If you use a wideband loop antenna, a cheap Distill AM Bandstop Filter reduces SDR overload from local medium wave signals.

Last summer, I wrote about the challenges with using wideband active loops on wideband SDR receivers. My main problem was overload from local AM or Medium Wave transmitters nearby. You may recall two issues. First, the harmonics above medium wave from strong signals. Second, and most important, was overload on the analog digital converter (ADC) which reduced overall receiver performance.

Most SDR have some built in ability to handle overload. Some, like my Flex 6300, have more bits in the ADC which raises ability to handle strong signals. Some, like my Perseus have built in band filters and attenuators. RSPduo in particular has a dedicated medium wave notch filter.

However, my main concern was my Afedri AFE822x dual channel challenge. Like the RSPduo, the Afedri is great for coherent reception and spatial filtering. The challenge is its digital AGC front end gain reduction. When you reduce front end gain because of strong AM signals, that gain remains reduced across all higher frequencies, making it somewhat deaf at HF.

I solved this problem by building my wideband loop helper, which contained switchable medium wave notch filters and attenuation. An alternative solution is use of a Distill AM Bandstop Filter. The barebones version of this Nooelec filter is on Amazon at a low price. It sells for US $10 or CDN $15. You can see two of these units above right. Since each has an SMA connector, you also need some adapters for BNC or other connections to radio and antennas.

I have been experimenting with this Distill AM Bandstop Filter for a few weeks and can report they work as specified. The notch is a bit broader and shallower than my DIY filter, but otherwise does the job with all my gear, particularly the Afedri AFE822x.

Distill AM Bandstop Filter Performance

You can see the filter attenuation between 200 kHz and 2.5 MHz above, measured on my NanoVNA. All three of the Distill AM Bandstop Filter units performed the same, with a 1500 kHz 3 dB bandwidth from 400 to 1900 kHz. Attenuation is less at the lower end of the AM broadcast band but still more than 20 dB. You can find detailed specs on the Nooelec web site.

Overall, I would say that this cheap unit gives you an average of 25 dB attenuation across the band. You will find the important thing, though, is that when used with a wideband loop antenna, you get an additional 20 – 25 dB headroom across HF which in most cases removes ADC overload from strong local radio stations. I proved this by monitoring signals between 5 to 15 MHz with the Distill AM Bandstop Filter inserted and found received signals to be 20-25 dB stronger.

Shortly, I will 3D print a case for these filters.