RadioBanter - View Single Post - Grounding Bar Length For Receiving Only Antenna

April 22nd 06, 02:02 AM posted to rec.radio.amateur.antenna

wrote:
. . .
A modest radial system will whip a deep ground rod, though. I'm about
10% tempted to break out J.D. Jackson' Classical Electrodynamics and
try to calculate the RF resistance vs. depth of a ground rod, but I
know it's going to be hard to deadly.
. . .

If you decide to, I'll be interested in seeing how you do it and what
results you get. Reg is the only one I've seen so far who can come up
with the impedance of a one-terminal device.

For those of us stuck with needing two terminals to determine a
resistance, we find that the RF resistance of a ground rod depends on
what's above the ground. That is, the RF resistance of the ground rod is
a function of the nature of the antenna. For example, here's the
resistance of a four foot ground rod in average soil at 14 MHz, beneath
vertical antennas of various heights:

Ant ht ft Ground rod R ohms
16.8 46.2
8.4 34.8
4.2 18.3

These were determined by using NEC-4 and comparing the feedpoint
resistance of the vertical over perfect ground with the resistance when
connected to a ground rod in average ground. I assume the change in
apparent ground rod resistance is due to the change in the field in the
ground from the antenna as the antenna height varies.

Quantitative is always best in this sort of discussion, for sure.

Indeed.

Roy Lewallen, W7EL