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Groundplane Antennas at Various Heights

B

BOOTY MONSTER

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i was crusing the WWW and found this page . it discussing the pattern of an omni at various . make note that this is at 14.1 MHZ and CB is centered aroung 27.201 MHz . i know antennas vary in pattern and performance when the frequency is changed , but i dont know how much of this applies to 11 meters . just thought it would get some interesting comments and could be a fun topic for some folks .

Groundplane Antennas at Various Heights

this quote kinda busted my bubble because 1/2-3/4 wavelength is the easiest and most economical height for me :(

"Conclusion:

Do not mount groundplane antennas at heights between 0.25 and 1.25 wavelength. At those levels above ground most of the energy will be radiated at angles of 27° to 45° into the ionosphere. This phenomenon seems to be independent to the number of radials or other counterpoises. Further simulations indicates that this is true for all other variants of vertical antenna systems too. "


your thoughts ????
 

BM, this is a very interesting article and the evidence presented may be right on the money in the purest sense, but in the real world where we have all kinds of stuff on the surface of the earth and other interfering radiation around our antennas and in the far field as well---such conclusions as noted in the article cannot IMO be considered as categorical regarding antenna success.


You may have noticed that the author's basic premise in observations were probably true, but he also moved his antenna a considerable distance when evaluating his comparisons. He also talked about the earths conductivity in his modeling and used average ground, but in his real world experience his results were developed from two totally different locations.


Ask yourself, what if his trip took him to an area of very good soil based on the ms/m figures he modeled, and while he was at home the soil conditions were over extremely poor soil? The modeling will not analyze those differences and therefore consideration must be made by the reader for those differences. Even if everything in the article is absolute truth, this should give you pause to reconsider.

BTW, on the last topic above, IMO there is a difference in performance between different antennas when over different soil conditions. The article suggest that all antennas respond the same to height changes, but what if the location changes. In the real world, you can't have your analysis both ways---and I'm surprised he made that statement in light of his moving his antenna.

Now, with my own experience I find just like is often claimed--- higher is better and that may always be true with local ground waves and direct line of sight signals not-with-standing interfering stuf on the ground.

Recently I got back to comparing my antennas since hurricane Ike blew through here. I mounted an A99 on a 10' mast and leaned it up against my house. My eves are 8'6" high and the roof peak is 17'8" high. Just like earlier observations I had at least two issues concerning height and stuff that interferes with my antenna signals. With my A99 feed point at about 10' high and leaning up close to my roof the antenna worked, but it was noticeably attenuated to signals on the opposite side of my roof and in fact was perceived to even be a little directional. I raised it up to 15' and I saw noticeable improvement, but IMO it was not due to the increase in height---the antenna was reacting negatively to being too close to my roof---wood and tar based shingles. It was not due to wiring, duct work, or plumbing in the attic either, because there were none of such items in that area or even nearby. I move the antenna at 10' high out away from the house and the improvement was very similar to the antenna at 15', but I still sensed some attenuation, so I raised the antenna to 25' and boom all of the radio signals to the north were noticeably better. I figured this was due to the increase in height, but the "shadow affect of my roof being higher than the base of my antenna," as mentioned in a patent on the Avanti AstroPlane antenna or in the antenna manual, was the cause of the problem I was seeing and height only had a minor effect in that case.

I did eventually get my AstroPlane up and it too reacted just like the A99 had done when put it through similar paces. The surprise for me was that the AP did not resolve the "shadow affect." The way I read the info was that the AP would solve the problem, but maybe I put the antenna to a unfair level of consideration, because the Avanti images describing the "shadow affect" show the base of both antennas well above the roof peak of the house where the antennas were located.

I say all of this, because I just got through going over many of the observations that likely relate to the height questions in your thread. For me the undesirable range limit of zero - 1.5 wavelength in the article probably covers just about the limits of most CB operators working range for installs, so what are we to do?


In my area, I have 60'> trees everywhere and I'm still able to work most of what I would like with little tale-tale signs of attenuation. Would 100' be better? Of course that would put the base of any antenna I put up well over all the trees and that would be better. I've have had antennas up there several times over the years and that usually did make a difference, and when it didn't I figured it may have been because going higher changed the tune enough and has some attenuating affects. I saw this happen with an A99 that had a good tune at about 30' feet, but the tune went south on raising to over 60' and that surprised me.
 
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that end fed vertical article freecell posted is awesome!

i loved it.

the part about feedline radiation in relation to 1/4 wave groundplane antennas showed exactly why avanti chose to put the coax inside a metal sleeve.

no, it doesnt fully accomplish what its intended to do, but avanti saw the need and at least tried to do SOMETHING about it. LOL

so, the question becomes; how many radials does it take to equal the effect of adding a choke balun at the feedpoint?

would adding another 4 horizontal radials 45* off from the original 4, on an antenna such as the maco V-5/8 eliminate the need for a choke balun?

we are assuming that the antenna is isolated from the support structure.

if not, how many would it take?
is 102" long enough?

would it have to be a complete horizontal disc at the feedpoint with a radius of 102"?

folks, we are getting at something here that seems to be all but unheard of in the CB circles.

most CB'ers run verticals these days, and many have problems with TVI and such. (lets not get into all the other reasons TVI exists LOL)

just imagine how many CB'ers are going to "prove" me wrong when i mention these ideas to them. LOL

again, a great article.
LC
 
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"how many radials does it take to equal the effect of adding a choke balun at the feedpoint?"

keep reading because as long as the Unbalanced Feedline IS NOT ISOLATED from the Balanced Antenna it doesn't matter how many radials are added or how long they are.
 
Something to remember about radiation patterns and 'take off angles'. Antennas are not limited to just one small clump of degrees of radiation. Those angles range from whatever the lowest one is to almost straight up. I don't think there's any way to limit those 'TOA's to only what you think are most usable. You get all of them, like it or not, and that's good. There's no one 'TOA' that's always going to be best, it will vary with propagation and where the one you want to talk to is located.
Antenna height and 'groundplanes'. Those radials radiate too, you know, so the space under a 'groundplane' is not radiation free by any means. So it amounts to as much height as you can get within reason. There are several other things to consider in that regard. An antenna at 1/2 mile above ground would be nice, but I think the feed line would be sort of a limiting factor, don't you? 'Course, you could always mount the transmitter just under the antenna and remote the whole mess.
- 'Doc
 
ive always wanted to use some big helium filled balloons (not baluns! LOL) to raise a long wire way up into the air.

i wonder how much power i would have to run through it to pop the balloons.
LC
 
ive always wanted to use some big helium filled balloons (not baluns! LOL) to raise a long wire way up into the air.

i wonder how much power i would have to run through it to pop the balloons.
LC

Probably going to depend a lot on what kind of balloon you use. A standard latex/rubber balloon shouldn't be affected at all. Now mylar balloons, on the other hand, being metallic and therefore conductive, might be another story altogether. Let us know.
 

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