[600MRG] Ground variations

[Rudy Severns]

It’s very interesting that Larry sees no effect of weather on his 630m
antenna tuning and I think his observation is important.  The effect on my
antenna is quite dramatic and I think very real.  My measurements are made
with good instrumentation and hopefully, careful technique.  In my case the
source of the variation in antenna tuning is clearly due to the 10:1
variation in conductivity and similar large variations in permittivity as
the seasons change during the year.  As I mentioned in my note, even over
the space of a few days I see 2:1 variations in soil conductivity.

 

>From Larry’s comments it would appear that this effect can be very site/soil
specific.  To increase our data base and improve our knowledge of this
effect I suggest Larry take soil measurements at his QTH.  I described my
approach in detail in my QEX article on ground characteristic measurements.
The article is posted on my web page.  It’s actually very easy, all you need
is a rod probe and a piece of hardware netting 2’-3’ square.  All you have
to do is measure the capacitance of screen-probe combination in air, then
lay the screen on the ground with a hole in the middle, insert the probe
into the soil through the hole and measure the impedance between the screen
and the probe.   I believe Larry has a VNA which makes the measurement duck
soup.  The conversion of the impedance measurement to conductivity and
permittivity uses simple algebra.  

 

I should add that my 160m vertical does not show significant variation in
tuning (only a few kHz) but then it has a large radial field, 64 150’
radials.  This isolates the antenna from ground effects.  In addition, the
antenna is very nearly ¼-wave, electrically much taller than my 630m
vertical.  As I explained in my earlier note, one of the reasons for the
increased sensitivity to soil characteristics at MF is due to the use of
short top-loaded verticals with their greatly increased E-fields.   The lack
of significant effect at HF with the usual ¼-wave verticals and ground
systems is no surprise. 

 

73, Rudy