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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