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"Bal uhn" or "bayl uhn"?
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. 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. But that is now how it was presented. Another post indicated the current the flows on the inside of the coax shield splits at the antenna feed point and part flows down the outside of the shield. An easy way to distinguish the two cases is to remove the antenna element from the shield. Of course this is no longer a balanced antenna, but still, what happens to the current on the shield? I assume it still flows on the shield inside in an amount equal to the center conductor current and then flows to ground on the outside of the shield? -- Rick |
#2
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"Bal uhn" or "bayl uhn"?
On 7/30/2015 4:55 PM, 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. 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. But that is now how it was presented. Another post indicated the current the flows on the inside of the coax shield splits at the antenna feed point and part flows down the outside of the shield. An easy way to distinguish the two cases is to remove the antenna element from the shield. Of course this is no longer a balanced antenna, but still, what happens to the current on the shield? I assume it still flows on the shield inside in an amount equal to the center conductor current and then flows to ground on the outside of the shield? Yes. Remember that the end of the coax connected to the antenna is now the generator (the source, as jimp said correctly). If the center of the coax carries X current, then the inside of the shield carries X current. It MUST go somewhere. So, with nothing there but the outside of the shield it runs down the outside of the shield with no 'knowledge' of what is happening inside the coax. The shield has become the other element of the antenna whether you like it or not and whether it goes to ground or not. Look up coaxial antenna. |
#3
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"Bal uhn" or "bayl uhn"?
On 7/30/2015 6:05 PM, John S wrote:
On 7/30/2015 4:55 PM, 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. 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. But that is now how it was presented. Another post indicated the current the flows on the inside of the coax shield splits at the antenna feed point and part flows down the outside of the shield. An easy way to distinguish the two cases is to remove the antenna element from the shield. Of course this is no longer a balanced antenna, but still, what happens to the current on the shield? I assume it still flows on the shield inside in an amount equal to the center conductor current and then flows to ground on the outside of the shield? Yes. Remember that the end of the coax connected to the antenna is now the generator (the source, as jimp said correctly). If the center of the coax carries X current, then the inside of the shield carries X current. It MUST go somewhere. So, with nothing there but the outside of the shield it runs down the outside of the shield with no 'knowledge' of what is happening inside the coax. The shield has become the other element of the antenna whether you like it or not and whether it goes to ground or not. The point is that the current flows on the outside of the shield because the current follows the lowest impedance path it has to follow, same as in all other cases. If you connect a balun to the end of the coax with no antenna connection on the side from the shield, there will be no current flow to the antenna element. So all the inner shield current will *still* flow back down the outside of the shield showing that there is no need to "explain" anything about a reflected wave from the antenna end causing the current to flow down the coax. -- Rick |
#4
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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 |
#6
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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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