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#181
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Roy Lewallen wrote: Read again the fourth sentence of the posting you quoted. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: If that two-terminal box contains an inductor, then the current out has to equal the current in -- that's the only way the sum of currents at the two terminals can sum to zero. What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the current become the same at both ends? 73, Jim AC6XG Okay. I read it. I'll try asking the question another way. What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the sum of the currents at both ends become equal to zero? 73, Jim AC6XG |
#182
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Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal network. It appears to be a three or four terminal network with a virtual ground. The only way I can see to model it and get these particular results would be to have it as a device which increases in impedance at every point along its length, and which has a current path to ground at each of an infinite number of such points. 73, Jim AC6XG |
#183
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Jim Kelley wrote: What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the sum of the currents at both ends become equal to zero? Sorry to be obtuse, Roy. The point is only that antenna circuits obviously present a problem to the assumption that such two terminal black boxes will necessarily have equal currents at both terminals. A solenoid should produce a field in the direction of its axis, should it not? 73, Jim AC6XG |
#184
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Jim Kelley wrote:
What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the sum of the currents at both ends become equal to zero? The two-terminal black box must have the same zero dimensions as the inductance. :-) -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#185
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You also need to go back and read the fourth sentence of my posting.
Sheesh. Roy Lewallen, W7EL Cecil Moore wrote: . . . Are you saying I cannot coil up 1/2WL of coax inside a black box and observe current flowing into both terminals for 1/2 cycle and current flowing out of both terminals for 1/2 cycle? That would be quite a revelation. |
#186
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Jim Kelley wrote:
Cecil Moore wrote: In any case, it appears to me that an antenna is not a two terminal network. It appears to be a three or four terminal network with a virtual ground. The only way I can see to model it and get these particular results would be to have it as a device which increases in impedance at every point along its length, and which has a current path to ground at each of an infinite number of such points. For a 1/4WL vertical, that would be true even if the path to ground is through displacement currents. But, what about free space? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#187
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On 3-Nov-2003, Wes Stewart wrote: .... Whatever we were doing required the use of a "magic" T. I, the ever inquisitive student, asked; "Doc, how does a magic T work?" Doc, former professor and the author of "Intermediate Mathematics of Electromagnetics", replied, "It's magic." In the US Navy, we usually explained that such devices worked on the principle of PFM. First word is Pure, Last word is Magic -ken- |
#188
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It would be even nicer if readers would read it.
An antenna could be regarded to have any number of terminals you'd like. EZNEC does not make voltage measurements. Please go back and read the lengthy recent thread regarding the meaning of voltage and voltage measurement. I recall you were a major contributor. But it's really not necessary to re-post it. Please don't. EZNEC does show the voltages across sources and loads, both of which are two-terminal lumped components. Roy Lewallen, W7EL Cecil Moore wrote: Roy Lewallen wrote: Read again the fourth sentence of the posting you quoted. It would be nice just to repeat it so 5000 readers don't have to go searching for it. In any case, it appears to me that an antenna is not a two terminal network. It appears to be a three or four terminal network with a virtual ground. To what is EZNEC referencing voltage measurements on an antenna in free space? |
#189
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Assume the box is non-conducting. There would be a current difference
between the two terminals unless you shrunk the length of antenna it contains to a vanishingly small length. In the context of the antenna, even a few feet of an antenna is a significant fraction of a wavelength. Put the box around a few feet of a power transmission line carrying 60 Hz current, and you won't be able to measure any difference between the current going in and coming out. In that situation, a few feet isn't a significant part of a wavelength. If you use a conducting box, you'll end up with current on the outside of the box that gets into the problem. Its value depends on coupling to the wire inside and to the antenna outside, so it gets stickier than I want to deal with. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: Read again the fourth sentence of the posting you quoted. Roy Lewallen, W7EL Jim Kelley wrote: Roy Lewallen wrote: If that two-terminal box contains an inductor, then the current out has to equal the current in -- that's the only way the sum of currents at the two terminals can sum to zero. What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the current become the same at both ends? 73, Jim AC6XG Okay. I read it. I'll try asking the question another way. What if you draw a two terminal black box around the middle few feet of a 1/4 wave vertical? What makes the sum of the currents at both ends become equal to zero? 73, Jim AC6XG |
#190
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Cecil Moore wrote: Jim Kelley wrote: Cecil Moore wrote: In any case, it appears to me that an antenna is not a two terminal network. It appears to be a three or four terminal network with a virtual ground. The only way I can see to model it and get these particular results would be to have it as a device which increases in impedance at every point along its length, and which has a current path to ground at each of an infinite number of such points. For a 1/4WL vertical, that would be true even if the path to ground is through displacement currents. But, what about free space? Evidently there's some kind of an equivalence between electric current and the electric smoke liberated by the antenna. ;-) 73, Jim AC6XG |
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