Nov 11, 2016
The majority of population wants easy to hear morning radio in their car, so signals from local broadcast FM stations can be so powerful that they easily overload a sensitive radio receiver. To cut to the chase, you need an FM trap for better receive performance - they are also called FM bandstop filters and FM notch filters, do the same job: reduce interference.
A wide variety of FM traps are available online and in stores; the two samples below are designed especially for radio signal reception use. Neither of these filters require external power - simply screw them on. Also, both are usable for bias-T applications - tested with three different LNAs and two bias-T dongles, both worked flawlessly.
I bought an FM trap from rtl-sdr.com ($15 incl. shipping) and eBay seller novakx5 ($20 incl. shipping) sent his version for testing, so might as well do a comparison at the same time.
Rtl-sdr.com FM filter
Offically called "RTL-SDR Blog Broadcast FM Band-Stop Filter", available from the manufacturer for $15 including shipping from China. Technical notes here if you like graphs.
Shipping took three weeks from China, regular bubble wrap envelope, small black metal box with two female SMA connectors and a Male-Male barrel connector. Washers and nut on the SMA connectors, aluminum metal box - a quality package.Usable out of the box with SMA standard receivers, such as v.3, Nooelec SMArt, SDRPlay and every other device sharing that standard. Older dongles with small MCX connectors require MCX to SMA adapters or pigtails.
FM notch FM band stop filter from Novakx5
Available on ebay for $15 (plus $5 for worldwide shipping) from a seemingly small-scale manufacturer, shipping took 5 days to Ireland. Antistatic bag stapled together, instructions on a sheet of paper for use and providing filter characteristics. It's incredibly small, but doesn't come with a metal case nor with washers and nuts for mounting.
v.3 as receiver from rtl-sdr.com, 38.6dB gain on strong local station, to the point that dongle grossly overloaded - it was necessary to see the effectiveness of filtering, less white/signal in the image is better.
For ADS-B, I tested rtl-sdr.com's filter with identical v.3 receivers, and over 24 hours, the filtered v.3 dongle had 2.77% more position reports.
For any SDR and front-end combination
Not only for RTL-SDRs, any receiver can - and probably will - benefit from no FM signals.
SDRPlay RSP with latest 1.1 SDRUno in images below, no, I shouldn't see Today FM at 286 MHz:
If you check the above waterfall screenshot, it's evident that both filters do a great job of removing some BCFM interference.
All in all, rtl-sdr.com's BCFM filter had better performance in this test, costs less, and comes in a metal case with nuts and washers. Novakx5's filter is significantly smaller and ships faster from Europe. Apart from nitpicking for a review, I'm sure either one will enhance your listening experience.