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I've built the M0AYF active loop antenna for my RTL-SDR, using BC847C for the amplifier transistors.

There are a couple very strong AM stations here, usually playing music. What I'm noticing is that whenever I tune to something strong, like another station with spoken word, I hear the music in the background. I can even hear it in obvious QRM signals (buzzing is modulated to the music).

this didn't happen with passive antennas, so I suspect the amplifier circuit.

What is this effect properly called, what could be causing it and how to fix my receiver? Is the amplifier gain too strong? (How would I lower it - reduce the supply voltage? Add a voltage divider in the signal path?)

Here is an audio sample (ogg)

Edit - the solution: Following the answer and comments, I built a 30MHz low-pass filter using the schematic from here: https://www.hfsignals.com/index.php/ubitx-v6/

enter image description here

The filter is connected directly to the antenna, before going into the amplifier. Neither side is grounded, the "GND" in the filter diagram is actually one leg of the loop.

This largely removed TETRA interference and FM stations and the overloading problem is completely solved. There is still some QRM left, but that's probably unavoidable when receiving in urban areas.

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    $\begingroup$ It's a form of front end overload. To fix it, you need to put a filter between the antenna and the amplifier. Sometimes it is possible to tune the antenna to be your filter. $\endgroup$
    – user10489
    Commented Jan 23, 2022 at 5:52
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    $\begingroup$ Buzzing?...suggests that your remote +12V supply is causing problems. You likely have a long run between 12V and loop-amplifier. This wire may be an antenna, with the diode in the power supply detecting AM modulation. Try a +12V battery right at the loop amplifier. It is also possible that your SDR is seeing too much signal. Does an RF attenuator at its input reduce QRM faster than desired signals? $\endgroup$
    – glen_geek
    Commented Jan 23, 2022 at 17:28
  • $\begingroup$ With buzzing, I didn't mean ground loop, it looks like some kind of continuous QPSK or trunked radio, I wasn't able to identify it, it shouldn't be at 20 MHz and below... mind, TETRA is very strong here, but it's at 430MHz. My amp is powered by a 12V car battery through coax, following the M0AYF schematic. Grounding to a radiator pipe didn't have any effect on this, though it reduces noise floor a little. Here's a screenshot of the buzzing signal: share.ondrovo.com/2022-01/qrm_buzzer.png $\endgroup$
    – MightyPork
    Commented Jan 24, 2022 at 10:20
  • $\begingroup$ Thanks for sharing the solution to your problem! $\endgroup$
    – rclocher3
    Commented Feb 7, 2022 at 20:12

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If you hear the crosstalk when using Direct Sampling mode, then you are likely hearing intermodulation or overload occurring in the LNA and/or in the ADC of the RTL-SDR, due to strong signals overdriving those circuits.

To reduce or eliminate intermodulation noise, you can try using an AM and/or FM broadcast band blocking filter (when not listening to the AM or FM bands, respectively), or bypass the LNA, or insert an attenuator in front of the LNA, which likely defeats the purpose of having an LNA, or perhaps use a directional (loop?) antenna that can put the strong interfering RF in an antenna null.

If you hear the crosstalk at VHF and higher frequencies (e.g. when using the RTL-SDR down-converting tuner chip), first try turning off the RTL-SDR AGC and lowering the RF gain. That may help, or you may still need to reduce AM band overloading, per the above.

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  • $\begingroup$ A narrow band antenna can also act like a filter. The small loop (very high Q) is a very sharp filter. $\endgroup$
    – user10489
    Commented Jan 23, 2022 at 17:15
  • $\begingroup$ The crosstalk is indeed in direct sampling, but I think the LNA bypass is already integrated (the model is Nooelec NeSDR Smart). When I added the jumper wire to one of the pads as suggested in online guides, direct sampling practically stopped working. Would a simple resistor divider work as an attenuator? $\endgroup$
    – MightyPork
    Commented Jan 24, 2022 at 10:12
  • $\begingroup$ Regarding smaller antenna, @user10489 do you mean a smaller loop antenna will have smaller bandwidth, or just smaller gain? How do I tune it if it's the former? I'm mainly interested in receiving the HAM bands (around 1800kHz, 4000kHz, 7000kHz), but I also like hunting for foreign AM stations. Would a low-pass filter rolling off around 30MHz (to remove FM broadcast, TETRA and other VHF/UHF) help with the overload? $\endgroup$
    – MightyPork
    Commented Jan 24, 2022 at 10:14
  • $\begingroup$ Not smaller antenna. Smaller bandwidth antenna. A "small loop" is a very specific antenna with a very specific tuning method. Most antennas you tune when you build them, so that they work on a single band. The small loop is special in that it is extremely narrow bandwidth (100KHz receive!) and can (and must) be tuned live across multiple bands. Yes, a low pass filter would help if you are flooded by FM stations. But for AM stations you need a high pass (or band pass or band reject) filter. In the US AM is 500-1800 khz, well below 30MHz. $\endgroup$
    – user10489
    Commented Jan 24, 2022 at 12:14
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    $\begingroup$ So, I built a 30MHz low-pass filter following the schematic shown here: hfsignals.com/index.php/ubitx-v6 This removed TETRA interference and FM stations, and the overloading problem is now completely solved! I now have a very clear reception of shortwave radios and the sub-10MHz ham bands! $\endgroup$
    – MightyPork
    Commented Feb 6, 2022 at 18:55

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