> What did you do to the ground system that made the base
> impedance go up but
> yet increased the field strength?
In the latest case we were testing elevated radials vs.
buried radials with exactly the same radiator on 40 meters.
We moved the radiator higher or lower to match the height of
the radials so the effective height stayed the same.
A small buried radial system producing 50 or 60 ohms base
feedpoint impedance had slightly more FS than an elevated
system using four radials. The elevated radial system was
around the high thirty ohm range.
This is really pretty common. It is very common when the
radials are longer than 90 degrees electrical. For example
if I install a radial system with 180 degree radials the
effect is often to increase the feedpoint impedance even
though the FS does not decrease over the same number of
shorter radials that have a low feedpoint impedance. I can
also use ONE very short resonant radial with a very low ESR
inductor and produce a ground with very low terminal
impedance yet very poor efficiency.
There are multiple things that can throw a wrench into the
idea what we measure at the feedpoint somehow tells us how
the entire system behaves. The root problem is when a
complex system has transmission lines with SWR impedances
are transformed. This means what we read at one point has
nothing to do with losses at points remote from the place we
are measuring. Remember every conductor in the antenna
system is to some extent a transmission line with surge
impedance, even the radials.
The idea we can determine efficiency by watching the
feedpoint is false when we try to determine distributed
losses only at the feedpoint. The old Rradiation over
Rground+Rradiation = efficiency is only a good formula when
we understand the system and all losses and the radiation
resistance is normalized to the feedpoint, but not when we
don't or can't do that.
The only way I can think of to normalize losses to one point
is to measure feedpoint resistance and then enclose the
entire radiating system in a closed conductive dome in the
farfield of the radiator, and then measure the change in
resistance at that single point when we force all radiation
to stop. This would normalize everything to the single
point. Since few of us can do that, looking at the feedpoint
is a waste of time.
If you want to see another example where people misapply
ground losses, look at articles on mobile antennas. There
are articles that conclude, because they only measure
feedpoint resistance changes, some mounting spots on a small
vehicle can produce ground losses of a couple ohms on 80
meters. Good luck with that! Why install 50 or 60 1/4 wave
radials when a $100 junk Toyota parked under the tower would
be the same?
If we want to know the FS change, we need to measure the FS
change. If we want to know ground losses we have to change
the ground and watch FS. Measuring the feedpoint is not
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