[TowerTalk] antenna impedence and ground impedence??

[Jim Smith]

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 at 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.
>
>
>_______________________________________________
>
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