My homebrew wideband loop helper performance demonstrates what you can achieve with simple circuits and a bit of effort at the workbench.
I am amazed by how this simple Butterworth bandstop filter provides great wideband loop helper performance. Yes, not perfect, but a good deep null to reduce overload from strong local broadcast signals at the flip of a switch.
My third order Butterworth filter, shown above, has -6 dB points of about 530 kHz to 1500 kHz, with a deep -50 dB null around 900 kHz. You can see that both channels are not perfectly matched, but reasonably close. Outside the broadcast band, insertion loss drops to about 0.2 dB so essentially the filter disappears. Butterworth filters are known for smooth passband responses.
As you recall, the main purpose of the wideband loop helper is to knock down medium wave signals entering the front end of a wideband SDR receiver. I can still hear some medium wave harmonics and mixing products into the 2 MHz range. But eliminating these would require a much more complex, perhaps seventh-order filter like these.
My main strong signal offenders are CFFR 660, CHQR 770 and CKMX 1060 kHz. All of these show up at more then -10 dBm or S9+70 dB. The wideband loop helper performance shows -20 dB on 660, and around -40 dB reduction at 1060 kHz. And if I switch in the attenuator, these levels sink another -18 dB.
I measured these response curves using NanoVNA. I am not sure why the curves are not symmetrical, but I suspect there is stray inductance and capacitance involved with the simple circuit.
Wideband Loop Helper Performance
Anyway, this little project solves my problem of using my Afedri dual-channel receiver with a wideband loop. No longer do the AM locals reduce front end gain on HF.
Now, to be fair, you could probably get comparable performance using the Distill Barebones Broadcast AM Bandstop Filter from Nooelec. This filter costs only US$10 and you might need to add some SMA-BNC adaptors. Given this, you could build a dual channel unit for around $30. Based on the data sheets, performance is about the same.
But building my own was more fun and I got to use all my tools including CNC and 3D printer.