[Mldxcc] Dipole VSWR

[JVarney]

​Of possible interest to Emily and Rick N6RK is a 1973 paper, "Integrated
Measurement of Soil Moisture by Radio Waves."  The authors did studies at
27 and 170 mHz using vertically polarized antennas to measure changes in
field strength as a function of soil moisture in the top few feet of soil.

For Emily's situation, a low dipole over ground, there is a section in the
report discussing the effect of moisture on the soil's virtual capacitance.
It's possible this explains the effect that Emily is observing: as the soil
dries out, the ground capacitance interaction changes, thus affecting the
dipole's tuning.

Another section of the report details an experiment where the authors
measured field strength with verticals over dry ground. They then heavily
and deeply watered the ground and measured again, finding a rather large
increase in signal strength. Not only that, charts in the report show that
the expected signal improvement is much larger for HF and lower
frequencies.  To raise DXCC and contest scores, turn on the sprinklers?
It's not clear from this report if a system of good radials would provide
the same benefits as watering does.

Unfortunately, the authors began a series of experiments of dry vs. wet
soil using horizontally polarized antennas but abandoned them.

http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1570&context=water_rep

​73 Jim K6OK​

p.s. -- To be clear I'm not advocating using a lot of water during the
drought for dxing and contesting


On Wed, Jul 15, 2015 at 3:28 PM, Emileigh Starbrook <estarbrook at gmail.com>
wrote:

> This is going to sound like I am crazy, but I have noticed something that
> is troubling on one hand and curious to say the least.  What I have noticed
> since spring is the antenna height where the optimum VSWR for my dipole has
> been dropping - a lot.  20m now bottoms out at 1:1.1 roughly 8 feet lower,
> 15M has dropped about 10 feet.
>
> Is this being caused by a drop in the water table?  The ground here is
> extremely dessicated.  My yard doesn't have grass - it's zeroscaped with
> succulents that are drip watered (about 1/2 gallon per week) so I'm not
> putting water on/into the ground.
>
> Emily N1DID
>
>
>
> ------------
> "There are two ways to be fooled.  One is to believe what isn't true; the
> other is to disbelieve what is true."
> - Soren Kierkegaard
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