eddie,
heres something you may like to read, it explains common mode currents and why mr cebik told me that getting accurate results when modeling the sigma style antennas with nec would be very difficult,
it also explains why just grounding your coax as some folk do and advise on forums may not be the best way forward,
i think the article also gives a clue as to what he meant by "there is more going on in that design than is apparent to most people" and the other things he told me.
[URL]http://www.cebik.com/content/a10/trans/cmp.html[/URL]
Thanks Bob.
I haven't digested this report as yet, but with a casual reading I see Cebik discussing stuff I depend on with my own modeling. This is in spite of the discouraging words I get concerning the product's viability. I continue to proceed, thinking I'll learn something and maybe understand antennas a little better doing so, right or wrong.
Do you believe Cebik is shooting straight with us and making sense in this article "CM Current Picture Show?"
If so, then how could we explain the high gain values of 6.25 dbi - 7.68 dbi for a center fed 1/2 wave dipole in his Table 3, on page #9? Are we reading an old Solarcon report for their original A99? This is meant to be rhetorical, while calling attention to the data we see that does not appear creditable.
I can't say Cebik is wrong and that is just a single issue, but is this data accurate? Cebik may well be providing us a valuable resource for our modeling efforts, but he repeatedly cautions the reader of errors and sometimes seemingly impossible solutions that will ward-off such errors...but IMP he repeatedly fails to indicate what that error value is. Is it 2-5% error or is it meaningful like 80-100%. I would like to know what the difference means, when I hear that there is a difference in a model when adding the feedline and mast.
I think that if he talks about an error, then he no doubt knows or has an idea from the process of that discovery...how much of an error is important.
Concerning errors, we note in this article that Cebik talks about using the Average Gain function. I consider and check that to gauge all of my models and the iterations I make...when I feel the need to fine tune or tweak a particular model. With the Sigma4 model I did, I was not concerned with anything but trying to make the model dimensionally accurate, which no one else in our group had done, and I did not tweak the model, per-se and as a result of more accurate the model proved modestly good, while everyone else was contending that the Sigma4 model would not show good gain unless very thin wires were used throughout. Or, is that just my imagination?
When I do a model, I typically get no feed back from any of the guys that claimed to do Eznec modeling. It was the same for previous modeling efforts until I started asking questions. Bob, why do you think that happens? Is it because no one is interested in modeling?
As a result, I have nothing to gauge my errors except this AG function and how the antenna physically looks compared to the real thing, including accurate dimensions when available.
Just yesterday I modeled my New Top One. I think I may have discovered what Avanti meant in their discussed in the Patent regarding the lower angle of radiation it produces and their use of the words "tilt, or slight tilt." See attached:
View attachment New Top One Patterns.pdf
It is to be noted that a real free space model will not have a mast.
However, due to my statements above, I'm just not sure I'm right. At this point I think this tilt or low angle has
little to do with using a shorter or longer mast in the antenna...as was suggested by freecell sometime back. He was determined to test that idea back then, but as far as I know...he never did, at least I never heard of any such results. And now it looks like all it takes to make a fact is to repeat an unproven idea.
IMO that 4' foot below the hoop idea suggested in Avanti's manual was noted only as a minimal install dimension...relative to the Earth, large profile towers, conductive guys, or a roof when applicable. My actual testing of this response suggest to me that the antenna is far more affected than noted in the manual or Patent in this regard, and is due primarily to the heavy amperage flow in the hoop. I recently installed my Old Top One using an old 12' foot R/S Crossbow boom as the center mast that was connected to the hub, and that 4" x 8" boom to mast bracket noticeably affected the tune for the antenna. As you have noted Bob, the mast for the A/P is critical to performance, and in addition to the length and diameter, the symmetry may be of importance as well. I can't test my idea, but based on the free space model the basic A/P shows a -5* degree slant down from the horizon and I don't see that in a similar center fed 1/2 wave model.
This may be why the antenna is prone to TVI toward the Earth. However, if the antenna is installed very low to the Earth it may not be as noticable, and if mounted high that may also mitigate the problem somewhat.
I also think modeling helped convince me of what I saw when the bottom hoop was 20' feet above the ground and my I-10K was laying right below it. The tune looked terrible on my analyzer/SWR bandwidth curves. I moved the 1-10K and the BWC's where normal.
