[600MRG] Ground variations

[Larry]

When Rudy's referenced QEX article was first published, I read it with 
great interest. It was, and still is, the only method of evaluating 
ground characteristics that made sense to me (that I'm aware of).

I never actually tried it for a number of reasons, but the biggest was 
that I always installed a very large radial system that reduced the 
ground related losses to the bare minimum possible. Unless you are doing 
research or evaluating a potential new QTH, it seemed like a lot of 
effort for no reasonable return.

Now, with 630 meter antennas and their practical limitations, it may be 
time to re-visit this.

The problem I have here is that the type of dry-land grass I have under 
the vertical is not compatible with the measurement system. I would have 
to move out of the grassed in area. My "dirt" is the inverse of 
homogenous, and I'm not sure that anything less than a couple dozen 
measurement sites would provide useful data. I will need to re-read the 
article and see what, if anything, I can do.

Larry - W7IUV / WH2XGP


On 07-Feb-15 10:39 AM, Rudy Severns wrote:
> 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
>
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