"If NEC2 is wrong, so is NEC4." - I don't think so since a different
ground modeling approach is available in NEC4.2. Running EZNEC Pro4
antennas both in 2 and 4.2 sometimes shows significant differences.
My .02. Exactly where is ground? It sure isn't the flat smooth plane
in NEC. And where does "real" ground start considering the huge variety
of stuff going on in the first few inches from 100% air to solid dirt?
I used 4.2 to model the inductive loaded BOG idea and found very small
(0.2") differences in BOG height (0.2 to 2") made large differences in
pattern and gain, even reversing the pattern. Conclusion - not
practical, even though the "best" model was neat.
Given the posts re NEC doesn't work at small above ground distances: the
N6LF data in his "lost" BOG performance article and some modeling I did
at small distances above and below ground convinced me that 4.2 is very
good at the small distances. Of course the conductor can't be in the
ground plane. The mathematics may blow up then but mother nature is
continuous.
Then I was surprised by the difference a mild freeze made in my DC
ground field resistance (the freeze was just a few inches). The change
had to be in the earth resistance of the rod interconnections (bare #2 &
#6), not the 8 & 10ft long rods. Ice has much lower conductivity than
water so even a frost, freezing the grass, might have a noticeable
effect on a BOG.
As the N6LF work shows and perhaps this is another example of "you get
what you pay for" - ie BOGs are small, easy, and useful, but much more
sensitive to the environment than elevated Beverages.
Bing found this cool essentially "Engineer's Handbook for AM Antennas".
http://crawfordbroadcasting.com/Eng_Files/AM%20Antenna%20Systems.pdf
Included is this:
"14.2 Antenna Resistance
Periodic checks should be made of the resistance at the common point or
non-directional antenna base. The frequency of this resistance check
will vary from station to station, and will probably be determined by
the nature of the environment around the antenna. Areas with poor ground
conductivity or sites with poor or deteriorated ground systems may see a
considerable shift in base or common point resistance with changes in
the amount of water in the soil. Similar (and sometimes more dramatic)
changes can occur when the ground freezes."
Grant KZ1W
On 3/24/2020 19:46, 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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