I recently put up a doublet antenna, and I quickly realized that I would have the best efficiency with a proper balanced antenna coupler. (Transmatch/Antenna Tuner)
After some searching on the web I came across the K1JJ design, posted to the AM forums. It is very simple and I was able to assemble the circuit in afternoon. The hard part was winding the 4" coil out of 8 gauge wire, which took 3 days! But that is a different story.
Schematic. The balanced feedline connects to a parallel-tuned circuit, which is link coupled to coax going to the transmitter. The link coupling is adjusted using a loading capacitor.
A few images of the coil winding process.
I later had issues with arcing, presumably due to a high impedance at the end of the transmission line, especially on 20m. This at first manifested as an increase SWR vs. time after applying power, culminating in the arc shown here. I mitigated the issue by unwrapping the wire on the wooden coil form and replacing it with plastic zip ties. No more issues!
I also found that in order to not have arcing on 80m with my linear amplifer I had to update the capacitors for higher voltage. Here's my most up to date configuration.
I had almost enough balanced feedline given to me for free to make it to the tuner. To get the rest of the way there I had make the rest myself. It's a nice rainbow!
Created 18 July 2020
Edited 12 Sept 2020
end of line.