be extended by bending or removing a variable capacitor
plate. Modified copper pipe U hold-downs are strong and
were used to mount the capacitors.
Homebrew high voltage capacitor ideas are in the
ARRL Antenna Book and on the Internet; just remember
for 100W about 4KV is needed. During initial testing at
100W, the variable capacitor arced until I discovered a
small leaf had fallen on it.
A popular way to match the antenna is with a small
1/5 diameter loop (eight inches of RG8X in this case).
One end of the coax is soldered to the center and shell of
the SO239 connector in the usual manner, but the other
end has the braid not connected with the center coax
wire soldered to the outside shell making a DC short.
Some say this method is better because it provides an
improved electrostatic shield and better pattern.
The feed loop can be put on the top or bottom.
Mounting at the top makes the loop less top-heavy.
However, RF is then coupled to the coax line hanging
down and needs to be choked off by toroids or some
No RF on the feedline was verified by moving my
hands up and down the coax line close to the connector
with no changes in SWR, which should result in better
nulls. With the capacitor up higher, there is also less
chance of detuning because of surrounding objects.
I got most of the parts from Home Depot, including
74 July/August 2018
Top mount detail.
K. Siwiak KE4PT, “RF Exposure Compliance Distances for
Transmitting Loops, and Transmitting Loop Current” QST
Technical Correspondence May 2017 pp 64-65.
K. Siwiak KE4PT, “Near Fields of an Electrically Small Loop
Can Affect Direction Finding” QST July 2015 pp 63-64.
Jim Lux W6RMK in an extensive technical analysis letter to
Steve Ford on Nov 29, 2006 about RF compliance with
Table 17 in FCC OET65B; www.fcc.gov/general/oet-bulletins-line.
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