A low pass filter for 40m

I’m currently finishing a 20w amplifier from EI9GQ’s book “Building a Transceiver” to pair with the “Simple SSB” 40 meter transceiver I recently completed.

The amplifier was mostly complete but I still needed a low pass filter for it. I had a look at the filter design suggested in Eamon’s book, but I didn’t have all the capacitor values or toroid types recommended. I therefore decided to use LT Spice to tweak the values to see if I could get by with what was in the junk box.

What I came up with was the following:

I then went on to building the filter, using what I had specified in LT Spice. I paralleled two 430pf capacitors for both C2 and C3 and used three T68-2 toroids with 16 turns each to give the approximate inductance.

I then soldered on some temporary SMA connections and hooked up the NanoVNA.

This looked pretty good to me, with attenuation at the second harmonic being down around -50dB and a cutoff frequency at around 8MHz.

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