|8:1 and Air Variable Cap
Today I made a 8:1 Transformer out of a piece of 18 ga. house wire. 24 turns as Secondary and 3 turns as primary on a T240 core. Using the Air Variable that I recently purchased on eBay that has about 3/8" plate spacing the half wave 40M wire tunes to 1.2:1 with the plates of the cap meshed about 2/3 of the way. The wire is probably still a bit long as I cut it to 468/F + .05% and then added about a foot to be sure it would not be too short.
Although it tuned well using a tuner and the 4:1 Unun on 80-10M the SWR and RF in the shack was too much for the CAT port on a FT-1000MP Mk V. The MAX232CWE chip failed. These are available from Digikey for less than $4 and I ordered two but installing them will be a challange for later. In the meantime I have pulled the MK V off the op bench and replaced it with the back up FT-1000(almost a 'D'). Several contacts were made on 40M with the 8:1/cap matcher today with good results. I did not notice any RFI and the SWR was low but this was with only 200W. Later I switched back to the 4:1 Unun and internal tuner for the 10M contest and made another 20 or so contacts some on QRP and some on CW. Comparing the 8:1/Cap combo and the 4:1 and internal tuner on 10M it looked as though the 4:1 had a one 'S' unit or so advantage.
On the right is the 4:1 Unun (Rybakov style) in a radio shack box.