[TowerTalk] Cutting a dipole

[Jim Lux]

On 9/4/13 7:26 PM, David Gilbert wrote:
>
> That's probably because none of us have a true idea just where our
> effective "ground" is.  It's not like there is some high conductivity
> zone in the top few inches of soil, or that everyone's soil conductivity
> profile is at all similar.  I'd bet that in some low conductivity soils
> the effective ground is several meters below the surface, and I'll also
> bet that it is at least to some extent dependent upon the frequency of
> interest.  I use and depend upon EZNEC and HFTA all the time, but I
> don't pretend they are any more accurate than the data I feed them.
>

There was some work back in the 70s by George Hagn (at SRI then) to 
measure soil parameters by measuring the Z of a dipole close to the 
ground, or laying on it.  He found it doesn't work very well, which is 
why the "open wire line" (OWL) technique was developed (basically drive 
two rods forming a parallel transmission line into the soil).

You're right that very few hams know what their local soil parameters 
are, and, of course, they vary pretty dramatically over a small 
distance, depending mostly on soil moisture content, but also on the 
relative concentration of sand/loam/clay.

This is why trying to get "a few degrees" phasing on something like a 
4-square with a fixed network is well nigh impossible without a lot of 
instrumentation and tweaking.  It's also why some people swear by a four 
square and others swear at them.  A fairly small inhomogeneity in the 
soil can turn a 20 dB null into a 5 dB null.

Where NEC comes in handy is seeing the effect of changes in soil 
parameters.  If your antenna has radical changes in behavior when you 
change the conductivity or epsilon, watch out.