When an FM Notch Filter Improves SDR Reception

[Pavel]

Short answer: An FM broadcast notch helps when strong 88–108 MHz stations overload a wideband receiver and create images, intermodulation or a raised noise floor outside the FM band. It will not improve every station, and it can reduce wanted signals near its stopband.

Before you start​

• Define the target frequency and polarization before choosing an antenna, LNA or filter. A wide tuning range on the SDR does not make one antenna efficient across that range.
• Sketch the RF path from antenna to receiver and include connector adapters, coax length, DC bias and every active component.

Step-by-step method​

1. Step 1: Test with manual gain and note whether strong FM peaks dominate the display. Disconnecting the antenna should remove genuine broadcast signals; persistent fixed spurs may have another source.
2. Step 2: Reduce gain first. If distant-band reception improves as gain falls but becomes sensitivity-limited, front-end overload is likely and filtering may allow useful gain without the broadcast energy.
3. Step 3: Insert the notch before the receiver and before a broadband LNA that could itself overload. If a band-specific antenna-side LNA is used, confirm whether the filter belongs before or after it from the local signal environment.
4. Step 4: Repeat the same target-signal test with identical gain and bandwidth. Compare signal-to-noise ratio and unwanted products, not only the lower absolute noise floor caused by filter insertion loss.

Concrete example​

A 1090 MHz ADS-B receiver near a high-power FM transmitter may show many false peaks and poor aircraft counts at normal gain. If inserting an FM notch removes the products and allows a clean gain increase, the resulting message count can improve even though the filter has some insertion loss.

How to judge the result​

The filter is helping when overload products disappear and the wanted out-of-band signal gains usable signal-to-noise ratio or decode count. If only all levels drop equally, the filter added loss without solving the limiting problem.

What to record​

• Compare the same signal at the same time, receiver gain and bandwidth; record signal level and noise floor separately.
• For a receive antenna, decoded message count or usable signal-to-noise ratio is usually more meaningful than waterfall brightness.
• An accessory helps only when the wanted signal improves more than interference, noise or overload products.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• An FM notch is not a universal noise filter; it specifically rejects the broadcast-FM band and cannot remove interference at unrelated frequencies.

Final check​

Return the RF chain to its original state and repeat the comparison. Keep the antenna or accessory change only when the wanted-signal result repeats without overload, unsafe DC or an unexplained noise increase.