KZ1W said: "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.
I didn't mean to imply there was no difference between NEC2 and NEC4 for
all situations. I only examined a couple of particular BOG situations
where I was able to get the same answer in NEC2 as someone else got in
NEC4. In those cases either both answers are correct or they are both
wrong.
Jerry, K4SAV
On 3/25/2020 11:17 AM, Grant Saviers wrote:
"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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