EM Waveguide Modes: What are they and how are they used

[bladesong]

Title pretty much says it all. In EM waveguides (rectangular, circular, what have you), I understand there are various "modes." I'm gathering that these represent specific, discrete solutions to the Hemholtz equation, and therefore there are only specific frequencies that will propagate?

What happens between these frequencies?

What determines which mode you would use in a waveguide and why?

Finally, the cutoff frequency - modes with frequencies BELOW this number are the ones that don't propagate, correct (i.e. a HPF vs an LPF)?

Thank you in advance - additionally, any general explanation of modes and their physical meaning and relevance would be greatly appreciated. My material seems to just jump right into how to determine these without bothering to explain what they are, where they come from or how they're used.

 

[bladesong]

Anyone?

 

[FailedLaunch]

Imagine waveguides as oscilators, resonance circuits or antennas. You can force any frequency on them but only those with the correct wavelengths will resonate ("fit into the waveguide"). For the rest of frequencies the impedance will be high to the point of being infinite for some frequencies.

What determines which mode you would use in a waveguide and why?

Don't people almost always choose the lowest mode because the resulting waveguide is the largest (i.e. easy to manufacture with given precision)? I may also recall something about maximum power transferred in the lowest modes as opposed to the higher modes but I can't guarantee that. That may be important for radars, transmission antennas, etc.

Finally, the cutoff frequency - modes with frequencies BELOW this number are the ones that don't propagate, correct (i.e. a HPF vs an LPF)?

Their wavelengths (actually half of the wavelength) are too "long" to fit in the waveguide.

Thank you in advance - additionally, any general explanation of modes and their physical meaning and relevance would be greatly appreciated.

Hopefully, I didn't mess up too much. It's been over a decade since the last time I dealt with waveguides.Sources:
Wikipedia: Transverse mode
Microwaves101: http://www.microwaves101.com/encyclopedia/TEM.cfm