Even the most professional antenna installations can't always stop interference from disturbing the picture on your TV set. This is particularly so when your antenna is picking up a distant TV station with a weaker signal, boosted by an antenna amplifier.
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An FM trap, installed between the antenna and the antenna's signal amplifier, prevents strong, local FM (Frequency Modulation) radio signals from overwhelming the TV signal and interrupting the video and audio on your television picture.
Most VHF TV antenna amplifiers come pre-fitted with an FM trap to reduce FM interference. You can buy standalone FM traps to fit to existing antenna/amplifier set-ups.
Usually you can adjust FM traps to block out only the problematic, interference-causing FM frequency. It's also possible to disable them completely.
You'll only need an FM trap if you have an amplified antenna. It's always better to have a larger, highly directional antenna capable of getting strong TV signals rather than a smaller, less directional antenna fitted with an amplifier.