Some time ago I was wondering if there were any updates on Steve Hunt’s (G3TXQ) common mode choke measurements because the information I was using, originated back from 2012 already. I was sad to find an in memoriam article on GM3SEK’s blog of 1st of January 2019 announcing that G3TXQ passed away. I’ve been using and G3TXQ’s recipes for my CM chokes and promoted it with many other fellow hams so may his name and callsign be remembered! From now on, I’m calling them TXQ chokes.
In the same article, there was a link to an article that was published in RadCom back in 2015, archived by GM3SEK’s: https://gm3sek.files.wordpress.com/2019/01/G3TXQ-RC.pdf. Indeed there are some updates compared to the 2012 chart. I advise anyone to read the complete article, but if you want a quick look at some recommended common mode chokes according to G3TXQ’s measurements, here they are:
G3TXQ’s 2015 chart of some good common mode chokes (and some very bad ones)
I’ve been using 17t on single FT240-43 for many years for my 40/80 antennas (the 2nd one in the chart) and 11 turns on 2 x FT240-52 on my antennas for 14 MHz and higher frequencies (the 7th in the chart). Looking at the chart, these configurations still seem to be the best choice but G3TXQ has added some interesting new configurations e.g. the 9 turns on 3 x FT240-52 that has greater than 8k impedance up to 30 MHz. I’m thinking of using that one in future for 14 MHz and up.
Within the online amateur radio community a design using multiple FT240-31 toroids has become popular. This is kind of contradicting G3TXQ’s measurements because it seems that he only found one decent solution using 31 material for 160m and 80m. All other 31 material based designs that did appear in the 2012 chart (below) were dropped in the 2015 chart probably because the figures weren’t too good. Not sure why this design has become so popular and what the theory behind it is but I reckon that if you put a number of mediocre chokes in series, it will probably make one good choke HI. I rather stick with the G3TXQ designs though, simple and effective. Here’s G3TXQ’s chart from 2012 for reference.
G3TXQ’s chart from 2012 showing some designs using 31 material
73 de PA3HHO (Pleun)