Explain what causes overload, interference, distorted audio and low power, and the first fix to try for each.
11 pool questions1 on the exam
Syllabus: Symptoms, causes, and cures of common transmitter and receiver problems: overload and overdrive, distortion, interference and consumer electronics, RF feedback
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Compendium articles on the ideas in this lesson, beyond what the exam asks.
A strong signal can swamp a receiver that can't reject it
Fix it at the receiver that is being overloaded: block the strong signal before it gets in.
A broadcast radio or TV that picks up your signal has a front end that can't reject strong signals outside its band. Your own 2 m radio can suffer the same from a nearby commercial FM station.
Fix it at the victim receiver's antenna input: a band-reject filter blocks the unwanted signal. Changing your mode doesn't help.
Cause
What it is
Fundamental overload
your signal is simply too strong for their receiver
Harmonics
extra signals at multiples of your frequency
Spurious emissions
other unwanted signals from a transmitter
All three can cause RFI (radio frequency interference).
RememberBlock the strong signal at the front door of the receiver that is overloaded.
Troubleshoot the simple things first
Situation
Do first
Neighbor says you interfere with their TV or radio
Check your own station works properly and doesn't interfere with your own TV or radio on the same channel
Cable TV interference
Check that all coax connectors are installed properly. Leaky connectors let your signal in
A device in a neighbor's home interferes with you
All of these: work with the neighbor to find the device, politely explain FCC rules, and make sure your own station meets good amateur practice
Distorted audio: too loud, or RF getting into the radio
On FM, a louder voice (or one closer to the mic) swings the frequency further. Too far is over-deviation: it spills past the channel and sounds distorted.
Louder or closer to the mic = wider frequency swing (deviation).
DeviationToo wide
Fix: talk farther from the microphone. Being slightly off frequency or in a bad location can also garble your audio.
The other cause is RF feedback: your own signal gets onto the mic cable and back into the transmitter.
Distorted voice? RF on the mic cable may be feeding back. A clip-on ferrite choke stops it.
RememberToo much voice? Back off the mic. RF in the cable? Clip on a ferrite choke.
High SWR can cut your power output
A poor antenna match reflects power back toward a solid-state transmitter. To protect itself, it turns its output power down.
The farther the antenna is from the line's 50 Ω, the bigger the reflected wave.
SWR4 : 1
Power reflected36%
RememberLow output power from a solid-state radio: suspect high SWR.