Difference between TEM and Quasi TEM mode

Hi

I am confuse about the TEM and Quasi TEM mode.
I have read a few comment and some book but still can't get a good understanding.
I need to know the different between these 2 and when will this occur?

I wish someone out that can help me to understand it.

Thanks for your time.

camelRF

Quasi TEM stands only when cross-section geo size is much smaller than wave length.

Hello Camel_RF,

TEM is a mode where E and H fields are perpendicular to each other and to the direction of propogation. This mode can exist between two excited conductors with a dielectric material filling the region between then.
Another important characteristic of TEM mode is a constant phase velocity, which means that β=k (propogation constant=wavenumber).

A good example of TEM mode waveguide is a coaxial cable where the E-field vector is the radial direction, and the H-field lines are perpendicular so their lines create concentric circles around the center conductor. The direction of propogation is along the cable.

Hi

The Quasi-TEM mode is best illusrated with the microstrip. As Pushhead writes, TEM is when the electromagnetic waves propagates with the same phase velocity, because the wave is in one medium. A microstrip on the other hand progates within two mediums: the substarte & air. As we know the wave will progate with different speed in the two mediums. Therefore it is called Quasi-TEM. The is why a dielectric constant for microstrip is between 1(air) and the material.

Regards

The TEM mode can be found whenever there are two conductors in only 1
medium. But for the case of microstrip lines, the wave is not completely confined in the dielectric, a portion of the wave propagates in the free space, as a result a longitudenal component of th field can be found , so it is not a TEM mode, but luckily this component is too small and can be neglected during analysis and this is
known as a quasi TEM because it was approximated to the TEM mode. u can check Pozar book for more details.

Another thing about Quasi-TEM mode is that althought the actual wave propogates in 2 different medias in a hybrid (mixed) mode, an approximate static solution is used and an effective dielec. const. (εeff) and propogation const. is calculated.
Now you can analyse the wave properties as if it was propogating through a homogenous effective media (εeff) supported by 2 conductors (just like TEM),

P.

There are NO direct-axis components of E and H fields in TEM wave unlike Quasi-TEM. I think this is the main differece...

TEM mode is used for only conductor to conductor tranfer of waves in a single medium like in coplanar wave guide or other wave guides. where as quasi TEM is used when there are 2 mediums for wave to travel..means like in microstrip lne when from substrate m.w has to travel in air...then quasi Tem is used...