----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
Subject: Re: [TowerTalk] antenna impedence and ground impedence??
>
>
> Some years ago, Jack Belrose, VE2CV suggested that one of the better ways
> to approximate ground characteristics at a given frequency would be to
> erect a precisely measured low dipole just high enough above ground to
> accommodate the limitations of your modeling software, and then tweak the
> ground characteristics in the model till the resonant frequency of the
real
> dipole and that of its model match. This sounds like an approach that
> would largely overcome both the inhomogeneity problem and the frequency
> problem. Critique?
>
> 73, Pete N4ZR
This was the approach that George Hagn started with. He was looking at
propagation and antenna designs in SouthEast Asia, especially for field
expedient antennas (i.e. no multi element beams on tall towers). It would
work OK if you happen to be in a treeless field, and you have enough room to
erect a suitable test antenna. For lower frequencies (say HF, as opposed to
VHF) the dipole gets pretty big, and things like the supports and feedline
start to perturb the results as much as the soil.
The eventual technique that Hagn came upon was to use a open-wire
transmission line (for which there are also precise analytical numbers).
This gives you "point" measurements, of course. His transmission line
consists of two parallel rods driven into the soil. You need to do several
different length rods at each point.
Another approach is to lay the openwire line on the surface (the nice
analytical solutions are for conductors half immersed in the soil, but
there's some analysis to show that the difference isn't huge).
As you go lower in frequency, other techniques start to be more useful
(notably, using coils and magnetic fields).
What you're really interested in, RF measurement wise, is the skin depth.
Not much point in measuring the EM properties of the soil more than a few
skin depths deep. Fortunately, skin depth is probably on the order of a
meter or so in most places (because of the lossiness).
By the way, this whole RF impedance of the soil thing is why a lot of those
ground radial analyses leave something to be desired. They tend to rely on
DC analysis, and ignore things like capacitive coupling from wire to ground,
etc.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|