Symptoms, causes, and cures of common transmitter and receiver problems: overload and overdrive, distortion, interference and consumer electronics, RF feedback
This group covers common transmitter and receiver problems: signals that are distorted, interference to neighbors’ equipment, interference to your own station, and what to do about each.
Distortion and over-deviation
An FM signal is over-deviating when its frequency swings wider than allowed, usually from too much audio. Talk farther from the microphone, or speak more softly. Speaking too loudly or too close to the microphone also makes your audio distorted or unintelligible through a repeater.
Other things that can distort a signal through a repeater include being slightly off frequency or being in a bad location with a weak signal.
RF feedback happens when your transmitted signal gets into the microphone cable or audio circuits, distorting voice. A clip-on ferrite choke on the microphone cable keeps RF out of the transmitter. Adding feed line, power cycling, or turning up squelch will not fix it.
Low power output
High SWR can cause a solid-state transceiver to reduce its power output automatically to protect its output transistors. Low SWR is the goal, and amplifier noise figure and linearity do not explain low power.
Types of interference
Radio frequency interference can come from:
Fundamental overload: a very strong signal overwhelming another receiver.
Harmonics: energy at multiples of your frequency.
Spurious emissions: unwanted signals outside the intended frequency.
A broadcast AM or FM radio hears your transmission unintentionally when it cannot reject strong signals outside its own band. The fault lies in the receiver front end, not in your microphone gain or deviation.
Curing interference
Block the amateur signal with a filter at the antenna input of the affected receiver. Filtering your own transmitter does not cure overload in someone else’s receiver, and mode changes generally do not either.
For a 2-meter receiver overloaded by a nearby commercial FM station, install a band-reject filter. A preamplifier only boosts the unwanted signal too.
For cable TV interference, first make sure all coaxial connectors on the TV feed line are installed properly. Loose or poor connectors let RF in. Check this before adding filters.
When a neighbor says you interfere with their radio or TV, make sure your station functions properly and doesn’t cause interference to your own equipment tuned to the same channel. Do not install a “harmonic doubler”, which would add harmonics.
When a neighbor’s device interferes with you
Work with your neighbor to identify the offending device, and make sure your station meets good amateur practice. Informing the neighbor politely that the rules prohibit harmful interference is also reasonable.
Key facts to memorize
Over-deviation: talk farther from the microphone.
RF feedback: ferrite choke on the microphone cable.
High SWR can cause low power from a solid-state transmitter.
Receiver overload fix: filter at the affected receiver’s antenna input.
Band-reject filter for strong commercial FM broadcast signals.
Cable TV interference: first check coaxial connectors.
Interference causes: fundamental overload, harmonics, spurious emissions.
Memory hooks
Overload is fixed at the victim: when your clean signal swamps a receiver, the filter goes on the receiver that is bothered. Harmonics and spurious emissions are different; they come from your transmitter and are fixed there.
Check the cheap, simple things first, like connectors.
Questions in this group (11): answers shown
T7B01
What can you do if you are told your FM handheld or mobile transceiver is over-deviating?
A. Talk louder into the microphone
B. Let the transceiver cool off
C. Change to a higher power level
D. Talk farther away from the microphone ✓ Correct answer
Over-deviation means an FM signal is swinging too wide in frequency, which is usually caused by too much microphone audio. Talking farther from the microphone, or speaking more softly, reduces the audio level and the deviation.
T7B02
What would cause a broadcast AM or FM radio to receive an amateur radio transmission unintentionally?
A. The receiver is unable to reject strong signals outside the AM or FM band ✓ Correct answer
B. The microphone gain of the transmitter is turned up too high
C. The audio amplifier of the transmitter is overloaded
D. The deviation of an FM transmitter is set too low
A broadcast receiver can pick up an amateur signal when its front end cannot reject strong signals outside its intended band, which is overload. The problem lies in the receiver's design, not your transmitter's microphone gain or deviation.
T7B03
Which of the following can cause radio frequency interference?
A. Fundamental overload
B. Harmonics
C. Spurious emissions
D. All these choices are correct ✓ Correct answer
Fundamental overload hits a receiver with a very strong signal, while harmonics and other spurious emissions put energy on unintended frequencies. Each of these can cause interference, so all the choices apply.
T7B04
Which of the following might be the cause of low RF power output from a solid-state transceiver?
A. Poor amplifier noise figure
B. Poor amplifier linearity
C. Low SWR
D. High SWR ✓ Correct answer
High SWR indicates a mismatch, and many solid-state transceivers automatically cut output power to protect their output transistors. Low SWR is the desired condition, and noise figure and linearity do not directly lower power output.
T7B05
Which of the following might reduce interference by an amateur station to a non-amateur over-the-air radio receiver?
A. Block the amateur signal with a filter at the antenna input of the affected receiver ✓ Correct answer
B. Block the interfering signal with a filter on the amateur transmitter
C. Switch the transmitter from FM to SSB
D. Switch the transmitter to a narrow-band mode
Putting a filter at the antenna input of the affected receiver blocks the amateur signal before it overloads the receiver. Filtering your own transmitter does not help when the problem is receiver overload, and mode changes generally do not either.
T7B06
Which of the following actions should you take if a neighbor tells you that your station’s transmissions are interfering with their radio or TV reception?
A. Make sure that your station is functioning properly and that it does not cause interference to your own radio or television when it is tuned to the same channel ✓ Correct answer
B. Immediately turn off your transmitter and contact the nearest FCC office for assistance
C. Install a harmonic doubler on the output of your transmitter and tune it until the interference is eliminated
D. All these choices are correct
First make sure your own station works properly and does not interfere with your own radio or TV on the same channel. Shutting down and calling the FCC is premature, and a harmonic doubler would make matters worse.
T7B07
Which of the following can reduce interference to a 2-meter band transceiver from a nearby commercial FM station?
A. Installing an RF preamplifier
B. Using double-shielded coaxial cable
C. Installing bypass capacitors on the microphone cable
D. Installing a band-reject filter ✓ Correct answer
A band-reject filter attenuates the strong commercial FM broadcast signal that is overloading your 2-meter receiver while still passing the 2-meter band. A preamplifier would amplify the unwanted signal as well, and shielded coax or bypass capacitors do not address it.
T7B08
What should you do if something in a neighbor’s home is causing harmful interference to your amateur station?
A. Work with your neighbor to identify the offending device
B. Politely inform your neighbor that FCC rules prohibit the use of devices that cause interference
C. Make sure your station meets the standards of good amateur practice
D. All these choices are correct ✓ Correct answer
Work cooperatively with your neighbor to find the offending device, and be sure your own station meets good amateur practice. Pointing out that rules prohibit interference is also reasonable, and together these steps make all the choices correct.
T7B09
What should be the first step to resolve non-fiber optic cable TV interference caused by your amateur radio transmission?
A. Add a low-pass filter to the TV antenna input
B. Add a high-pass filter to the TV antenna input
C. Add a preamplifier to the TV antenna input
D. Be sure all TV feed line coaxial connectors are installed properly ✓ Correct answer
Start with the simple fix: loose or poorly installed coax connectors on the TV feed line can let RF in and cause interference. Check these before adding filters or preamplifiers.
T7B10
What might be a problem if you receive a report that your audio signal through an FM repeater is distorted or unintelligible?
A. Your transmitter is slightly off frequency
B. You are speaking too loudly or too close to the microphone
C. You are in a bad location
D. All these choices are correct ✓ Correct answer
Distorted audio through a repeater can come from several causes: being slightly off frequency, speaking too loudly or too close to the microphone, or a poor location with a weak signal. All of these choices can contribute.
T7B11
Which of the following can eliminate distorted voice transmissions?
A. Adding extra feedline to the antenna to lower SWR
B. Turning the radio on and off to reset the computer-controlled circuitry
C. Adding a clip-on ferrite “choke” to the microphone cable to prevent the transmitted signal from feeding back into the transmitter ✓ Correct answer
D. Turning the squelch control fully clockwise to prevent the transmitted signal from triggering the squelch circuit
Distorted voice can result from RF feedback, where your own signal enters the microphone cable. A clip-on ferrite choke on the microphone cable blocks that RF. Extra feed line, power cycling and squelch changes do not cure it.