[600MRG] ground

[Rudy Severns]

It’s not surprising your antenna current wanders around as the seasons
change.  My antenna has 128, 150’ radials and a top hat 240’ in diameter but
I still see significant changes in tuning even during the winter when the
soil characteristics wander around only a factor of two or so.   From winter
to summer the detuning is much greater and I can watch it drift higher in
frequency after the rains end and, as the weeks pass, the soil dries out.
The reason for this sensitivity is that the short heavily loaded verticals
we use at MF have much more intense E-fields than the ¼-wave verticals
typical at HF.  In a ¼-wave vertical the E-field component is small, the
soil losses are driven by the H-field which the radial fan very effectively
shields.  While a radial system is effective for the H-field component it’s
much less so for the E-field component which contributes to making the
antenna very sensitive to changes in soil characteristics.  This is why in
commercial/military short MF/LF antennas it not uncommon to use 500 radials
which stretch out some distance beyond the top-loading hat.  As part of my
experiment I plan to go to 256 radials this summer after I’ve documented
this year’s behavior. 

 

I’m currently conducting regular measurements of my soil characteristics
from 100 kHz to 10 MHz.  I’ve set up a test spot in the field behind the
shack.  I leave the ground screen, the probe and a pre-calibrated cable to
the shack in place.  All I have to do is measure the impedance with a
network analyzer at shack end of the cable and pop the measurement into an
EXCEL spreadsheet to generate pretty graphs.  After the end of a dry spell a
couple of weeks ago I made a measurement.  Then we had a couple inches of
rain spread over a few days.  The conductivity and permittivity jumped up
almost a factor of two.  We’re currently having another soaking and when it
ends in about three days I’ll measure my soil again to see if the
characteristics have stabilized yet.  This time of year even after a couple
of weeks without rain (rare!) the soil is still very moist.  I had no
problem pushing the measurement probe a couple of feet into the ground.

 

During the summer we have a long dry season and the soil becomes like
concrete.  I have to use a long drill to make a hole for the measurement
probe and my conductivity is down by a factor of ten!  Your variations in
antenna current may very well reflect a large change in the ground system
impedance.  But!  You may also be experiencing detuning of the antenna due
to both the conductivity and the permittivity changing out beyond the ground
system.  

 

Lots of fun to had learning about MF.  73, Rudy