Topband: Adding radials, a question

[Richard (Rick) Karlquist (N6RK)]

> 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.

Rick N6RK