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Thanks for clicking on my question.

I do not understand how an unbalanced transmission line works. Take for instance coaxial cable which is the subject of my confusion. This confusion arises from the apparently conflicting information I've found online which I'll dot point for clarity:

  • I've seen explanations online that talk about how the outer shield is "just grounded" and how it is common.

  • How, though a matching current flows in the outer conductor, the voltage is taken to be 0 (contradicts how it can be common?).

  • I've seen sources stating that opposing signals are sent down each conductor and seen some imply only the center conductor is active.

  • I've seen experiments where the shield was disconnected and the cable started radiating. I'd normally immediately assume this would certainly imply cancelling currents but what's with the other sources?

Among my confusions is say for instance we connect a dipole antenna, one pole linked to the core and the other pole linked to the shield, i.e. a varying voltage source. Since the impedance for the outer and inner conductors differs (individually, if it is even possible to consider impedance of a single line), how should the signals travel at the same speed since the characteristic impedance is different, is it because of the interaction between the two causing it to have a uniform impedance between the two?

I'd like to resolve all these confusions without getting overly mathematical or even too deep into the physics if possible, understandably quite ironic as I'm asking this question on a physics forum. None the less, could someone please clarify everything above, and/or possibly point towards more complete and clear sources?

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2 Answers 2

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The voltage of the outer shield of the coaxial cable is not zero, instead it has the voltage distribution same as that of the inner core, The outer shielding provides the path for the returning current that coupled from the inner core. Therefore it is required to be ground terminated at both ends ( better at both ends ) so as to let the current go.

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  • $\begingroup$ Thanks for your explanation, but how does this relate to the concept of transmission lines? I take that the signal is sent down the core relative to the shielding voltage but despite many ideas as to what might happen I don't seem to be able to form good idea of what's happening. Could you or someone else perhaps clarify? $\endgroup$ Commented Sep 9, 2022 at 12:00
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Good question. This is the way I understand transmission lines, balanced or unbalanced.

Image a pair of conductors that form the transmission line. A source is connected across the pair at one end and a load (say a dipole antenna) is connected to the pair at the other end.

The source will drive current into one conductor and collect and equal current at the other conductor. The current will flow down one conductor into the load and back to the source through the other conductor. Where the currents will flow down the transmission line depends on its geometry.

Balanced lines tend to have a symmetrical geometry like a balance pair (two parallel conductors). One line is driven by positive by the source and the other is negative. The symmetry allows to the electromagnetic field distribution to be shared equally and symmetrically between the two conductors. There is an imaginary line of zero potential equidistant between the two lines.

The current in geometrically opposite parts of the line are equal and opposite. This means that there is no net radiation.

For an unbalanced line, say a coaxial cable, the RF skin effect means that the return current in the shield is split between the inner and outer surface of the shield. Its as if the shield is formed by two separate conductors that cannot communicated by electromagnetic field. The current in the outer shield can radiate.

I'm not totally sure of this part but if an unbalanced source is used, i.e. one conductor is grounded, then the electromagnetic fields between the conductors is distorted leading to radiation.

One of the problems with circuit analysis of transmission lines is the earth symbol. On a circuit diagram it implies that the voltage is zero and that current disappears from the problem. However, real current flows round a circuit i.e. both parts of the transmission line. A circuit may be built correctly according to a circuit diagram but the earth points are incorrectly placed leading to earth loops which is a manifestation of an unbalanced line.

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  • $\begingroup$ Insightful answer, how does this relate the speed of transmission given the inner and outer conductors having different geometries, wouldn't this mean there is different propagation speed, or is there an interaction between the two netting a different value for the speed. $\endgroup$ Commented Sep 9, 2022 at 22:32

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