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"Bal uhn" or "bayl uhn"?
On 7/30/2015 4:28 PM, rickman wrote:
On 7/30/2015 2:01 PM, Dave Platt wrote: In article , rickman wrote: Yes, I read that, but it doesn't really explain this current. Later they make the statement, "the current on the braid outside side is the sum of currents other than transmission line currents on the entire coaxial cable structure". This is pretty clear, but still does not explain the source, or maybe I should say "why" the current flows on the braid and not the antenna. Don't ask "Why does current flow on the braid?". Ask "What would *stop* current from flowing on the braid?". Current flows on *all* paths that have less than an infinite impedance. That's its nature. Remember, I = E / R (or, for AC/RF, I = E / Z). "I" (current on the braid) will be nonzero, if the voltage at that point is nonzero (E != 0) and the impedance down the braid at that point is not infinite. The effect of a balun is to place a high "choking" impedance in series with the outside of the feedline braid, thus "choking off" the current flow. I can't say I agree with your "choking" impedance idea. The coax connects to the balun in the same way it connects to the antenna. The balun can have no effect on the impedance of the coax shield. Just as you ask, "What would *stop* current from flowing on the braid?" when connected to the antenna what will stop the current from flowing on the braid when connected to the balun? The balun is an impedance that the RF sees as it starts to travel down the outside of the coax toward the transmitter. But you know about common mode currents, I think. The only thing that will stop the current from flowing on the outside of the shield when connected to the balun is if the balun presents a much lower impedance path for the current than does the shield. Think of a common mode choke. However it is mechanically implemented, its purpose is to provide a block to common mode currents and allow differential currents only. The only way your suggestion makes sense is if the current actually comes *from* the antenna and the balun prevents that current from returning to the feed line. Actually, your statement is the key. The feed point of the antenna IS the current source. One end of the current source is applied to the 'hot' side of the antenna and the other end is applied to the other element plus coax shield. As far as the feed point is concerned it has one element on one side and two elements on the other side. |
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