Topband: Adding radials, a question

[W5UN]

I have a question for any of you who have experience with adding radials
and making measurements of the changing impedance on a 1/4 wl ground plane.

I have a 1/4 wl 80 meter GP made from #12 AWG copper wire, with a 3/16 wl
radius ground screen beneath it on the ground surface. The vertical is
suspended from a Dacron line strung between two towers approximately 250
feet apart. As it is, the impedance of the GP measures 43+j0 ohms at 3.525
MHz. I am thinking of converting this antenna to a "T" vertical on 160
meters. I know that the ground plane needs some improvement for this, so I
cut some radials to 1/4 wl on 1.8 MHz, and began to add them in groups of
four. While doing this, I thought I might as well see how the additional
radials were affecting impedance of the 3.5 MHz GP. To my great surprise,
the impedance went from 43+j0 ohms with only the ground screen, to 48+j0
with the ground screen plus four radials.  Then jumped to 51+j0 with eight
of these new radials plus original ground screen. The resonant frequency
did not change, nor did I expect it to.

This seems to fly in the face of theory, which says that the antenna
impedance should approach 36 ohms, or so, as losses are reduced to near
zero. Can someone explain what is going on here?

FYI, the ground screen consists of a grid of #14 AWG house wiring, on four
foot centers, bonded at each crossing intersection.



73, Dave - W5UN						w5un at wt.net

homepage: http://web.wt.net/~w5un    Grid: EM23mg

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