Higher order modes waveguides

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fule_1

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Hi,

I was wandering, why are waveguides always designed as TE01 waveguides?
Why dont we use higher order modes in waveguides? I know everything about cut-off frequency. Lets suppose we are working with oversized guides.

For example. I never saw waveguides with feed like this...



Thanks
 

Each mode in a waveguide propagates at a different speed. If you want to keep a signal intact, you want to utilize just one mode. The easiest way to do this is to reduce the dimensions of the waveguide until only one mode is supported, the TE01. There are people working on multi-mode waveguides where they use each mode as a separate channel. This is challenging because power can easily couple from one mode to the another when there are any discontinuities in the waveguide. For very short distances, the speed difference between the modes is usually small enough that it is not a problem. This is why you see multimode optical fiber used within buildings, but single-mode fiber for long hauls.
 
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    fule_1

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Excellent answer. Thanks.
What does it mean "For very short distances, ..." ? How long is "very short"? 1 lambda, 10 lambda, ...?
 

For multimode fiber, short distance is maybe 100 meters or more.
 

Higher order waveguides are sometimes used in circular waveguides such as TE21 mode couplers for target acquisition in radars or satellite communication systems.As stated before they can couple to other modes if you can't extract them properly.
 

yep, you CAN use higher order modes.
You run the risk of your energy propagating down the guide in more than one mode, though, and that would cause havoc in some systems. In a communications system, it would look like a constant dispersive fade.

If you wanted to run a TE20 mode, you could probably do it safely if you SUPPRESSED the TE10 mode, with screws or a septum. A guide with a TE20 mode, would have the left energy at 0 degrees, and the right energy at 180 degrees....so it might make a perfect broadband balun, for instance
 

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