Let me correct any confusion I caused...
In my contribution to the discussion of tower equivalent cross-section, all
my references should be RADIUS, not diameter.
Thus, the reference uncovered by Bill W4ZV is certainly in the ballpark of
the Cummins paper I cited, and is very close to the value for the radius of
a circle with the same enclosed area.
I managed to get myself turned around, since the data entered into EZNEC is
diameter. And yes, I managed to put the 'radius' numbers into the program as
if they were diameter. Then I mixed up the two in my post to the reflector.
The difference between my erroneous value of 5" diameter and the supposedly
correct value of 10" diameter is significant, mainly in the reactance. At
1850 kHz, the model shows:
5" dia. --> 41.5 +j65 ohms
10" dia. --> 39.8 +j41 ohms
In the case of my 160M vertical, this does not get the model any closer to
the measurement. I have not made direct R-X measurements at the tower base
(too cold to drag out the GR-1606). However, at 1850, I get 1:1 VSWR --
through 225 ft. of 7/8" hardline -- while matching with 1440 pF series
capacitance. Thus, I estimate the impedance to be very close to 50 +j60
ohms.
As noted before, my inverted-L of the past two winters agreed well with the
EZNEC model, and used the current ground system. So the large difference is
a bit puzzling. As WX gets warmer, I'll make further measurements, then
remove the on-ground radials and work on an extensive buried ground system.
Perhaps an explanation for the differences will become apparent.
73, Gary
K9AY
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