[Mldxcc] Dipole VSWR

[Richard (Rick) Karlquist]

FWIW, I have been working with N6LF on making ground resistance
measurements.  His work is on 80 meters, but it could be adapted
to 20/15/10 meters.  What is involved (on 80 meters) is to build
a dipole of a particular length (around 120 feet) at a height
of 4 feet above ground, using standard electric fence wire and
supports.  The resonant frequency and driving point resistance
at resonance are measured.  A lookup table gives the NEC4
ground parameters.  Then NEC4 can be used to model a dipole
at an arbitrary height.  The NEC4 ground parameters can be
studied in terms of changing over time.  It is important to
understand that NEC4 models the ground as a homogenous half
space, and doesn't have an provisions for modeling a water
table.  From what you have described, there could be a dropping
water table, but there could just as well be a homogenous
ground that is simply changing over time.

I am in the process of setting up NEC4 on my computer so that
I can implement the N6LF ground resistance procedure.  I have
the latest version of NEC4 with enhanced ground modeling.  If
you can interested in measuring your ground as described above
by building a low dipole and logging data periodically, I can
provide NEC4 support.  We could also model your dipole with
the measured ground constants and see if NEC4 is correlated with
your SWR measurements.

Rick N6RK

On 7/15/2015 3:28 PM, Emileigh Starbrook 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
>
>
>
> ------------
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> other is to disbelieve what is true."
> - Soren Kierkegaard
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