In the German QRPForum someone mentioned a group of QPR enthusiasts QRPGuys. They have some neat QRP kits - transceivers as well as antenna-tuners.
I gave these kits a try: QRPGuys Portable Tri-Band Vertical Antenna and the QRPGuys 40m-10m UnUnTenna Plus
This post deals with the first one - the QRPGuys Portable Tri-Band Vertical Antenna:
This antenna tuner consists more or less of two switchable inductors:
First I've build the antenna as described in the user guide.
Testing the antenna in my backyard shows a far too low resonance on all bands. The length of the vertical element is given with 5.18m or 17'. I had to shorten it to 4.90m to get a 1.4:1 SWR on 20m.
Switching L1 in series with the vertical element give also a very good SWR of 1.5:1 on 30m. But the 40m resonance was about 600kHz to low - so I removed 5 turns from L2 (the 40m inductor).
Now I have on all three bands a good SWR of 1.5:1 or less. For the radials I used four wires, each of 3.05m or 10' layed on the ground in a star pattern.
The antenna has her first mission behind. I've used her on the summit "Kaltes Feld" and she worked very well.