Henry, WD5JFR wrote:
"Microwave waveguide is just an infinite number of 1/4 wave stubs
connected mouth to mouth."

It`s true that a waveguide is the same as any number of shorted 1/4-wave
shorted stubs connected in parallel. Their parallel connection between
the same two points will not affect their resonant frequency. That`s
why, as Richard Clark noted, if the 1/4-wave shorted stubs are circled
around into the form of a closed container, it makes a resonant cavity.
The diameter of the drum formed by the large number of 1/4-wave stubs
has a diameter of 1/2-wavelength at the resonant frequency.

Rectangular waveguide is more or less aperoiodic down to the low
frequency limit of its propagation. This occurs where its larger
cross-section dimension is 1/2-wavelength. Attenuation rises as
frequency through the waveghuide is lowered towards its cut-off at the
1/2-wavelength dimension. Below cut-off, attenuation is very high.

In the waveguide`s passband it is preferred to coax because
air-insulated coax has about 3X the loss of waveguide. But, waveguides
must be at least 1/2-wavelength wide which limits practical use to
microwaves. A waveguide for 30 MHz would be at least 17 feet wide.

Best regards, Richard Harrison, KB5WZI