Topband: Broadbanding a Vertical

[Jim Brown]

I'm sure this will come as no surprise to the old timers on the 
list, but I'm passing this along to the newer guys. 

In contests, activity often goes pretty high in the band, and it's 
nice to be able to operate up there without tweaking an antenna 
tuner. My antenna is a simple top-loaded Tee vertical, with the top 
flat section tuned a bit longer than resonance, and a series 
capacitance added in series with the feedpoint to bring it back to 
resonance. That was giving me 1.2:1 below 1845, but was hitting 2:1 
around 1880 or so. Today, I lowered the antenna and added a second 
vertical wire in parallel with the first one, spaced with a foot or 
so between them. There are parallel conductors ONLY in the vertical 
portion of the antenna -- they're tied together where they hit the 
center of the Tee.

I measured the base impedance and found that I needed a much larger 
capacitor to bring it to resonance. When I added it, I'm now below 
1.2:1 from about 1820 to 1880, below  1.5:1 from 1800 to about 1925 
kHz, and hit 2:1 at about 1980 kHz. 

The same sort of technique can be used on most resonant or semi-
resonant antennas, including inverted Ls and dipoles. It IS 
important to keep the parallel conductors spaced apart -- the 
principle is to simply make the conductor "fatter". And if you're 
loading your tower, you're already fat, so you should find it 
fairly broad. :) 

73,

Jim Brown K9YC