nanovna-H – measure 40m low pass filter for WSPRlite flex

This article demonstrates the use of a nanovna-H to measure the response of a low pass filter designed to pass 7MHz frequencies but attenuate harmonics. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI for use in a 50Ω system.

Implementation

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 nanovna-H displaying s11 and s21. The sweep is pretty much as you might expect of a compromise between rejection at 14MHz and Insertion Loss at 7MHz. Return Loss at 7.02MHz is 18.5dB, so the VSWR plot will be good.

Above is a screen shot from the nanovna-H. It is running ttrftech’s v0.5.4 firmware, hence the Smith chart marker value in R+jX format… though it does not remember that across power off, it comes up in hammy sammy mode.

Above is the VSWR response. A very good response, a tribute to Richard’s design.

I should note that the nanovna-H has a 10dB attenuator on its rx port (Port 2 or in nanovna speak ch0). This improves the impedance seen by the filter which affects its response.