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|Garry Nichols||Posted - 19 October 2012 9:58 |
Now Boys . . . .
I don't think you have stated "how" Zim's center fed dipole antenna will be effected, only "that" it will be effected. Maybe you can list the reasons?
I have a feeling that Zim is centering on pattern factors and John is centering on resonance and efficiency factors.
Which would make both of you correct!
Chair-throwing is not allowed on this forum. Only in the psycho-acoustics lab that I used to work in . . . .
|gzimmer||Posted - 19 October 2012 10:14 |
> I have a feeling that Zim is centering on pattern factors and John is centering on resonance and efficiency factors.
Well, I'm just trying to describe how a simple dipole operates when fed with tuned feeders.
I've no idea what John is trying to say. He's made a long list of wild claims, each of which evaporate when I contest them.
Time for a long break from rap 'n tap I think.
Edited by - gzimmer on 10/19/2012 10:15:13 AM
|Garry Nichols||Posted - 19 October 2012 11:36 |
I don't see anything amiss in what you've said about the center fed dipole using a tuned feeder. It fits my amateur radio understanding of how this would work.
Let's wait to see if John clarifies his comments. I could not follow his comments from what was typed out. There is probably something going on "between the lines" that needs to be brought to light.
|_J_||Posted - 19 October 2012 14:46 |
I am trying to understand too, I think Zim is doing something I missed.
I don't know quantitatively how the feedline affects the antenna, I just think the feedline affects the dipole specifically by tuning it. The dipole will need tuning because it is short and close to ground, so it will not be close to the feedline's Z. Then the feedline will further transform that Z to something else, frequency dependent that will need to be tuned out by the antenna tuner.
Guess I am not clear on the motivation for the dipole, being so close to ground, it looses a lot of its normal pattern. Using it as a T seems like a good idea though.
Is any of this clear?
|Garry Nichols||Posted - 19 October 2012 15:4 |
Hey, John is on the same side as us Zim!
Despite the low height, Zim says that more distant stations come in well and locals are, of course, down.
A balanced approach to life.
|_J_||Posted - 19 October 2012 15:10 |
OK. Do you think that has a theoretical foundation, experimental results has lots of uncertainty for antennas. Since skip arrives at a low angle and rotates to vertical a lot, that would seem to be hard to differentiate from locals.
Edited by - _J_ on 10/19/2012 3:22:44 PM
|Garry Nichols||Posted - 19 October 2012 18:34 |
As Ronald Reagan used to say, "There you go again . . . . ".
Until you mentioned it, I had never heard of low angle horizontally polarized waves "rotating" to vertical polarization. But, I only read amateur literature, and don't get into theory. Perhaps you can expand on this point.
I do know that a low horizontal antenna accepts vertical polarization off the ends more readily than horizontal or vertical polarization broadside. I have a model of an 80M dipole at 6 feet above average ground and it shows this. It was prepared for me by Jack Belrose, VE3CV, an antenna engineer with the Canadian government and who has done antenna work for the military. (This was in response to a letter I had sent him about a QST article.)
And I've looked over a lot of the practical literature and modeling put out by Eyring, Inc on their ELPA-302 on-the-ground antenna. This usually takes the form of a center fed with quite long legs. Vertical polarization off the ends. But with fairly high losses due to being on the ground. A military tactical antenna.
That just about dumps all my knowledge out on the table.
I think Zim had said (or implied) that the sigs he was receiving were likely fairly high angle skywave. Does that change the picture for you at all on his antenna? You seem to be mostly commenting on low angle horizontally polarized waves.
I deduced from Zim's comments that he was likely receiving distant stations via multi-hop high angle waves. (At MW can we have multi-hop?)
Maybe Zim can comment on which direction wrt the dipole the signals he is hearing are coming from.
|homebrew||Posted - 19 October 2012 20:0 |
I would think that multi hop signals at BCB frequencies would be attenuated way too much for crystal set reception under normal circumstances over land.
|_J_||Posted - 19 October 2012 21:0 |
Guess I missed the point about 'where I go again' -whatever that meant? And I don't think Zim intends to throw anything at me, or do anything, and I am certainly not throwing anything. So that went right over my head too??
But I think so, high angle skywaves could be favored, especially if they hit a lobe of the antenna. Those lobes and nulls will depend on the dipole height, frequency and dirt conductivity.
But high angle bounce is kinda rare, especially at MW.
Lot of strange things happen to MW waves near the earth. Waves rotate to perpendicular to a conductive boundary and become perpendicular polarized because they can't propagate near and parallel to it, the conductive boundary sort of "shorts out" the parallel component. (You can't maintain a voltage field parallel to and adjacent to a conductor. You have to visualize this, It will end up being one or more fields perpendicular to the conductor.) The wave's energy gets divided between heating the boundary and reconstituting as a wave that travels along the boundary.
Wave polarization also gets tumbled when they get refracted (bent and bounced) by the ionosphere.
HB: I think that is right.
|gzimmer||Posted - 19 October 2012 21:43 |
John has been insisting that horizontal dipoles don't work at MW.
I tried to point out that while a low dipole will will be fairly inefficient, and will have high angle lobes, it can work simply because there are plenty of strong high angle signals present in the evening.
The rest of John's claims: that a low dipole can't be matched, that the line had too much capacitance, that my feed-line isn't actually working as a feed-line, that a short feed-line will prevent the dipole working, that a feed-line will badly affect the dipole, that a mismatch on open-lines is bad, etc, are all completely incomprehensible to me.
I think that John is so desperately committed to the idea that "horizontal dipoles don't work on MW", that he is blind to the simple concept of a horizontal dipole with tuned feeders. But I'm only guessing because he hasn't yet explained exactly what it is he is trying to say.
The whole point of this experiment was to try and find a simple antenna which could give some physical selectivity between stations when switched between different configurations. At the very least it would be interesting to switch between polarisations and observe the result.
|gzimmer||Posted - 19 October 2012 22:27 |
Here's a good article showing radiation patterns for horizontal dipoles at various heights.
|_J_||Posted - 20 October 2012 1:4 |
Im not thinking they 'dont work' -I just think there are a lot of things that make them not work much like a purist dipole. You seem to agree with me on most of them, at least when I can communicate them.
|golfguru||Posted - 20 October 2012 1:16 |
Edited by - golfguru on 10/20/2012 1:17:24 AM
|gzimmer||Posted - 20 October 2012 1:36 |
Edited by - gzimmer on 10/20/2012 4:10:39 AM
|Garry Nichols||Posted - 20 October 2012 9:41 |
"There you go again . . . . " was a famous line from the Reagan presidential debates. I guess it came to mind in light of all the commentary on the present debates and sound bite re-runs of previous debates. I was referring to your re-mentioning the rotation to vertical polarization of low angle horizontally polarized waves. I've always found this very curious because I had only heard about it from you. The usual comment on this is that low angle horizontally polarized waves cancel themselves out due to phase differences between ground reflected and direct waves arriving at the antenna.
I thought that the discussion was becoming a bit "contesty", so thought I'd throw in some humor and divert attention a bit with my having thrown a chair out into the hallway when I worked as a tech in a psycho-acoustics lab many years ago. I always assume (perhaps incorrectly) that I can get away with idiosyncratic comments as long as they are sufficiently obtuse as to be humorous.
Everything that I have seen on dipoles close to the ground and shorter than half wave indicates that they have no lobing. Zim's interesting link also shows this for the dipole at 0.1 wave height. The pattern in 3D is similar to the cap of an ordinary white mushroom, although with a bit more vertical dimension.
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