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#21
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Vertical problem
"826" wrote in
: Hi, Sorry I didn't rely sooner. The weather hasn't been too good hear. This morning I disconnected the coax from the shack and measured the antenna with the six foot length of coax. It looked OK. I then connected just the shield of the long coax going to the shack and still measuring with the short coax it shifted just like measured before in the shack. I have then made up a choke balun and installed it at the base of the antenna and it improved everything by about 60%. I will now make another one for the shack end. I think everything is under control now. Thanks to all for heading me in the right direction. Vern M0WQR Vern, whilst the choke made a difference, the fact that there remains a difference in the feedpoint impedance with the shield connected and disconnected, even when using the choke, indicates that there is some effects observable from common mode feed line current in your scenario. If you want a measurement of the antenna as it would be used, measure it as it would be used... make sure the shield current path is as it would be used. Owen |
#22
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Vertical problem
On Sun, 27 Jul 2008 21:07:25 -0700, John Smith
wrote: Ed wrote: ... About as well as it flows up the center conductor, possibly. You don't think that just because the cable is buried that RF flow on the shield conductor is prevented, do you? Ed I sure can imagine one heck of a "capacitive load" on that outer conductor to ground! What, thousands/tens-of-thousands of pf? Regards, JS 95% plus of the transmitter or received energy is safely contained between center conductor and shield. That is how the physics works out. All of the feed line center conductor current is pretty well matched by the same current in the shield. It is the closely coupled concentric magnetic fields that make coax work so well. |
#23
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Vertical problem
JosephKK wrote:
... 95% plus of the transmitter or received energy is safely contained between center conductor and shield. That is how the physics works out. All of the feed line center conductor current is pretty well matched by the same current in the shield. It is the closely coupled concentric magnetic fields that make coax work so well. Or, simply put, for ever action, an equal and opposite reaction occurs .... yes, basic physics ... however, what is in question--since the other braid has and "inside" and an "outside", where does that equal opposite reaction occur ... Regard, JS |
#24
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Vertical problem
John Smith wrote:
JosephKK wrote: ... 95% plus of the transmitter or received energy is safely contained between center conductor and shield. That is how the physics works out. All of the feed line center conductor current is pretty well matched by the same current in the shield. It is the closely coupled concentric magnetic fields that make coax work so well. Or, simply put, for ever action, an equal and opposite reaction occurs ... yes, basic physics ... however, what is in question--since the other braid has and "inside" and an "outside", where does that equal opposite reaction occur ... Regard, JS Gawd, I am getting hasty in my old age ... No electrical field can occur without a magnet field, no magnetic field can occur with out a corresponding electrical component, again, basic physics; you are attempting to say the magnetic component of the current on the outside of the braid is NOT SEEN by the center conductor .... that may be in error ... but, I would be willing to listen how tinned copper (the braid) is some sort of shield to magnetic fields ... Regards, JS |
#25
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Vertical problem
John Smith wrote:
John Smith wrote: JosephKK wrote: ... 95% plus of the transmitter or received energy is safely contained between center conductor and shield. That is how the physics works out. All of the feed line center conductor current is pretty well matched by the same current in the shield. It is the closely coupled concentric magnetic fields that make coax work so well. Or, simply put, for ever action, an equal and opposite reaction occurs ... yes, basic physics ... however, what is in question--since the other braid has and "inside" and an "outside", where does that equal opposite reaction occur ... Regard, JS Gawd, I am getting hasty in my old age ... No electrical field can occur without a magnet field, no magnetic field can occur with out a corresponding electrical component, again, basic physics; you are attempting to say the magnetic component of the current on the outside of the braid is NOT SEEN by the center conductor ... that may be in error ... but, I would be willing to listen how tinned copper (the braid) is some sort of shield to magnetic fields ... Regards, JS Geesh, too old, I am telling you ... Disregard "static fields", however, that is NOT what is in question here ... Regards, JS |
#26
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Vertical problem
JosephKK wrote:
95% plus of the transmitter or received energy is safely contained between center conductor and shield. That is how the physics works out. All of the feed line center conductor current is pretty well matched by the same current in the shield. It is the closely coupled concentric magnetic fields that make coax work so well. It's unfortunate that this isn't true. If it were, it would make our lives a lot easier. Anyone interested in learning more about this topic might take a look at http://eznec.com/Amateur/Articles/Baluns.pdf. Roy Lewallen, W7EL |
#27
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Vertical problem
"John Smith" wrote in message ... John Smith wrote: John Smith wrote: JosephKK wrote: ... 95% plus of the transmitter or received energy is safely contained between center conductor and shield. That is how the physics works out. All of the feed line center conductor current is pretty well matched by the same current in the shield. It is the closely coupled concentric magnetic fields that make coax work so well. Or, simply put, for ever action, an equal and opposite reaction occurs ... yes, basic physics ... however, what is in question--since the other braid has and "inside" and an "outside", where does that equal opposite reaction occur ... Regard, JS Gawd, I am getting hasty in my old age ... No electrical field can occur without a magnet field, no magnetic field can occur with out a corresponding electrical component, again, basic physics; you are attempting to say the magnetic component of the current on the outside of the braid is NOT SEEN by the center conductor ... that may be in error ... but, I would be willing to listen how tinned copper (the braid) is some sort of shield to magnetic fields ... Regards, JS Geesh, too old, I am telling you ... Disregard "static fields", however, that is NOT what is in question here ... Regards, JS --------- Rats! You beat me to it! Ed, NM2K |
#28
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Vertical problem
Ed Cregger wrote:
... Rats! You beat me to it! Ed, NM2K It's about time! Some of these guys have held my feet to the fire! :-) Regards, JS |
#29
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Vertical problem
Basic physics is fine, but it's possible to reach all sorts of invalid
conclusions if misapplied. Roy Lewallen, W7EL |
#30
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Vertical problem
Roy Lewallen wrote:
Basic physics is fine, but it's possible to reach all sorts of invalid conclusions if misapplied. Roy Lewallen, W7EL I have been thinking about this "comment." And, absolutely!; Things only get more interesting when we can "bend" the laws ... ;-) Regards, JS |
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