Interference: Diagnosing It and Being Neighbourly About It

The distinction that decides everything

When a neighbour’s device misbehaves while you transmit, there are two possibilities and they need opposite fixes.

Your transmitter is emitting where it should not. Harmonics, spurious emissions, splatter from overdriven audio. Your problem, fixed at your station.

Their receiver cannot handle a strong signal that is entirely legal. This is called overload or front-end overload, and it is far more common.

What would cause a broadcast AM or FM radio to receive an amateur radio transmission unintentionally?

The receiver is unable to reject strong signals outside the AM or FM band

A consumer receiver has almost no filtering ahead of its first amplifier. A legal, clean 50 W signal a few metres away is enormous by its standards, and it simply cannot cope. Nothing is wrong with your transmission.

Which end gets the filter

This follows directly from the distinction:

Which of the following might reduce interference by an amateur station to a non-amateur over-the-air radio receiver?

Block the amateur signal with a filter at the antenna input of the affected receiver

At their receiver, because that is where the problem is. A filter at your transmitter cannot help — it is already only transmitting in its own band, and that is exactly the signal their receiver is failing to reject.

The reverse case, when you are the victim:

Which of the following can reduce interference to a 2-meter band transceiver from a nearby commercial FM station?

Installing a band-reject filter

A notch that removes the broadcast band before your receiver’s front end sees it.

And for audio-frequency trouble caused by RF getting into a cable:

Which of the following can eliminate distorted voice transmissions?

Adding a clip-on ferrite “choke” to the microphone cable to prevent the transmitted signal from feeding back into the transmitter

RF on a microphone cable gets rectified inside the radio and comes back as distortion. A ferrite choke raises the cable’s impedance to RF without affecting audio.

Handling the complaint

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?

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

Check your own house first. This is both good engineering and good diplomacy: it establishes whether the problem is yours, and it demonstrates good faith before any conversation about their equipment.

What should you do if something in a neighbor’s home is causing harmful interference to your amateur station?

All these choices are correct — work with your neighbour to identify the offending device, politely inform them that the FCC rules require them to stop using it if it causes interference, and check your station to ensure it meets the standards.

Note the order in that answer. Identify cooperatively, mention the rules politely, and verify your own station. Leading with the rules is how a solvable problem becomes a permanent feud.

For cable television specifically:

What should be the first step to resolve non-fiber optic cable TV interference caused by your amateur radio transmission?

Be sure all TV feed line coaxial connectors are installed properly

Cable systems are shielded, and interference means the shield is compromised — usually a loose or badly-fitted connector. Fixing the connector fixes both directions: your signal stops getting in, and the cable system’s signal stops leaking out.

Faults on your side

Low RF output from a solid-state transmitter may be caused by high SWR — the protection circuit reducing power to save the finals. Check the antenna system before suspecting the radio.

Over-deviation on FM: talk farther away from the microphone. Same fix as before, from the other direction.

Audio reports through a repeater — “all these choices are correct” — can mean your transmitter is slightly off frequency, your batteries are low, or you are in a poor location. All three degrade a signal into the repeater without any fault in the radio itself.

Radio frequency interference can be caused by — again “all these choices” — fundamental overload, harmonics, and spurious emissions. The first is the receiver’s limitation, the other two are the transmitter’s, which is why diagnosis has to come before any fix.

A diagnostic order

1. Does it happen with your own receivers, in your own house? If yes, suspect your station.
2. Does it affect only one device, or everything? One device points to overload in that device.
3. Does it happen on every band you transmit on, or only one? Only one suggests a harmonic relationship.
4. Does a ferrite choke on the affected device’s leads change it? That confirms conducted pickup rather than direct radiation.

Check yourself

1. Your 2 metre transmission comes through a neighbour’s stereo. Where does the filter go, and why?
2. Your solid-state radio suddenly makes half its rated power. First check?
3. A neighbour’s new LED floodlight wipes out 2 metres. What is the first step?