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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 :/

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  • $\begingroup$ Your ground leads are far too long and it seems you only have one ground, not two. This would give strange results even in an audio system at 1 kHz! Keep them to a few cm at most. Find ground pads near the filter and connect the coax to them. Each coax ground needs to be connected near its centre conductor, perhaps the ground side of one of the capacitors. The leakage you see is probably coupling around the filter, through the huge loops. Well done for measuring the filters before carrying on with the construction! It would have been hard to find the soldering bug later, without a VNA! $\endgroup$
    – tomnexus
    Commented Dec 16, 2021 at 19:30

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I found the fault. It was due to the enamel not being stripped back far enough on one of the inductors, preventing a good soldered connection.

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    $\begingroup$ Thanks for sharing the fix! $\endgroup$
    – rclocher3
    Commented Dec 16, 2021 at 15:50
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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

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