microwave. it couples the waveguide to open space.

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.

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.

• waveguide is a metal pipe that contains and guides microwaves from place to place in a microwave system (e.g. oscillators, amplifiers, mixers, modulators, filters, antennas)
• horn antenna has a waveguide connected at its focus, in transmit mode the waveguide feeds the horn which then emits a microwave beam, in receive mode the horn collects a microwave beam and concentrates it int the waveguide