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#1
September 5th 08, 03:34 AM
posted to rec.radio.amateur.antenna
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Baluns?
On Sep 4, 7:51*am, Cecil Moore wrote:
wrote: Again, you seem to have the IEEE definitions of "differential" and "common-mode" exactly reversed in your head. Really? Do you believe the currents in a resonant 1/2 wave dipole are common mode or differential mode? Assume it is a 1/2WL wire in free space. Where is the common reference? There is none unless you choose to define one. You could go halfway down the output choke and call it "common refrerence" or "isolated ground" or whatever you want to call it. Since there usually no need to reference it axcept perhaps for tutorial purposes, I wouldn't call it anything. You could also reference it to earth ground but only if you physically connected it to the aforementioned centertap. I've heard antenna currents called "common-mode" currents but neither Kraus nor Balanis call those currents "common-mode". They are usually called "antenna currents" (in phase, radiating) vs "transmission line currents" (out of phase, non- radiating). In any case, differential currents on a transmission line are 180 degrees out of phase and ideally, non- radiating. Common-mode currents on a transmission line are in phase and radiate. The currents in a folded dipole are in phase and radiate. The ideal transmission line is common mode and does not radiate because the fields cancel as you said earlier. The dipole antenna is ALSO common mode but the fileds do NOT cancel because the conductors are physically 180 degrees apart from each other so they cannot interfere with each other; instead the fields radiate into free space rather than cancel each other out. It is rather simple really. It is correct to call antenna currents "common mode currents". If the currents on a transmission line are differential, how would (COULD) they be converted to common mode currents on the antenna? We would need a 180 degree phase shift somewhere. Answer: the currents on both the transmission line and the antenna are common mode. Within a ferrite toroid wired in a 1:1 current-choke- balun configuration, common-mode current induces flux in the toroid with virtually none from differential mode. If the device is made out of turns of coax, the differential currents never see the choking impedance. Which explains why you should call it a balun and not a choke. If the balun and source impedances match there should be no choking impedance; maximum power trnasfer should occur. -- 73, Cecil *http://www.w5dxp.com |
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#3
September 6th 08, 03:54 AM
posted to rec.radio.amateur.antenna
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Baluns?
On Sep 5, 7:23*am, Cecil Moore wrote:
wrote: The ideal transmission line is common mode and does not radiate because the fields cancel as you said earlier. The dipole antenna is ALSO common mode but the fileds do NOT cancel because the conductors are physically 180 degrees apart from each other so they cannot interfere with each other; instead the fields radiate into free space rather than cancel each other out. If transmission line currents were common-mode, they would radiate like crazy and would be an antenna instead of a transmission line. When one shorts the two wires together and feeds the system Marconi style against ground, then the currents are common-mode. (Please switch your news-reader to fixed font). A '+' indicates a connection, not polarity. * +-------------------- * | * current reference (V) * | * Differential currents to antenna * +-------------------- * * * Normal Transmission Line mode designed not to radiate * +----+------------------ * | * * | *current reference (V) * *| * | * * | *Common-mode currents (becomes antenna) * | * * +------------------ GND * * * Marconi Style feed designed to radiate If the currents on a transmission line are differential, how would (COULD) they be converted to common mode currents on the antenna? We would need a 180 degree phase shift somewhere. I'm glad you asked. When we take the last 1/4WL of transmission line and open it up into a dipole, we have rotated one wire by -90 degrees and the other wire by +90 degrees. That's a 180 degree difference. The transmission line currents are 180 degrees out of phase. The extra 180 degrees of physical rotation subtracts from the 180 degrees in the transmission line for a total of zero degrees (in phase) at the antenna feedpoint. This is explained in detail in "Antenna Theory", by Balanis, 2nd edition, page 18. I will try to duplicate it here using fixed font ASCII graphics. * * * * * * * * In Phase Antenna Currents --------------------+ *+-------------------- * * * * * * * * * * * * *| *| * * * * * * * * * * * * *| *| * +------------------+ *| (V) * * Differential * * * | * +--------------------+ * Transmission Line Currents Note the out of phase currents in the transmission line results in in-phase currents in the dipole antenna. -- 73, Cecil *http://www.w5dxp.com Upon doing independent research, it turns out my theory is exactly the same as your's but my terminology was reversed. The situation is summed up in an article by W8ji: (quote)...the line will not radiate or contain substantial electric or magnetic fields external to the line area. The lack of external fields, even at a very small distances, is rooted in two conditions: 1.) All outgoing currents on one conductor are matched by equal level and exactly opposite phase currents on a return conductor at any given point along the line. (The reason I said the line will not radiate) This causes an exactly equal and opposite magnetic field along each conductor. The opposing magnetic fields caused by equal currents flowing opposite directions cancel magnetic fields outside the general area of the two conductors. 2.) All voltages from each conductor of the line to the outside environment surrounding the line are either contained within a closed shield, or are exactly equal and opposite an imaginary neutral reference point representing the environment around the line (balanced lines) (I discussed this imaginary reference point in some detail as well). We always have a constant ****differential*** voltage across the line (between the conductors) and that voltage changes only with standing wave ratio as we move along the line. 3.) The vector product of differential current flowing in conductors and voltage between line conductors at any point along the line always equals the power transmitted in transmission line mode. (unquote) So I simply reversed the terminology and I agree that your use of the term "differential" was correct. I was calling the mirror image current pattern "common mode" when it should be differential. But I still shy away from matching impedances with a "choke". I would match impedances with a balun or RF transformer. I agree that you must isolate, either mechancially or electrically, the input from output on a balun. I made the simple mistake of confusing the terms differential and common mode. Thank you for the correction. And I have tried using the CM mode marconi style feed (shorting the antenna at the transceiver input terminals) to try to get my 80m G5RV to radiate at 2Mhz some years ago, as suggested by MFJ in their instruction manual, to no avail. I am sure it can radiate quite well at some frequency however! |
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#4
September 6th 08, 02:46 PM
posted to rec.radio.amateur.antenna
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Baluns?
wrote:
Upon doing independent research, it turns out my theory is exactly the same as your's but my terminology was reversed. As proven a week ago by my definition quotations from "The IEEE Dictionary". But I still shy away from matching impedances with a "choke". I would match impedances with a balun or RF transformer. Jerry Sevick, W2FMI, says a *balun IS a choke* plus a transmission line. He wrote in "Building and Using Baluns and Ununs": "If the writers had accepted the correct model for these devices (given to us by Guanella and Ruthroff), which shows that *they are really chokes* (lumped elements) and configurations of transmission lines (distributed elements), ..." (emphasis mine) -- 73, Cecil http://www.w5dxp.com |
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#5
September 6th 08, 06:35 PM
posted to rec.radio.amateur.antenna
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Baluns?
On Sep 6, 9:46*am, Cecil Moore wrote:
wrote: Upon doing independent research, it turns out my theory is exactly the same as your's but my terminology was reversed. As proven a week ago by my definition quotations from "The IEEE Dictionary". I shall not try to pop the bubble of your glory in the triumph of knowing the correct terminology. But I still shy away from matching impedances with a "choke". I would match impedances with a balun or RF transformer. Jerry Sevick, W2FMI, says a *balun IS a choke* plus a transmission line. He wrote in "Building and Using Baluns and Ununs": "If the writers had accepted the correct model for these devices (given to us by Guanella and Ruthroff), which shows that *they are really chokes* (lumped elements) and configurations of transmission lines (distributed elements), ..." (emphasis mine) -- 73, Cecil *http://www.w5dxp.com The lumped element equivalent of the transmission line itself is a repetitive pattern of inductors and capacitors. If all inductors are chokes, then the transmission line itself is composed of chokes. |
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#6
September 6th 08, 08:33 PM
posted to rec.radio.amateur.antenna
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Baluns?
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#7
September 6th 08, 04:06 AM
posted to rec.radio.amateur.antenna
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Baluns?
Cecil Moore wrote:
wrote: ... As soon as he completes his "reversal", he will be back at the beginning, where his error(s) in thinking first began ... let's take a nap and let him accomplish his work ... grin Regards, JS |
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#8
September 6th 08, 06:28 PM
posted to rec.radio.amateur.antenna
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Baluns?
On Sep 5, 11:06*pm, John Smith wrote:
Cecil Moore wrote: wrote: * ... As soon as he completes his "reversal", he will be back at the beginning, where his error(s) in thinking first began ... let's take a nap and let him accomplish his work ... grin Regards, JS Yes John, go swallow an ambien and take your nap. Those Alzheimers- raddled brain cells could use some rest so they will be stronger for you to conduct future random senility eruptions. |
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#9
September 6th 08, 08:34 PM
posted to rec.radio.amateur.antenna
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Baluns?
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Hybrid Mode
