Creating a DIY 88-108 MHz FM Trap

One of the most problematic strong signals you can encounter is regular 88 – 108 MHz broadcast FM stations. They transmit at high power and can cause overloading and intermodulation problems on simple receivers such as the RTL-SDR. This means that FM stations can prevent you from receiving signals even when you are tuned far away from the broadcast band.

The simplest solution to reducing strong FM stations is to build an FM trap. This is simply a band stop filter that blocks frequencies between 88 – 108 MHz from entering your radio. Adam (9A4QV), the creator of the popular LNA4ALL and several other RTL-SDR compatible products has recently uploaded an article showing how to build a home made FM trap out of cheap common parts.

Adams article goes through and explains the design of a FM trap and how to use freeware software to aide in the calculations. The final FM trap designed by Adam uses just 3 common SMD capacitors and 3 hand wound coils. His filter attenuates more than 30dB in the 88-108 MHz range with an insertion loss of less than 1dB up to 1.7 GHz.

A DIY FM Trap
A DIY FM Trap

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