I'm building my first kit, it's the mcHF SDR radio. I've started with the low pass filters. The radio has four of them:

mcHF Low pass filters

mcHF low pass filters schematic

I tested the first filter with a NanoVNA and got a result that resembled what it should be, (except that the line should stay flat after the cut off and not rise slightly as it's doing):

mcHF 40m low pass filter, strange sweep results

I moved onto the next filter, wound the toroids, tested each one for the right impedence, and soldered in the toroids. But the sweep analysis of the filter is giving completely wrong results.

mcHF 40m filter, bad sweep response

I tested the joints over and over, recalibrated the NanoVNA. I'm at a complete loss for what's going on :/

New contributor
rcx935 is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.

Probably the connection wires (yellow, green and black) ar far, far too long! Solder the coax wires as short as possible to the filter connections.

Also check the number of windings (in error): 23, 27, 24 and for the second 17, 19 and 18. This should still be a low pass filter, but not as good as the design.enter image description here


Your Answer

rcx935 is a new contributor. Be nice, and check out our Code of Conduct.

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.