Upper cutoff frequency for rectangular waveguide

[CPL.Luke]

So I know there is an upper cutoff frequency (at least in the microwave band) for a rectangular waveguide. However for the life of me I can't find why one would exist in the equations, the internet is full of references to it based off of the dimensions of the waveguide, but I'm curious whether or not the various calculators and tables are assuming a certain material is being used to construct the waveguide and thus they calculate the cutoff based on that, or if there is a real EM reaso as to why an upper cutoff exists.

 

[lbrits]

Well, roughly I think it's the point where the longitudinal component goes from being imaginary to real (passing through zero). When it goes real, the wave attenuates. Here's a good explanation: http://en.wikipedia.org/wiki/Cutoff_frequency

 

[astrokreng]

The upper cutoff frequency for a rectangular waveguide is a fundamental property of the waveguide and is not dependent on the material used to construct it. It is determined by the dimensions of the waveguide, specifically the width and height of the cross section. This cutoff frequency is related to the mode of propagation within the waveguide, and as the frequency approaches the cutoff, the mode becomes increasingly attenuated until it can no longer propagate. This is due to the waveguide's geometry and the boundary conditions imposed by the walls of the waveguide.

The existence of the upper cutoff frequency is a result of the wave nature of electromagnetic radiation. As the frequency increases, the wavelength decreases, and for a given waveguide geometry, there comes a point where the wavelength becomes too small to support a mode of propagation within the waveguide. This is where the upper cutoff frequency is reached.

It is important to note that the upper cutoff frequency is not a sharp cutoff, but rather a gradual decrease in the amplitude of the propagated mode. This is due to the finite conductivity of the waveguide walls, which allows some energy to leak out of the waveguide at higher frequencies.

In summary, the upper cutoff frequency for a rectangular waveguide is a fundamental property determined by the waveguide's geometry and is not dependent on the material used to construct it. It is a result of the wave nature of electromagnetic radiation and is a gradual decrease in the amplitude of the propagated mode as the frequency approaches the cutoff.