40m filter for WSPRlite flex

The WSPRlite flex requires external low pass filters for each band of operation.

Since my experiments will be conducted on the 40m band, the following low pass filter meets the requirement. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI for use in a 50Ω system.


Above, the filter is assembled on a piece of matrix board with two BNC connectors. The inductors are fixed with hot melt adhesive, and the whole thing served over with heatshrink tube. It is not waterproof.

Above is a sweep of the filter on a VNA. The sweep is pretty much as you might expect of a compromise between rejection at 14MHz and Insertion Loss at 7MHz.

Loss analysis

Measured Insertion Loss at 7.1MHz is 0.33dB (-|S21|) , and the measured input impedance implies a Mismatch Loss of 0.04dB, so loss (or transmission loss) is 0.33-0.04=0.29dB indicating quality inductors and capacitors.