Help me understand how current flows through a transmission line as EM waves

Hi All,

We know that for high speeds ... current flows through a transmission line as EM waves.
However there is also a flow of eletrons (very slow compared to the EM waves).
I'm confused here, how does both these flow of current together constitutes the total flow of current. Woulnt we need 2 equations to describe current flow here????

regards,
newbie

* I think im missing an important point here.. just cant seem to grasp it

Re: Current Flow

High speed ?
Do you mean high frequency ?

EM waves are probably generated when high frequency signals pass through the feeder wire. And the wire probably acts as the wave guide.

The delay time of electron movements do not add.(I think) Electrons flow at a few mm per second. So if it had to flow through a 100 m long wire it would take 1/2 a minute...But it does not.

The EM in the transmission line is not supposed to cause any effect as they are killed by the shielding.

Or am I talking crap ? :?:

Re: Current Flow

The traveling EM wave is the cause of the current on the conductors. There is the equivalent of wave tilt. The EM wave is slightly tilted toward the conductor.

This wave tilt on ground waves is what makes Beverage antennas work.

Re: Current Flow

Maxwell's equations are a set of equations found by trial and error to allow us to predict what we actually observe in electrical experiments. Maxwell's most important contribution to Maxwell's equations is the concept of an abstract field (more important than displacement current).

Today we think of E and H fields as real things. They are not. They are abstract mathematical concepts that let us predict things like current flow, power, etc. All this is described in detail in an article to be published in the June issue of IEEE Microwave Magazine.

For AC signals (like the spectral components of a high speed signal), the current flow is electrons moving. But remember, for AC, the electrons are moving back and forth. One electron never makes it from one end of the wire to the other. Current flow is a real physical thing, just count the electrons moving back and forth.

However, all the fields are abstract mathematical concepts, they are not real. Why do we calculate these abstract fields (something I have been doing for over 25 years)? Because once calculated, they let us predict things like cross talk and reflections, etc. Do not pretend for even one second that we actually understand WHY these things happen.

So, we should view the current as the cause of the EM wave. But the EM wave is only an imaginary mathematical helper that helps us figure out what happens when that current flows. And what happens, if you think about it, is really pretty amazing...and no one has even the faintest idea why it happens.

Hi flatulent -- There is no wave tilt for TEM waves. There is for Beverage antennas, and I do think (but am not sure), that that tilt is critical for their proper operation. And Beverages do work very very well. I have three of them.

Re: Current Flow

I was not thinking of the gross angles of TE and TM waves. what I had in mind was the small change in the electric field near the conductors which caused a component of the energy flow to go perpendicular into the conductor. This energy flow is what supplies the resistance loss. Another way of looking at it is that the current in the wire produces a voltage drop (in phase) in the wire resistance along the wire. In order to match the boundary conditions of electric fields being identical across the boundary (less the free charges on the boundary) the EM wave has to have a component along the wire at the boundary.

Re: Current Flow

flatulent -- Absolutely correct. The tilt in the fields (even if they are abstract) is a critical part of propagation on a lossy line, and it is indeed caused by the metal loss in the line. The tangential E field does indeed represent a wave traveling perpendicular to the transmission line, in this case, directly into the metal itself. This tilt also causes everything (Zo, velocity) to vary with frequency (dispersion).

The additional point (adding to, not correcting what you state), that is important not to overlook, is that for the (non-physical) lossless case, there is no tilt, but the wave still propagates. If we extend to free space, lossless is very close to the actual sitatuion with TEM waves (and zero tilt) not seeing significant loss for millions or even billions of years of propagation. No tilt, but they still propagate.

Re: Current Flow

Thanks All,




When the voltage switches from 0 to say 5v as in the diagram above, we have a 5v signal propagating along the transmission line.
This is not an AC signal and the speed of propagation is which is much faster than electron flow speed (1cm/s)

Well apparently from the book im reading (one way to look at this)... this is due to EM field charging the capacitance along the transmission line... but to do so.. we would have to physically move an electron one side of the plate and remove an electron from the other and this would take too much time.

So how actually does the signal propagate down the line

regards,
newbie

Re: Current Flow

The electrons are so numerous that they only have to move a very small distance to make a big charge distribution change. Most metals have something like 10^23 per cubic cm.

What is happening to confuse you is what is commonly called a "teacher's metaphor." This is an overly simplified description that makes one aspect of the situation easier to visualize. It is not meant to be the literal truth. The equivalent LC network is easier to visualize than the vector solution of simultaneous differential equations which are more accurate descriptions of the circuit.