The rule of thumb for calculating the lowest frequency that can pass though a wave guide (rectangular) is given by 30/2a (G Hz) where a is the largest dimension.

fc = speed of light in cm /2a

 

I've forgotten more than most people ever know about waveguide theory. I was a Mk92 Firecontrolman in the Navy. We pushed a quarter million watts down the plumbing and when needed, we could flood the tube with sulfur hexaflouride gas and push the output up to a cool million watts. With a psuedo random frequency dithering option, we were pretty much 'jam' proof as well. Almost unlimited track while scanning capabilities and dual target designation from one tracker. State of the art in the late 70's and early 80's

 

The main thing I remember about rectangular wave guides was that they were equivalent to an infinite number of 1/4 wave shorted stubs in parallel. We just used descant dried and filtered compressed air to pressurize the wave guides on the APS-20 and APS-45 systems on the plane in my avatar. Squadron was de-commissioned in 1965 due to over-the-horizon radar being developed.

 

Nice topic and very helpful informations!

Isn't it though?