorbs transmitter power as heat instead of radiating it, so you can test or adjust a transmitter without sending a signal on the air. It does not affect modulation, antenna efficiency or receiver noise.
T7C02
Which of the following is used to determine if an antenna is resonant at the desired operating frequency?
A. A VTVM
B. An antenna analyzer ✓ Correct answer
C. A Q meter
D. A frequency counter
An antenna analyzer measures SWR and impedance across a range of frequencies, so it shows where the antenna is resonant. A VTVM, Q meter and frequency counter do not show resonance of an installed antenna.
T7C03
What does a typical RF dummy load consist of?
A. A low-voltage power supply and an AC relay
B. A 50-ohm non-inductive resistor mounted on a heat sink ✓ Correct answer
C. A low-voltage power supply and a DC relay
D. A 50-ohm inductive reactance mounted in a shielded enclosure
A typical dummy load is a non-inductive 50-ohm resistor mounted on a heat sink to dissipate power. It does not need a power supply or relay, and it is resistive, not inductive reactance.
T7C04
What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
A. 50:50
B. Zero
C. 1:1 ✓ Correct answer
D. Full Scale
An SWR of 1:1 means all power is delivered to the load, with no reflected power, which is a perfect match. Zero is impossible for SWR, and 50:50 is not a standard reading.
T7C05
Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?
A. To protect the RF output amplifier transistors ✓ Correct answer
B. To comply with FCC rules on spectral purity
C. Because power supplies cannot supply enough current at high SWR
D. To lower the SWR on the transmission line
High SWR can cause excessive current or voltage stress on the output transistors, so most solid-state transmitters automatically reduce power to protect them. This is not required by FCC rules, and the reduction does not lower the SWR.
T7C06
What does an SWR reading of 4:1 indicate?
A. Loss of -4 dB
B. Good impedance match
C. Gain of +4 dB
D. Impedance mismatch ✓ Correct answer
An SWR of 4:1 means a significant mismatch between feed line and antenna impedance. A good match is close to 1:1. SWR is a ratio, not a measure of gain or loss in decibels.
T7C07
What happens to power lost in a feed line?
A. It increases the SWR
B. It is radiated as harmonics
C. It is converted into heat ✓ Correct answer
D. It distorts the signal
Power lost in a feed line is dissipated as heat in the conductors and dielectric. It is not radiated as harmonics, and it does not by itself raise the SWR.
T7C08
Which instrument can be used to determine SWR?
A. Voltmeter
B. Ohmmeter
C. Iambic pentameter
D. Directional wattmeter ✓ Correct answer
A directional wattmeter measures forward and reflected power, from which SWR is determined. Voltmeters and ohmmeters measure voltage and resistance, and iambic pentameter is a poetry term.
T7C09
Which of the following causes failure of coaxial cables?
A. Moisture contamination ✓ Correct answer
B. Solder flux contamination
C. Rapid fluctuation in transmitter output power
D. Operation at 100% duty cycle for an extended period
Moisture getting into coax corrodes the conductors and degrades the dielectric, a leading cause of cable failure. Rapid power changes or full-duty operation within the cable's rating do not wear the cable out.
T7C10
Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
A. Ultraviolet light can increase the resistance of the conductors
B. Ultraviolet light can increase losses in the cable’s jacket
C. Ultraviolet and RF signals can mix, causing interference
D. Ultraviolet light can damage the jacket and allow water to enter the cable ✓ Correct answer
Ultraviolet light from the sun can crack and degrade the cable jacket, allowing water to enter, which damages the cable. The UV resistance is about protecting the jacket, not about conductor resistance or signal mixing.
T7C11
What is an advantage of foam-dielectric versus solid-dielectric coaxial cable?
A. It is more resistant to moisture contamination
B. It has higher voltage breakdown
C. It has less loss per foot ✓ Correct answer
D. It has a better impedance match to 50 ohms
Foam dielectric has less loss per foot than solid dielectric coax of similar size. Foam cable is actually more vulnerable to moisture if the jacket is damaged, and neither type gives a better match to 50 ohms.