A. G. Hanson has written:

'Optical waveguide communications glossary' -- subject(s): Fiber optics, Dictionaries

Rectangular Waveguide - TE10; (TM11 in case of TM waves)

Circular Waveguide - TE11;

Circular waveguides offer implementation advantages over rectangular waveguide in Calculations for circular waveguide requires the application of Bessel.

A variable attenuator is a device used to adjust the signal strength of a radio frequency or optical signal. It can be used to reduce the intensity of the signal by an adjustable amount to prevent overloading or distortion in the receiving equipment. Variable attenuators are commonly used in communication systems, test and measurement equipment, and audio systems to control signal levels.

Moisture in the air in a waveguide can scatter the microwave energy the waveguide is designed to transport. This translates into signal loss or attenuation. The VSWR drops, and that is not a good thing.