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"Bal uhn" or "bayl uhn"?
rickman wrote:
On 7/30/2015 5:46 PM, John S wrote: 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. There is something fundamentally wrong with our communications. Are you saying the balun is *part* of the coax? I have seen baluns made by wrapping the coax around a core. I have been assuming the balun was a transformer between the feed line and the antenna. There are two types of baluns; voltage baluns and current baluns. A voltage balun is usually a transformer and it forces the output voltage to be equal. A current balun is something that increases the impedance of the outside the shield path. The common forms of choke balun are simply wrapping the coax into a coil, wrapping the coax into a coil around a ferrite rod, wrapping the coax into a coil on a ferrite toroid, or large ferrite beads strung on the coax. See www.eznec.com/Amateur/Articles/Baluns.pdf -- Jim Pennino |
#3
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"Bal uhn" or "bayl uhn"?
rickman wrote:
On 7/30/2015 6:09 PM, wrote: rickman wrote: On 7/30/2015 5:46 PM, John S wrote: 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. There is something fundamentally wrong with our communications. Are you saying the balun is *part* of the coax? I have seen baluns made by wrapping the coax around a core. I have been assuming the balun was a transformer between the feed line and the antenna. There are two types of baluns; voltage baluns and current baluns. A voltage balun is usually a transformer and it forces the output voltage to be equal. A current balun is something that increases the impedance of the outside the shield path. The common forms of choke balun are simply wrapping the coax into a coil, wrapping the coax into a coil around a ferrite rod, wrapping the coax into a coil on a ferrite toroid, or large ferrite beads strung on the coax. See www.eznec.com/Amateur/Articles/Baluns.pdf Refer to figure 8. The balun is inserted between the feed line and the antenna. Clearly this example adds no impedance to the shield of the feed line. Rather it must present a very low impedance to the flow of current from the shield to the antenna element. You can also look at the illustrations in Appendix 1, both the voltage balun and the current balun. In both cases they show transformers which must present a low impedance path for the Io current as they call it. The text here does talk about a construction of the current balun from coax and the high impedance to current flowing on the outside of the shield *in the balun*. But when considering the feed line, the balun provides a low impedance to the current flowing from the inside of the feed line shield (Ii) which means it will not follow any other path. If you want to argue about it, argue with the author of the article. There is little that you said that has anything to do with what I said. -- Jim Pennino |
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