Cozy evening in

    When it's warm and quiet at home, with no Shaheds or other explosives overhead, it's the perfect time to get lost in crafting: women are busy cross-stitching, while men poke at circuit boards with soldering irons. An older colleague suggested an idea to protect the SDR receiver's IMD (intermodulation distortion) dynamic range using a low-pass filter (LPF) that attenuates everything picked up by an effective antenna above 30 MHz.Since I live very close to powerful transmitters, I need heavy attenuation. That’s why I chose a Chebyshev filter over a Butterworth. Sure, it’s not as flat in the passband, but it gives you a razor-sharp roll-off. And it needs to be at least a 9- to 11-pole filter. Hand-winding that many coils takes ages, and I don't really have the materials anyway, so I settled on axial inductors. I’ve already had some experience using them in filters before.  Nope, we're not at the finish line just yet. First comes the design phase. I wanted to calculate the filter components myself to match the parts I already have on hand, so I fired up RFSim99. 

The capacitance of the coaxial cables connecting the RF jacks to the board, and then to the receiver, will serve as the outermost capacitors. As for the soldering itself—it’s like having your morning coffee on the porch in the garden: far more of a pleasure than a chore.

I didn't bother factoring in the Q-factor during the calculations because reality will make its own adjustments whether you remember them or not :-) However, the assembled model—even with a slightly wonky SWR—showed sufficient attenuation. The copper ground plane, which I made sure to add on the bottom side of the filter board, also played a crucial role. 

The photo of the finished build clearly shows that the attenuation exceeds the dynamic range floor of my NanoVNA. In my case, that’s better than 70 dB.

My hope that a dedicated software, NanoVNA Saver, would leverage its algorithms to give me a much deeper look into the filter's characteristics did come true, but it brought little satisfaction: at the end of the day, it's still bound by the raw measurements of the NanoVNA itself :-(
In practice - and this is by no means the first filter I’ve built - the difference is noticeable by ear. However, since the interference comes from all directions and takes many forms, it's impossible to tell exactly where I won or where I lost. I can’t afford to upgrade to the next model, the NanoVNA-F V3 with its wider dynamic range, so let's just wrap it up by stating a fact: it was an evening well spent! :-)