I built an elevated 40 meter ground plane last year. Some of the elevated
radials were near trees and my house so I was also concerned about how to
get exact resonance when the length of one or more radials may need to vary.
I used a technique I had read somewhere -wish I could remember whom to
credit, but I can't. In any case, it worked quite well.
Before even raising the vertical element, I resonated the radials by taking
two opposing wires and forming a dipole. I put the other two wires on the
ground. I fed this low dipole with some coax with a coaxial choke at the
feedpoint and measured the impedance at the feedpoint using the remote
sensing function of my AIM4170. Then I trimmed this dipole for resonance at
the desired frequency. Then I put these wires on the ground and did the
same thing with the other set of wires. Then when putting up the vertical
element I only had one variable to play with in terms of overall resonance -
the length of the vertical element. I set up the radials to be slightly
sloping and the impedance at resonance turned out to be in the mid 40s, so
no further matching required. It worked for me.
73 ... Craig AC0DS
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