Dave... would it be a fair extrapolation to take your last sentence, and
draw the conclusion that if adding radials changes feed impedance, then
there was actual ground loss in the near field? Or that if we add more
radials and feed impedance change is not seen, then we are at a minimum
for ground loss?
The above statements certainly align with my gut feeling, but my gut
feeling is different than a mathematical proof :-)
I'm not Dave, but maybe this will give you something else to think about.
Once energy leaves the antenna, an external change can have the same effect
on impedance no matter if it turns applied RF power to heat or more EM
energy. If we put an antenna in a lossy media, we don't know what portion of
resistance is from loss and what portion from radiation, unless we measure
radiation or heat.
Also, since the antenna and radial system has standing waves, a decrease in
resistance at one point can cause an increase in resistance at another
point. Take a 1/4 wave transmission line, decrease the termination
resistance at the far end, and the input end impedance will increase. Each
radial is a transmission line, and the antenna is a transmission line. How
do we know what is going on by looking at resistance at one point?
Even without any change in efficiency, we can move impedances all around
when we change things. A folded monopole with equal size conductors has 4x
the feed impedance of an identical monopole with a single conductor of the
same effective area. If we change the ratio of conductor sizes, we can move
feed impedance all over the place with no efficiency change.
What if we move the feedpoint? If I have a given antenna and move the
feedpoint around while not altering phase or current distribution, feedpoint
impedance will change all over the place but pattern and efficiency will
stay the same.
With all of that in mind, and remembering radials and antennas have standing
waves, why would we ever assume a resistance change at one point in a
complex system reliably tells us about behavior at other points in the
system? That would only be true if we understood all the complex
interactions caused by transmission line effects, or had a special case
where field strength and feed resistance were directly related (like by
adding a resistor at the feedpoint).
When measuring a 40M vertical, 4 elevated radials at 6 feet had about the
same field strength as a dozen or so radials in soil contact. The feed
resistance of the elevated radials was in the upper 30 ohm range. The feed
resistance of the earth-contact system was nearly 60 ohms. Field strength
was the same.
If you want to know field strength change, measure field strength change.
73 Tom
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It is undesirable to believe a proposition when there is no ground whatsoever for
supposing it is true. — Bertrand Russell
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