80m harmonic fix by changing the capacitance of the LPF

Howard WB2VXW previously mentioned that he would try to reduce the harmonics by changing the output to 25 ohms and adding a step up transformer to go back to 50 ohms for the output. This would allow tripling the values of the capacitors at the input and output of the filters, reducing the effect of the stray relay and layout capacitance.

In the end after more simulation, Howard decided just to change the filter characteristics and retain the 50 ohms termination impedance.

Howard was able to come up with a filter that doubles the capacitance.  At least on 80 meters, with this new filter design, the harmonics don’t exceed -45 dB in his tests. Not as much margin as he would like, but legal.

Howard changed the 3 inductors to 1.6 uH by adding 2 turns on each.  He added an extra 1000 pF cap in parallel with the input and output caps, bringing the total to 2000 pF.  He also added a 620 pF cap in parallel with the one of the two paralleled 1000 pF caps in the middle two sections for a total of 2620 pF. (750 pF would have been better, but he didn’t have one).

Howard is asking others to give this ago.   After validation of the 80m LPF redesign he plans to try a similar solution for the 40 meter band.

Reference

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