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A quiz question I recently saw essentially states the following:

In an ideal coaxial line, a radiofrequency signal will travel only through the inner conductor.

A coaxial cable can be thought of as a waveguide with conducting walls. It's well known that the field in these kinds of geometries exists within the dielectric medium (air, vacuum, etc), not within the conducting walls. The fact that there is no field within the conductor is what determines the geometrical boundary conditions for each kind of waveguide (rectangular, circular, etc). Given this, wouldn't the electromagnetic wave within the coaxial cable travel through the dielectric medium between the two conductors?

Now if instead of introducing an electromagnetic wave within the cable we're talking about connecting it to a current or voltage source, then yes, the current signal travels through the inner conductor. But this is distinct from the RF electromagnetic wave, isn't it?

Is this just a matter of terminology, of referring to the current signal while actually saying "RF signal"?

EDIT: Literal translation of the question:

Theoretically, in a perfectly adapted coaxial line, a radiofrequency signal tends to travel:

a) Only through the outer conductor

b) Only through the inner conductor

c) Equally through both conductors

d) Far away from the dielectric

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    $\begingroup$ Please provide the full text of the quiz question. From what you said it sounds wrong, but I'd want to see the full text to be sure. $\endgroup$
    – The Photon
    Commented Sep 26 at 18:25
  • $\begingroup$ Check the edit. $\endgroup$
    – agaminon
    Commented Sep 26 at 18:27
  • $\begingroup$ As given, there's no question, just a statement of (possibly incorrect) fact. Are you supposed to judge it true or false, or is there a further question you are supposed to answer based on this statement? $\endgroup$
    – The Photon
    Commented Sep 26 at 18:28
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    $\begingroup$ What are the other choices? $\endgroup$
    – Jagerber48
    Commented Sep 26 at 18:55
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    $\begingroup$ What topic are you studying? If this is about transmission lines in circuits (telegraphers equations, etc) then the best answer is probably "equally between the two conductors". If this is in an EM context then the question is bonkers and the answer should be "in the fields in the dielectric". $\endgroup$
    – The Photon
    Commented Sep 26 at 19:17

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For an impedance-matched coaxial line (source and load impedances both equal the line impedance) then the current through the inner conductor is equal to the current through the shield. In this case the shield current flows through its inner surface.

If there is an impedance mismatch between the cable and the load, part of the signal exiting the cable is reflected off the load. This current flows on the outside of the shield, and in the same direction as the current on the inner conductor. The sum of the inside and outside currents is equal to the current on the inner conductor.

All this is essential to understanding how to design and build transmitting antenna systems for amateur radio. The ARRL Antenna Book covers this in detail.

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  • $\begingroup$ Can you back up the claim that reflected current signals flow on the outside of the shield? Does that mean a reflection from a mismatch produces no signal on the center conductor? How would it work if the feedline is a twisted pair line (where there's no "outside" and "inside")? Are you talking about any mismatch or maybe some particular situation (like where there's a unbalanced-to-balanced line transition)? $\endgroup$
    – The Photon
    Commented Sep 27 at 4:36
  • $\begingroup$ If the reflected current signal flows on the outside of the shield, And the sum of inside and outside currents is equal, where do the field lines associated with thej reflected voltage signal (presumably also emenating from the outside of the shield) terminate to? $\endgroup$
    – The Photon
    Commented Sep 27 at 4:41
  • $\begingroup$ Hello niels nielsen, In the case of "an impedance matched coaxial cable", does "the shield current flowing through its inner surface" satisfy Ohm's law? If the answer is yes, does it cause the transmission energy loss of the coaxial cable? $\endgroup$
    – HEMMI
    Commented Sep 27 at 4:48
  • $\begingroup$ So yes, the current signals may travel only through the inner conductor, what I wonder is if speaking of these as RF signals is frequent or not. $\endgroup$
    – agaminon
    Commented Sep 27 at 9:56
  • $\begingroup$ @ThePhoton, in that case the electric field lines go out to infinity i.e., the coax line starts to radiate and becomes part of the antenna. This creates problems for the radio operator and his or her equipment and must be avoided, hence the emphasis on proper impedance matching. . $\endgroup$
    – niels nielsen
    Commented Sep 27 at 18:16

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