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852
Junior Member
Username: 852
Post Number: 23
Registered: 3-2006
| | Posted on Thursday, March 09, 2006 - 3:36 pm: | 
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Not knowing alot, about alot of things. Can someone explain to me in "simple" terms how this works? To the best of my reading knowledge the first wavelength for CB is 36 foot from the ground. They say this is where you should try to get the feedpoint of the Antrons, and such for max. performance. So how is it they say the Top One Astroplanes perform so well with the bottom hoop just being 10 foot above ground? "Keep it simple" |
Marconi
Advanced Member
Username: Marconi
Post Number: 627
Registered: 11-2001
| | Posted on Friday, March 10, 2006 - 5:56 am: |
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852 in a real sense I don't believe that there is a particular height that is better than all others. With the antennas that use the coax or the mast as a countepoise (RF ground), to the extend they do, maybe it is important in those setups to have this length or height at some reasonable lenght and maybe one lenght (height) may be better than another to some station out there far away. With antennas that have raised ground planes (GP) attached and raised up with the antenna, it may be less important how high the antenna is if the GP is doing a good job and has any current flowing in it.
I have found using observations with several antennas up and on a switch for instant switching, that two similar antennas at about the same height and far enough apart as to not affect each other, can react differently from one station to another and this is sometimes not explainable. You have to consider angles if we consider a signal as being a straight line from one point to another and angles are caused by reflections in signals by stuff on the ground that lays between certain stations and that does not for others. Atomspheric conditions also have an affect, and noise is a big one affecting antennas.
There is just really too much to consider to answer very directly what height has to do with performance. But, height has an advantage in getting over stuff and for sure the earth curves so height has an advantage to extending the visable horizon. On the ocean you can probably see the horizon at a distance of about 11 miles. So it you get above the horizon that distance is extended considerably.
I have found that on raising an A99 by incriments of about one foot or so, I can see slight differences in true resonace for the antenna. As I went up my center frequency went down a bit in true resonace. I do not believe you can see this without an analyzer and I'm not even real sure that what I saw the day I tested was what I thought I saw. At any case the drop in frequency was not much, and since the antenna is so broadbanded it likely made no difference except to observe that at about 24' I noticed this change in frequency stopped dropping. I can only assume that at about 24', in my case, the earth stopped having its affect on my A99.
Main thing is get the antenna high enough to get out and receive without possible noise, if possible. And, most important, only high enough so as not to worry much about it falling due to lack of control.
About the top one. I don't believe that antenna is hampered by raising it higher. I do believe that the AP may well work better than another at some low height, however and that is due to the design. I sense it has less of an affect of reacting to the earth as badly as maybe my A99 so it may well perform better at say 10' high than my A99 would. When an antenna is reacting to the earth you can have heavy losses in signal. Maybe the A99 needs that 24' of feed line and/or mast below it to decouple from the lossy earth it sits on. And maybe the Astro Plane does not. Also I feel the Astro Plane has about 40% of its radiating element in the horizontal plane and I think this has some considerable affect on this antenna's responses to vertical and horizontal signals, with the plus toward the horizontal. For local traffic this feature may make the antenna a bit less responsive to local vertical signals, but for DX signals this may well prove to be a very good positive. Reports seen to prove this is likely. Plus it is said that horizontal elements are less affected by the earth than are vertical elements and that maybe the case with the AP.
That is the way I see it. |
Tech833
Moderator
Username: Tech833
Post Number: 1267
Registered: 8-2002
| | Posted on Friday, March 10, 2006 - 10:43 am: |
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Well.....
The quick answer to the question about two antenna reacting differently to different stations is simple- Look at your signal meter as you copy a mobile driving around. Notice how it bounces up and down all the time. This is because of variations including, but not limited to- Reflections, Phase shifts from surrounding buildings, posts, etc., Ground conductivity differences, the list goes on.
Now, imagine that you happen to choose to mount your base in a spot where if a mobile were parked there, your signal would be lower to a certain station that it would be if you moved it another 20 feet some direction. You can see how one Earth mounted station can have a drastically different signal than an equal Earth mounted station just a few feet away from the first.
Next, on the Astro Plane (Top One) antenna... It is much less effected by ground height because it is 'upside down', so it doesn't see any ground under it at all. Simplified- Antennas don't look up, they only see down towards their feedpoint. Since the Top One feedpoint is on top, it can't see and ground under it. |
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