Thanks to all who contributed. W8UVZ gave me an idea that let me proceed
with testing. George said " why don't you pull the far end of each radial
back to the center, and connect it, thus making it an effective 1/4 wl
length consisting of two wires instead of one".
Radial resonance of 1/2 wl is apparently the problem that distorts the
readings. With both ends connected to the center, I measured the following:
4 radials: 52 ohms
8 radials: 45 ohms
16 radials: 43 ohms
16 radials + ground screen: 41 ohms
ground screen alone: 42 ohms
This makes more sense, and looks consistent with theory.
With ground screen only connected, resonance was at 3.524 MHz.
With radials and ground screen connected, resonance was at 3.594 MHz.
With radials only connected, resonance was at 3.594 MHz.
The radials tended to effectively shorten the antenna.
Lesson learned: stay away from 1/2 wl resonant radials if you want to make
understandable measurements
Two measurement methods were used:
(1) Autek VA-1 analyzer, connected directly at the base of the GP,
(2) passive NULL bridge (Micromatch) with higher power input to overcome RF
voltages on the antenna.
Both methods yielded the same results.
Having satisfied myself that things are as they should be, I will now
proceed with the conversion of the GP from an 80 meter GP to a 160 meter
"T" vertical.
73, Dave - W5UN w5un@wt.net
homepage: http://web.wt.net/~w5un Grid: EM23mg
RR 10, Box 27
Mount Pleasant, TX 75455
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