I wonder if anyone in the group has experience with QRP Labs Low Pass Filter kits? I recently built 10 of them - one for each band 160m thru 10m - and found that on some bands the "knee" of the filter response is very close to the upper band edge. It's particularly bad on 20m and 12m where the tops of the bands are at a point 1.5dB down the filter slope.
I've checked and double-checked the capacitor values and the number of turns on the inductors; I've also spaced the turns on the toroids carefully so as to minimize the inductance. But -1.5dB is the best I can achieve at the 20m and 12m upper band edges. Measurements were made using a 2-port VNA and checked with a TG/SA.
I guess I can reduce the number of turns on the inductors, but wondered if anyone else has experienced the same thing, and if it is a design error?
Steve G3TXQ
