Testing a FM Broadcast Bandstop Filter

Over on YouTube user Cameron Conover has been testing a simple FM broadcast bandstop filter with his HackRF. The same filter can just as easily be used with the RTL-SDR to remove broadcast FM interference and images. Cameron uses a MCM Electronics 88 – 108 MHz FM Trap which can be found very cheaply on Amazon or Ebay for around $15 USD. His video shows that the FM trap works very well and significantly reduces out of band FM interference.

8 comments

  1. paul

    Thank you for a great site and a great product. This thread is old but what the hell.
    I bought the above mentioned fm trap because i live in an fm saturated urban nightmare. I thought that removing the loud signals and harmonics would let me get a clearer reception of the other signals. So i plugged it all in and nothing improved. Trying to figure it out i realized that my coax out of the filter was acting as an antenna and letting the fm back in. Even the pigtails that connect the airspy to my rtl dongle act as antennas letting the fm back in. Then to completley push me over the edge i realized that even with nothing conected to the rtl dongle, no antenna, fm was still being picked up rather loudly. Even with a hundred fm traps strung together the dongle was still going to let fm hop back into the signal. My problem is that i dont listen to sdr underground in a perfectly shielded room. Even with nothing but a conecter after the filter, fm just hops back on. Other than wrap the whole thing in a giant ball of tin foil what am i supposed to do. Im actually sad because i thought this was a solution. Others with better listening rooms are going to be able to use this but so far fm is always going to get in and mess everything up. Any suggestions are welcome. Thanks again.

    • admin

      Search for the keyword ‘shielding’ on this blog using the search on the right column. Basically your idea to wrap it in tinfoil is on the right track. The newer dongles sold by us come with an aluminum case so should be already shielded, unless the signal is extremely strong. If you have an older model with SMA connectors sold by us, then we also sell the case separately, or if you have a different model with an MCX connector then a company called Nooelec also sell an appropriate case. If you prefer to DIY it, then you can use copper tape, or find another suitable metal enclosure yourself.

      Other than shielding your other option would be to go for a higher end radio with shielding and high dynamic range like an Airspy.

      • paul

        Thanks for the note. I meant to say spyverter not airspy. I bought your metal dongle because it looks like the best and i have no regrets. It is an amazing product. I like the idea that all these gadgets are small and modular and can be thrown into a backpack and run on sdr touch for my android. It is a perfect elegant and inexpensive solution for me to get into wideband radio listening. The problem is i live in toronto and we have something called the cn tower which blankets the city in fm, am, and vhf signals. I am reluctant to wrap everything in metal tape or build a metal case simply for esthetic reasons. But I probably will throw it all together with some batteries into a small cookie tin or something. Again, all the equipment I’m using including the fm trap are working perfectly as expected, it just that my location for running it all is less than ideal. From what I’ve learned about the radio hobby is that people are always trying to improve their setup and deal with all the tradeoffs and problems inherent in radio frequency. It’s never perfect for everyone and my situation is no different. Thanks for providing such a great site.

    • Well, elighten us a little plesae. Be more somewhat technical and use some specific examples to back up the "Yeah right" part of your comment.rt

      Well, enlighten us a little please. Be somewhat technical and use some specific examples to back up the “Yeah right” part of your comment. There are many people that purchased inexpensive devices as an entrance to SDR. We don’t expect a pinnacle of performance from these things; if that is what your comment is based about.

      • Tehrasha

        I believe that he was ridiculing the ‘so powerful’ description of the filter. ‘so powerful’ that it affects frequencies adjacent to the FM band. A more accurate statement would be that the filter is ‘so wide’ or ‘has such low Q’ that it affects frequencies outside of the desired notch.

        The MCM FM Trap that I bought on eBay works fine fro me. I do not notice it adversely affecting the 108-137Mhz Airband. I dont know about the channel 5/6 region below 87Mhz as there is nothing in that range local to me to receive anyway.

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