Stub "hybrid" lowpass filter to filter out amplifier harmonics

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 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, after recommendations from OK2ZAW, a coil of 7 turns with a diameter of 30mm for this 40M LPF, with 20M notch

After some tuning of the stubs, and coil adjustment, I managed  a very sharp -76 dB of rejection on 20M, and an insertion loss of 0.11 dB. It can be stagger tuned, to give a wider, but not as sharp, null.

I'm more than pleased. No more false spots, and one more build up the sleeve.











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