You know, Marconi, all that 'stuff' around an antenna doesn't necessarily mean other antennas.
- 'Doc
Well guys, 'Doc may wish for me to model the entire Harris County Metroplex in order to cover all contingencies for what happens to antennas with stuff nearby, but that is not going to happen. Just modeling these three antennas in an antenna farm at 36', 72', and 108' feet apart...is quite revealing. I would try and demonstrate this, as usual, with more PDF files of my model, but for two issues.
1. 'Doc is absolutely right about antennas being affected by stuff on the Earth. In my model here, it is evident even out to 108' feet with just two antennas side by side. I removed the A99 from the main model for this part of the project.
With the GM as the source antenna, the SD'r at any distance in the range from 36' - 108' feet...the GM pattern was affected just like 'Doc says. There might not be as much of an affect, in all aspects as 'Doc and other's suspect, but it did surprised me none the less. I was thinking that maybe 36' feet apart might be enough to kill most interaction. There are many possible variables even with only two antennas in the model, much less three.
2. I would post the models and try and explain what I think, but I'm not sure yet that I understand the resulting directional relationship of the Antenna View to the pattern view in the 3-dimensional display in free space. The free space models may be the only way to look at 'Doc's idea. It would be nice to be able to do this over real Earth, but I think Eznec only allows us to do the 3 dimensional view with the source model in free space, with no losses, and no connection to the Earth. In these real world Eznec models, I have all antenna masts attached to the earth. I haven't figured out the resulting directional factors for the free space models yet, and that difference that 'Doc mentions, is a skewing affect on the pattern. That affect allows the model to sill be omni-directional, but with modest gain differences at different compass headings.
All I can see here is that when two antennas are mounted close together, the affects are so complicated that my lack of understanding and trying to explain...would be almost futile.
Again, remember that I asked 'Doc how much difference stuff around an antenna really made. Again, I don't see that much change in the gain or the maximum angle of radiation...for these antennas mounted alone vs. the antenna farm, but there is certainly skewing to be noted in the free space patterns if I were to go there. And, when I remove the passive antennas from the antenna view, the skewed free space pattern returns to fully omni-directional...just as we might expect.
I did
not model these in free space, but I did go there to get Average Gain values for the models, and check if I could see more of a difference in the patterns. I think I did see more of a pattern difference using free space models. The Eznec real Earth models here hardly changed at all. So, maybe 'Doc was referring to free space affects, as making the big difference. Most "professional" RF guys tend to talk in terms of antenna efficiency and free space ideas...when they write or talk. For me, that must be considered in your understanding for whatever you read or hear said.
I conclude that my models, over real Earth, are not that far off from what I might be seeing here...with real antennas at my location. As a note, I also did not mess with changing the Earth conditions for these models, and modeled over Average Earth. I also tried to keep all wire segment lengths at near 1' foot.
Regarding my descriptors for my model entitled:
"Gain Master' SD A99 ISO 19.5'.ez". The (') following (Gain Master') simply means the GM is the source antenna for this model. If there is an "ISO" noted after the antenna's descriptions, it means the antenna has a 6" isolator at the antenna end of the mast...with the bottom end attached at the Earth. The (') was likewise used for the SD'r and the A99 when they were made active with a power source for this idea. As a note: in the Antenna View, the source antenna has the red circle at the feed point and is the radiating antenna.
One of the most remarkable things I noticed here, was when I isolated (ISO) the A99 from its mast. It is obvious in the three attached Antenna Views that the current flow, noted in red, originates from the GM. However, I ask that you note the considerably increased passive current flows on the A99 that is isolated vs. those on the A99 that is not isolated. I conclude that this quirk does not seem to disagree with what science might predicts, and that antenna construction and passive resonance on conductive elements...is also of much importance for 'Doc's issue.
Here is an example of what I saw the A99 doing:
View attachment 'Doc's antenna farm ideas.pdf
It was my thought that this happened due to the isolation of the 1/2 wave portion for the A99, and thus provided for the 1/2 wave to just be sitting out there in free space, so-to-speak. Conversely, when the mast was not isolated, the A99 looked like a wavelength or longer element with the mast attached. Thus it did not respond as well to the RF radiating from the GM...kind of like a proper reflector should do on a yagi multi-element beam. I was surprised to see Eznec indicate this, but again I think this is probably what science would predict. However, this was all very interesting to see happen in the model...even if the models are not right-on-the-mark.
I might have some more to say about this idea, but I don't see how I could really explain all the issues these models present. IMO, you guys would probably need to understand something about modeling, and then be able to manipulate the files yourself and see what is happening.
Else I'd would post some more of these Eznec examples showing how similar the patterns are and with the exception noted above...just how little difference there is at one wavelength in spacing between antennas.
