Some basic concepts need your attention

[junglerock]

1. Mode conversion: IS it stating that changing from TEM to TE or TM ??

2. TEM is a very good mode for EM energy transmission? then how about TE or TM mode ?

3. Why after a certain high frequency (cutoff freq), the TEM will change into TE or TM mode?

4. For a transmission line, the denser current density on the surface, the better insertion loss?

 

[elmolla]

As far as I know:

1. Mode conversion: The conversion from any mode to another. You can simply think of it as an example from TEM to TE or TM, or from TEM to quasi-TEM.

2. The TEM's operate at lower frequency than other modes causing less attenuation. You can't use TEM with single conductor wave guides. The best mode is usually the fundamental mode, but in the circulat waveguide, the fundamental TE11 isn't the best, we prefer the TE0N modes which have lower attenuation at higher frequency.

3. In single conductor WGs there are no TEM waves. In multiconductor waveguides, the or even in open waveguides the TEM doesn't convert to TE or TM modes, rather, the new TE and TM modes propagate along with the TEM. i.e. Its like you're going up a ladder, each time you get past a story you get one ticket but you don't loose the old tickets; Each story here is the cut-off frequency of the mode, so, for example, when you get past the TEM (cut-off=0Hz) and that of TE1 and TM2 you have all those three modes propagating in your WG simulataneously.

4. As far as I know, the insertion loss (IR)=-20log|Γ|. What does this have to do with the surface current density? You have better insertion loss the less the value of Γ. i.e. Less reflection, the best case is when we have maximum power transfer i.e. a matching load. If you want to interpret that as maximum surface current density, well, if you mean the surface current density deacreases due to the reflected amount and is best when there is no reflection you can say so. But I prefer you don't think of it this way.