Re: Toroid material for QRP HF tuner: "2" or "6"?
I tend to build z-matchers to cover low HF and hi HF. One covers 160 thru 20 meters with type 2 cores. Others covers 40 meters thru 10 meters using type 6 cores.
Also build a few single band versions such as 10 and 6 meters using type 10 cores (black).
I like the designs from Charlie Lofgren, W6JJZ, and find them to have a wide impedance matching range and good measured efficiency.
You can find his designs in ARRL Antenna Compendium #5 and here: http://www.seboldt.net/k0jd/z-match.html
For measurements the AI1H procedure found in QEX for Sep/Oct 2003
and PDF here:
Chuck Carpenter, W5USJ
EM22cv, Rains Co. TX