[Lowfer] re: GROUND RESISTANCE

[Alan Melia]

Hi Rick ground resistance is frequency sensitive so needs to be measured at
the operating frequency at LF, using either a commercial RF bridge or you
can construct a simple but effective unit....see
http://www.alan.melia.btinternet.co.uk/aelossbr.htm
There are also several articles on aerials for 136kHz that we used the
bridge to measure.

Note that "the resistance in the ground" is not the only thing you have to
consider. Current is driven through this loss resistance by the capacity of
the antenna to ground. It is found that increasing the antenna to ground
capacity (but only over "open" ground....not over lossy trees or buildings)
reduces the ground loss in the antenna circuit. However this capacity needs
to be from the "end" of the radiating portion, because increasing the
capacity of the vertical to ground shunts current away from the radiation
resistance and reduces the radiated power. Hence the capacity hats and
multiple top-load wires.

With rocky mountainous ground you have a problem increasing the
conductivity. This is where loops have been found useful. However the Loop
Maestro, Bill Ashlock, did some very interesting experiments (reported here
and should be in the archive) with verticals at his "winter retreat" in the
mountains and found that he achieved substantial improvements in vertical
antenna efficiency by using an elevated ground-plane. From the work on loops
it would seem that the ground-plane needs to be at least 3foot above the
terrain so that the currents carried in in the ground-plane wires are not
coupled into the lossy ground below. Access usually dictates that they must
be above head height, or in Laurence's case in Alaska above Moose antler
height!! :-))

There is no "one-solution-fits-all" to this problem, all sites have their
individual problems, you need to measure and try improvements, and
re-measure......you will then quickly find when extra ground rods give
little extra improvement.

Good luck
Alan G3NYK