Stub "hybrid" lowpass filter to filter out amplifier harmonics
I needed to knock down the second harmonic from my amplifier as I was getting spotted on 20 when conditions were good, when calling CQ on 40 using my multiband vertical. At first I was thinking about some sort of low impedance, series resonant circuit on 20, installed between vertical feedpoint and ground, but then I shifted to coax stubs, but I had no SO239 tee's. I wanted to it done right away so I figured I'd install panel mount SO239's on an aluminum box, and make the stub connection on the inside, when I read about W2VJN's hybrid lowpass filter. It's a three pole lowpass, but instead of capacitors, it uses coaxial cables with their ends open. Since they need to be left open, they are cut to 1/4 wave on the frequency that we want to supress. For 20M, this works out to be approx 3,4M. The series coil is about 1uH to resonate with the coax capacitance. I built this because it was an easy design, and no more work than to drill 4 holes in a 1590B box. I used, afte...