Topband: inverted L fed with coax

[Tom Williams]

Evan,

I have a 46' vertical, ~90' horizontal Inverted L with 97 on ground radials between 75 and 150' long. The antenna base is some 350' from my shack. I'm feeding it with LMR400 and RG-213. With a base impedance around 22 Ohms, I built a 2.25:1 UNUN and got an "acceptable" match. I have worked 173 countries on topband with various antennas, the last 60 or so using this antenna for two years.

The SWR plot always showed some weird impedances and I was never fully satisfied with the strange load settings on my amplifier. A week ago I removed the UNUN and replaced it with a simple shunt coil. My SWR is now 1.2:1 at the base and 1:1 back in the shack on 1830 kHz. It was nice to load the amplifer normally and get 1 kW output with only 3 W reflected! The 2:1 SWR bandwidth in the shack is 60 kHz. 

The coil was measured at about 2.7 uH and is wound from #12 AWG bare solid copper wire, 3" in diameter. I added a couple extra turns so I could use an adjustable tap to get the best match. I wound 8 turns, each spaced about 1/2". The tap to ground ended up being 6-1/4 turns from the antenna end.

I have pictures if you would like to see them. Email me at n2cu at roadrunner.com

73,
Tom N2CU <><