# EM waves inside a transmission line clarifications

I just wanted to ask several clarifying questions to make sure I have an accurate picture of what happens in a transmission line.

A transmission line is usually made up of conducting material. If you connect a voltage source to a transmission line, is the "voltage wave" that gets sent down the transmission line just a change in electric field inside the line with respect to time and position?

If it is indeed a change in electric field inside the line, how is that possible if electric field inside a conductor is supposed to be 0? Is it that perturbations caused by the voltage source make a transient change in the charge distribution in the conductor, resulting in the change in electric field? My understanding is that people say that electric field inside a conductor is 0 in a steady-state context: after a long time under the same conditions, electric field inside a conductor is 0.

If the EM wave is inside the conductor, is there also a wave inside the medium that is between the two wires? If there isn't a wave inside the medium as the EM wave propagates, why does the characteristic impedance of the medium matter? I ask this because the EM wave is trapped inside the conductor.

Suppose you have a transmission line with a short on one end and an open on the other end, and you power the line with a quick pulse, say by connecting and quickly disconnecting a voltage source. Is the EM wave that reflects inside the transmission line inside the medium, or inside the line?

• Crazy thing: Free space is a transmission line with a characteristic impedance of ~377 ohms. But there is nothing there to carry the wave. Try and wrap your head around that. Commented Jun 21, 2018 at 16:00

I just wanted to ask several clarifying questions to make sure I have an accurate picture of what happens in a transmission line.

A transmission line is usually made up of conducting material.

No, a transmission line is made of two pieces of conductive material, with a piece of non-conductive material, aka dielectric, between them.

If you connect a voltage source to a transmission line, is the "voltage wave" that gets sent down the transmission line just a change in electric field inside the line with respect to time and position?

If you connect a voltage source to the to conductors, then a wave is sent down the line, that consists of an electric field in the dielectric, and a current in the conductors. At high freqeuncies, the current does not penetrate far into the conductors, and can usually be thought of as travelling at the surface of the conductors. I am always nervous when somebody uses the word 'just', and it usually pressages a failure of understanding.

If it is indeed a change in electric field inside the line, how is that possible if electric field inside a conductor is supposed to be 0?

Because a 'line' is not the same as a 'conductor', see the definition above.

Is it that perturbations caused by the voltage source make a transient change in the charge distribution in the conductor, resulting in the change in electric field?

A current flows on the conductors, this is what charges up the conductors with respect to each other.

My understanding is that people say that electric field inside a conductor is 0 in a steady-state context: after a long time under the same conditions, electric field inside a conductor is 0.

If the EM wave is inside the conductor, is there also a wave inside the medium that is between the two wires?

There is an electric field in the dielectric, and a voltage wave travelling along the line in the dielectric. An ideal conductor will have no field within it. A high conductivity conductor will have negligible field in it. It makes no difference to the first order behaviour of the line whether the conductor is ideal, or very good. If the latter, the line will have some loss.

If there isn't a wave inside the medium as the EM wave propagates, why does the characteristic impedance of the medium matter? I ask this because the EM wave is trapped inside the conductor.

The voltage wave is across the dielectric, with a current wave on the conductors. The characteristic impedance, that is the ratio of the voltage wave to the current wave, is a parameter of the line, not of the dielectric medium. It is a combination of the geometry of the line, and the permeability of the dielectric.

Suppose you have a transmission line with a short on one end and an open on the other end, and you power the line with a quick pulse, say by connecting and quickly disconnecting a voltage source. Is the EM wave that reflects inside the transmission line inside the medium, or inside the line?

The EM wave is inside the line. The electric component of the EM wave is across the dielectric. The current component of the EM wave is on the surface of the conductor.

• Thanks for the response! A couple follow-up questions: What do you mean that the current charges up the transmission line conductors with respect to each other? Do you mean in terms of volts? Also, is the characteristic impedance of the dielectric then concerned when you're talking about EM waves propagating through a medium in the absence of a transmission line? Lastly, to clarify, the EM wave is inside the line, which is constituted of 2 pieces of conductive material and a dielectric, the electric field and voltage waves are across the dielectric, and the current wave is in the conductors Commented Mar 19, 2017 at 7:36
• With 'respect to' means 'measured from'. One conductor has a different voltage to the other, in volts. It gets that charge because a current flows to that region of conductor. The characteristic impedance is a function of the geometry of the transmission line. The medium alone has a dielectric constant. Yes, the EM wave is inside the line, the electric field wave is in the dielectric, the current wave in the surface of the conductors, which goes hand-in-hand with a magnetic field wave in the space between the conductors. Commented Mar 19, 2017 at 10:02
• To clarify the line impedance - two thin wires will have higher impedance than two fat wires the same distance apart. A coax line with a large outer/inner size radius ratio will have a higher impedance than one with a small radius ratio. Using a plastic dielectric for either type of line will reduce the impedance to about 65% of what it was when air-spaced (for typical plastics). Commented Mar 19, 2017 at 10:11

Think about transversal EM wave that travels into space. There is E-field and H-field. Similar situation you'll find in transmission line. The voltage becomes E-field, and the current is H-field. All the energy flow (EM wave) is positioned around the conductors and not in them.

The E-field in conducting material is zero, except there is a loss, so the E-field in the transmission line conductor has longitudinal direction, because the voltage at the end is smaller due to internal resistance. But this is a pure loss, that we can omit.

The major importance is the transverse direction of the E-field and H-field around the conductors. The product of H and E filed is also known as Poynting vector. It shows the power density and direction, that would be from source to load.

Important thing to learn is: The energy flows in the dielectric medium in the space around conductors and not in the conductor itself - it makes sense, since EM wave travels without conductors. The conductors are being used as a guide to trap the EM wave inside the boundary space at the expense of the energy loss due the resistance of conductors.