A transmission line is made of 2 conductors and a dielectric between them. Does the dielectric make these conductors different from a short circuit? Voltage drop across a short is 0, but it's the same for a lossless transmission line. Doesn't that mean that voltage and current in a short is maintained? I'm guessing that short circuits don't have characteristic impedance as a transmission line does, as characteristic impedance isn't the same as resistance. In a diagram, one can see that there is a difference between characteristic impedance of the line and impedance of the load, but I'm trying to understand why EM waves will reflect off a short after going through a transmission line if both shorts and transmission lines are wire, though transmission lines are 2 strips of wire, but with some dielectric between them.


1 Answer 1


You should notice the following: It's not current and voltage that propagates in the transmission line. It's a radiowave.

The wave is in the space between the conductors, not in the metal. The wave has simultaneous electric and magnetic fields that cannot be separated. They exist both, or it's not a wave.

The fields have quite complex spatial structure that can mathematically be expressed only as 3D vector fields. We, the practical electricians do not tease ourselves with 3D vector fields, but follow the wave by observing the voltage of its electric field and the current that is induced into the conductors.

The wave goes along the transmission line because it has such geometrical form. Not all wires or other pieces of metal quide the wave along it. For example, the antennas are optimized for throwing the radiowave into the space or catching the arriving radiowave as well as possible.

The radiowave reflects when it meets an incontinuity that does not be able to suck it. A short circuit in a transmission line is one possible incontinuity that can't store or dissipate the wave

The reflection does not occur, if the incontinuity allows the wave to retain the proportion between it's electric and magnetic field. In coaxial or pair cable that incontinuity is a proper termination resistor or other line with the same impedance (= the ratio E/H). Or a well matched antenna.

A short kills the electric field. The wave does not die, but generates another wave (=the reflected one) that has opposite electric field. Their sum at the short is =zero and no contradiction exists. The short works and the wave continues its life as the reflected wave.


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