Your neighbour's TV, stereo, phone or doorbell was never designed for a transmitter a few dozen feet away. Under strong RF it can pick up your signal in places the designers never expected. The result is a picture that breaks up, speakers that carry your voice, or a gadget that misbehaves each time you key up. It is the most common interference complaint, and often cured with a cheap part.
Four ways in
RF has four ways into a TV or stereo. The symptom usually tells you which one.
Whichever way RF gets in, the symptom tells you which path to treat. Treat the path that matches.
Is it you or the device?
Two different problems look alike from the living room, and the cure is at opposite ends.
Clue
Points to
Happens on every band you use, and on many TV channels
Overload: the device cannot reject a strong signal. Fix at the device.
Your own station may be emitting something. Fix that first.
Voice or buzz in speakers with the tuner off
Audio rectification on a cable.
Start with your own station and your own equipment, then ask to try things at the neighbour's.
Treating each way in
Antenna input: put a filter at the device's antenna jack: a high-pass filter that rejects HF but passes TV channels, or a band-reject (notch) filter tuned to your band, for example for 2 m interfering with FM radio.
Cable TV: make sure all coax connectors are installed properly and tight, because loose or corroded ones let your signal in. Cable operators may repair leaky lines.
Speakers and audio cables: the most frequent culprit. Wind the cable several turns through a ferrite core, and add a bypass capacitor at the input if needed. See Filters and ferrites for RFI.
Power cord: clamp a ferrite on the cord, or use a plug-in line filter.
Phones, baby monitors, doorbells: the same ferrite-and-filter approach. These devices are often poorly shielded, and sometimes you can only reduce the field or relocate the antenna.
Being a good neighbour
Treat the neighbour as a partner: explain what is happening, offer to supply filters, and try them together. Rules differ by country. In the US, an amateur station must be operated properly, and consumer devices typically carry a statement that they must accept interference they receive. Where those two collide the result depends on the facts. This article does not decide fault, but good will and a few ferrites are the usual way out.