TEM mode

[mirok]

Hi,

Can any body tell me why we have TEM mode in a coaxial cable but we can't have it in microstrip transmition line. In another word I want to know what are the physical constraints for TEM mode propagation. Thanks in advance.

Amir

 

[GOTWW]

mirok, I am just a little confused about your statements.
Sure, the fundemental mode in coax is the TEM, but just as well is that in microstrip, compensated for the effective dielectric constant. Now push the frequency up. Coax is no longer necessarily TEM, but may be TE or TM, but taken to the limit, would be TEM like. Not your stuborn mircostrip. It to, just like the coax will have diffusion, they call it quzi-tem. Fairly well modeled but generally lossy at this point and there for not really desirable. So we narrow the width, and thin the substrates. Since has been so extensively studied it is not really a problem. And Really would your intuition suggest that an evironment in which 1/2 or some proportion of the propogating EM fields travel slower than the rest demonstrate properties of a fully symmetric system?

 

[mirok]

Hi GOTWW,

What are the boundary conditions for TEM mode propagation? For example, the field near an antenna can never be TEM but at the same time the field in the far field will be very close to TEM mode. Why this happens? Why we can't have TEM mode in microstip line but it would be much better at coaxial cable. Thanks again for your attention.

Amir

 

[GOTWW]

For transmission lines:

To determine the field of a TEM wave, a suitable potential
PHI(x,y), must be found first. It is neccessary that PHI be a solution of
DEL_t^2 PHI = 0 and satisfy the boundary conditions

PHI = Vo/2 on Surface 2
= -Vo/2 on Surface 1

and DEL_t= transverse operator of DEL, assumming a z-dependence

.....= ax d/dx +ay d/dy.

For free space, in the near field of an antenna, the fields contain decaying reactive components, evenscent fields.

 

[Higgler]

TEM versus non TEM is:
straight versus curved fields.

When the Fields are arranged in a straight line from one conductor to another, it's referred as TEM. Microstrip has fields at it's edges that are curved between the conductor and the ground plane.
You nearly have TEM microstrip when you have a wide thin microstrip of high dieletric where the fields at the edges of the microstrip are nearly straight.
If you put a top cover on a microstrip device, you'll often see slight performance changes, that lets you know the fields have curved out of the microstrip into the air, otherwise the top cover would have no effect.
kch

 

[GOTWW]

There is such a thing as rectangular coax. And what about the good old 300 ohm twinlead. Many curved lines.

 

[Higgler]

Hmmn,
Good point. I often oversimplify things.

A short search on the web produced this;

"Note by Nigel Cook.

This is very important. The fact is that IEE published B Lago claiming that the TEM wave exists in electromagnetism today, when it does not."

Apparently there's been a debate on the existence of TEM waves that is still ongoing.

kch

 

[GOTWW]

Does it not basically boil down to the amount of dispersion (that I miss-worded as diffusion above, sorry) allowable for a given application? With-in a typical application, given rise-times, group-velocities and phase velocities, there is a certain amount of dispersion that can be tolerated and still have a working system. Free space "TEM" propagation is about as good as is gets, but still suffers from environmental factors that limit bandwidth.

 

[hacksaw]

food for thought:

TEM does not exist except with perfect conductors, but it is still a pretty good approximation...