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My understanding is that the desired mode of operation for a two wire transmission line is differential, where at any point along the line the addition of the voltage or current is zero, such that the electric fields cancel and net radiation is zero.

Due to skin effect, in coax, the two wires are the inside of the braid and the outside of the center conductor. Technically speaking in this case the outside of the braid isn't part of the two wire transmission line.

For transmission lines, if you add together together the voltage or current at any point along the line and then divide by two, the result is the common mode voltage or current, which is that portion which does not have an equal and opposite. Common mode current inside coax can only be caused i believe by irregularities in the physical properties of the coax, or if the transmitter output isn't exactly differential. Common mode current is not caused by antennas which aren't balanced.

For all antennas fed with coax, at the junction between coax and antenna, current flowing away from the transmitter towards the antenna, will, due to skin effect, split up between the side of the antenna connected to the braid and the outside of the coax. The ratio of current flow between antenna and outside of coax is directly proportional to the ratio of the impedance wrt ground of the antenna element connected to the braid and the outside of the braid.

If it so happens that the impedance of the antenna element is low compared to that of the braid, then most of the current will flow into the antenna.

Common mode current inside of coax and current flowing on the outside of coax are two completely different conditions. For differential systems, common mode refers to an error condition concerning two wires, the two wires in this case being inside the coax. Current flowing on the outside of coax is a direct result of the voltage divider which exists at the junction of the braid of the coax and the connected antenna element, and is not common mode current at all. The outside of the braid is effectively part of a single ended single wire circuit with ground as the common point.

So why do ham radio operators refer to current on the outside of coax as common mode current, when quite clearly it isn't ?

To me it seems they are getting the outside of the braid mixed up as part of the two wire transmission line inside the coax, and this just serves to confuse everyone.

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    $\begingroup$ "Common mode current is not caused by antennas which aren't balanced." that's certainly how most engineering references describe it. $\endgroup$
    – user21417
    Commented May 19, 2022 at 2:40
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    $\begingroup$ The current isn't split between the antenna and the coax braid at the feed point. The director and reflector in a Yagi have high currents and voltages on them, even though completely disconnected. Same with a coax feed line. It can pick up RF current even if disconnected from a radiating antenna (say powered by another feed or transmitter). $\endgroup$
    – hotpaw2
    Commented May 19, 2022 at 3:24
  • $\begingroup$ @hotpaw2 Then why do ham radio operators use a choke at the feed point ... ? $\endgroup$
    – Andrew
    Commented May 19, 2022 at 3:39
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    $\begingroup$ There are more differentials at play at the feed point than the split between the antenna and the coax. $\endgroup$
    – user21417
    Commented May 19, 2022 at 12:56
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    $\begingroup$ One of your misconceptions here is that the antenna is an entity that acts alone by itself. It is not. You have to consider the entire antenna system, which includes the antenna, any reflectors around it, the coax, ground radials, the ground itself, and possibly even the structure the antenna is attached to. Any imbalance anywhere in the antenna system or between parts in the system could cause common mode current. $\endgroup$
    – user10489
    Commented May 20, 2022 at 2:42

2 Answers 2

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Common mode current occurs when current in the shield flows in a direction common with current in the center conductor. "Common mode current" is just a simple descriptive label to distinguish it from differental current.

Common mode current is caused by all kinds of things. Irregularities in the coax is the least likely of those. Imbalance in the antenna is a possible cause. The most likely cause is interaction between the outside of the shield and part of the antenna and other metal objects near by.

Common mode current can be blocked by a balun, but also if you try to position the coax so that it is at right angles to the dipole in all planes in a volume within 2-3 wavelengths (near field) of the antenna, it also will reduce the common mode current. (In other words, balance it geometrically against the antenna it is feeding.)

Remember, this is about radio and radio waves. Those don't travel in wires all the time. They try to escape. Thinking about things like "voltage divider at the junction" is thinking in DC where the energy can't escape the wire so easily and all the currents will sum up. In RF with common mode current, they won't because some of the energy escapes before it gets to the junction. After all, you are feeding an antenna, you are trying to make the energy escape, and we do our best to try to make it escape through the antenna, but it's frequently just as happy escaping along the length of the coax if we give it the opportunity.

Why do hams use a choke at the feed point? Because this is one of the best places to block common mode current. The choke can be almost anywhere along the coax, but the further from the feed point it is placed, the more coax is available to radiate through common mode current. (There are nodal points on the coax where the choke would do nothing, and other points a (multiple of a) quarter wavelength away where it would have maximum effect.)

Common mode current isn't an "error condition". Usually we try to avoid it, but sometimes we take advantage of it, and use the coax as an antenna.

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  • $\begingroup$ Thanks for the answer, but you haven't really answered the question, CM current is a term used to describe the normally unwanted current that can exist in a differential system. Using that term to describe current in a single wire doesn't make sense. The outside of the braid of coax isn't part of a differential system with two wires with equal and opposite voltage and current. $\endgroup$
    – Andrew
    Commented May 19, 2022 at 23:30
  • $\begingroup$ You are correct -- common mode current isn't in a single wire. It's only in coax and other transmission lines. Without the center conductor, the current on the outside of the shield is uninteresting. You are wrong in thinking that the outside and inside of the braid can be "separated" when thinking about coax. $\endgroup$
    – user10489
    Commented May 19, 2022 at 23:55
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    $\begingroup$ The definition of common mode current I gave is always true, not conditionally true. It is conditional if common mode current exists in a particular situation, but the term still has the same meaning. Common mode current exists (or doesn't) in transmission lines; outside of that context, it isn't interesting. It should not exist in differential systems that are functioning as differential systems (although it might). $\endgroup$
    – user10489
    Commented May 20, 2022 at 5:22
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    $\begingroup$ The fact that common mode current is between the outside of the shield and the center conductor is how it manifests. It's existence in the outside of the shield vs. just conducted in the shield is irrelevant. In fact, "outside of the shield" is also an oversimplification, because skin depth requires it to actually be deeper than that, and the little tiny strands in the shield aren't really all that thick. $\endgroup$
    – user10489
    Commented May 20, 2022 at 5:25
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    $\begingroup$ Radiation from the antenna can also induce current in the outer shield. So a balun at the radio end of the feedline, not just the antenna end, can also be helpful. $\endgroup$
    – pidloop
    Commented May 24, 2022 at 19:32
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RF Current on the outside of a coax will radiate an EM field. You can test the amount of this EM with an RF field strength held up next to a coax at various locations along its length, and testing with and without a choke balun to an asymmetric antenna. Any EM field radiating from a feed line is energy not radiating from the antenna (conservation laws), which is usually undesirable.

According to Maxwell, RF magnetic field generation is only possible if the vector sum of all currents on the coax (both “outside” and “inside”) are non-zero, thus a common mode current.

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  • $\begingroup$ Everyone seems to be missing the point i am trying to make in the question, yes, induced RF in the outside of coax is real, but that too isn't a common mode mechanism. CM current is a term used to describe the normally unwanted current that can exist in a differential system. Using that term to describe current in a single wire doesn't make sense. The outside of the braid of coax isn't part of a differential system with two wires with equal and opposite voltage and current. $\endgroup$
    – Andrew
    Commented May 19, 2022 at 23:42
  • $\begingroup$ The only thing EM fields care about is the vector sum of all the currents, whether there is one conductor, or one hundred, and whether any or none are differential. One usually only cares about common mode current on a feed line because it might generate an external EM field. $\endgroup$
    – hotpaw2
    Commented May 20, 2022 at 1:35
  • $\begingroup$ Common mode current describes the mode of operation of transmission line where a portion of the current is not differential and the fields fail to cancel, so that it radiates. Sloppy language (and thinking) considers this to be only the current on the shield that radiates, but really it's an operation mode for the transmission line, and describes a situation where net current (across insulated wires) is non-zero. $\endgroup$
    – user10489
    Commented May 20, 2022 at 11:26
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    $\begingroup$ @Andrew I think one of the problems with your understanding is that there are rarely, in practice, "two wires with equal and opposite voltage and current" in any practical antenna system. CM problems arise precisely because real conductors under real situations behave in a very measurable manner. We can describe typical aspects of this phenomena in a number of ways, but engineers and amateurs alike are also very interested in mitigation because the phenomena are so common (no pun intended). But $I1 \neq I2$ just... happens. A lot. $\endgroup$
    – user21789
    Commented May 20, 2022 at 16:40
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    $\begingroup$ Saying "calling this common mode is stretching it" is like saying "I don't know why you call that red, it looks green to me." $\endgroup$
    – user10489
    Commented May 24, 2022 at 23:16

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