40m LPF analysis

My W6JL amp didn’t seem to quite meet the spurious emissions limits in Australia with the 2nd harmonic a few dB short of 50dB down, though this was at the limits of my test equipment. This was a bit of a mystery as a 5 element LPF should have easiy taken care of the remaining 2nd harmonic.

Using the scalar analyzer neatly reveals why:

That little kink around 12Mhz is worth  5dB or so and so explains the gap.

The filters are being rebuilt as 7 element pluggable units and with hopefully better construction this will be the end of that.


Measuring the new 20m LPF i became suspicious of the results and it turns out the buffer amp in the analyzer is not ideal and is being affected by the load. 

Remeasured filter but with a 6dB attenuator after the amp:

Looks perfect.

I’ve now convinced myself that what i’m seeing of the 2nd harmonic is actually artifacts of the test setup. In any case, I’m moving to higher order LPF anyway.

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