W4EF said: Reading that QRZ.com thread you linked to and reflecting on
K4SAV's measurements and how much they appeared to vary over a fairly
short period of time, got me wondering about what could be responsible.
You are not the only one wondering about that Mike. Those are some
pretty weird measurements. I have no explanation. That's the reason I
continued to take measurements over a one month period and continued to
get those big variations from one measurement to the next.
It's also the reason I am repeating the measurements again now, although
I moved the dipole about 50 feet away from where it was before. The
readings now seem to be much more reasonable and not so much variation
between each measurement. I see the resonant point moving around with
ground moisture content and the whole curve shifting as well but no big
changes like the first time I did this. I may try to move the dipole
back where it was before to see if any of the weirdness returns.
I still can't fit any of the measured curves to a NEC2 analysis by
varying ground parameters and height above ground in NEC2. Note that
attempting to do this also violates the NEC2 guidelines for wire heights
above ground. So why should I expect to be able to do this? I have
duplicated NEC4 patterns for a BOG in NEC2 to an insignificant
difference. If NEC2 is wrong, so is NEC4. That doesn't leave any tools
for simulating BOGs.
I have previously done other simulations for a BOG in NEC2 and the
results never agree with measurements. I keep looking for model patches
that may allow some useful information to be obtained from simulations.
Jerry, K4SAV
On 3/24/2020 9:46 PM, Michael Tope wrote:
Dan,
Reading that QRZ.com thread you linked to and reflecting on K4SAV's
measurements and how much they appeared to vary over a fairly short
period of time, got me wondering about what could be responsible. I am
fairly certain that at least part of the radial system for my 160
meter vertical is over the drain field of the septic system for my
cabin. Who knows, maybe a properly timed toilet flush could make the
difference between working a new one and coming up empty :-)
Okay, the toilet aided pileup busting is a stretch, but seriously, I
am not sure if I can think of any better explanations than septic
discharge for why ground characteristics could change that quickly. I
do know in some cases there can be water table very close to the
surface, but I don't know how much that typically varies over the
short term. My only other thought is that maybe in K4SAV's case the
lay of the grass under the wire is changing slightly over time and the
impedance is super-sensitive to the gap between the antenna and the
ground (although I think K4SAV said that his grass was all dead).
73, Mike W4EF..........
On 3/23/2020 10:18 PM, Dan Maguire via Topband wrote:
Grant wrote:
If you really want to know the parameters, see antennasbyn6lf.com
as Rudy describes techniques for ground RF properties measuring.
Turns out that very subject was being kicked around on a recent
qrz.com thread:
https://forums.qrz.com/index.php?threads/indirect-measurement-of-ground-constants-with-a-dipole.696955/
Dan, AC6LA
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