To: <topband@contesting.com>
> Date: Wed, 18 Mar 1998 18:53:18 -0700
Hi Eric,
> I don't pretend to be able to explain it but I have now been
> bitten several times by inferring relative effeciencies from
> feedpoint impedance measurements. And I was later sorely
> disappointed by actual field intensity measurements which were
> done to confirm the work.
I cited two examples in my other post where BW and feedpoint
efficiency can go the way we expect to improve things and the results
go the OPPOSITE way.
Let me explain why that happens.
When a short loaded resonant ground system is installed, the electric
field around the radial (where voltage and current are nearly 90
degrees out of phase) greatly increases. The field is simply more
concentrated at or near that point.
One effect of the greatly concentrated electric field (and magnetic
field) is an increase in loss in the earth near the radial. The
system has very large standing waves on the radial, and it transforms
that loss to a different impedance when viewed at the radials input
terminal.
A second bad effect is the concentrated electric field increases the
coupling between the radial and the bottom area of the radiator.
This increased coupling effectively "short-circuits" the radiator,
robbing the upper section of current while greatly increasing the
circulating currents in the lower area of the radiator.
The effect described just above is exactly like placing a series
L/C/R circuit across the feedpoint! You drive the base impedance low,
but that extra current only heats the loss resistances.
The result is you can have a 1/4 wl vertical with near or even below
theoretical impedance yet have rotten efficiency. We can also change
something that makes absolutely no change in feedpoint resistance
yet improves efficiency!
Any correlation is blind luck.
My mobile antenna is another example. If I use my antenna with a
low hat (five feet above the roof) the base resistance is only a few
ohms (essentially coil ESR alone), and BW is very narrow. If I move
the hat up to ten feet, the base impedance nearly triples (to 28
ohms) while the bandwidth nearly doubles. Efficiency goes up about
four times!
The concentrated fields and currents are why these short "ground
independent" vertical antennas we see advertised have performance
that stinks!
In a complex resonant system with standing waves on the conductors,
the current, resistance, and bandwidth we measure at one point has
precious little to do with the field strength.
If you want to know the FS change, measure it. It takes less time and
equipment than doing it wrong or guessing.
73, Tom W8JI
w8ji.tom@MCIONE.com
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