> and I can change the resonant frequency and base impedance
> significantly by adding one ground strap to my pickup truck
> between the cab and the bed, yet FS hardly changes at all.
I have observed similar effects to the one's Tom describes.
I have a 3ft by 3ft grid of wires forming a
230 ft diameter ground screen. I installed a 40M
vertical at the center and connected a signal
generator between the antenna and the center
crossover of the ground screen. The impedance
was 60 ohms! I then added 25 foot long radials and was able
to gradually reduce the impedance to 36 ohms with 60 radials.
As radials were added, I monitored the field strength
with a spectrum analyzer 800 feet away and noticed that it was
unaffected by the number of radials. The resonant
frequency was affected by the number of radials.
I measured the impedance using a homebrew wheatstone bridge
with my FT-757 as the detector; therefore zero error
due to AM radio stations. Also, the signal generator
was battery operated and located at the antenna, so no
feedline radiation issues.
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