Elementary Question of cutoff frequency in waveguide - RF Cafe Forums

 Post subject: Elementary Question of cutoff frequency in waveguide

Posted: Fri Jun 22, 2007 10:14 pm 

Got a little confused by the cutoff phenomena happened in waveguides, need your help please.
For example a rectangular waveguide has its first two cutoff freq to be TE10 ~ 6.56GHz and TE20 ~ 13.12GHz, and I feed in a wave of freq 14GHz (does this wave have to be a TE or TM wave so that it can actually propagate?) , then there should have at least two modes(TE10&TE20) co-exist inside the waveguide right? and, although the wave bears two different modes for propagation, it still maintains its own frequency, i.e. 14GHz, right? if I'm right so far, then the thing bothers me a lot is that, how can this monochromatic wave having two different wavelengths inside the waveguide? Thank you so much for your patient explanation!

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Posted: Mon Sep 24, 2007 8:18 am 

Simply put, different modes can take slightly different times to traverse the guide. Its the same phenomenon that leads to pulse spreading in multimode optical fiber.

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Posted: Fri Oct 05, 2007 7:22 am 

Another related point is that although the over-sized guide can support both modes, initially only the 14GHz TE10 mode would be excited by the launcher. Further down the guide, the mode might then de-generate into higher modes, simply because the geometry satisfies Maxwell's equation solutions and so they can be supported.