A rectangular waveguide is a hollow conductive metal pipe with a cross section of rectangular shape that guides electromagnetic waves with minimal loss of energy.

The inner dimensions of the waveguide determine the cutoff frequency. The cutoff frequency is the lowest frequency at which the waveguide can operate. In rectangular waveguide, the waveguide width determines the lower cutoff frequency.

The Electronic Industries Alliance (EIA) defined  WR designations for standard rectangular waveguides. The designator for a size consists of the letters WR followed by numerals. The letters “WR” stand for Waveguide Rectangular and the number that follows “WR” is the width of the waveguide opening in mils, divided by 10.

The table below gives the details on the various waveguide sizes and their recommended frequency range.