Re: [TowerTalk] antenna impedence and ground impedence??

[Jim Smith <jimsmith@shaw.ca>]

Hope this doesn't cause too much laughter but I keep thinking of that 
graph in the Ant Handbook which shows dipole Z vs height.  Could you put 
up a modelled dipole at a known height, measure its Zin and then fiddle 
the model ground characteristics to make the modelled Z the same?

There is the problem, of course, that the ground characteristics have 2 
variables so there will be no unique solution.  Maybe put the antenna at 
2 different heights or try it at 2 different frequencies? 

Of course the antenna would have to be "in the clear" etc.

73 de Jim Smith   VE7FO

Jim Lux wrote:

> ----- 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
>
>  

_______________________________________________

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