Waveguide

A waveguide is a hollow metal pipe — usually rectangular or circular — that carries microwave energy by bouncing it down the inside walls, with no centre conductor at all. Where a coaxial cable needs two conductors, a waveguide needs none: the electromagnetic wave simply zigzags along the metal tube like light down a mirrored corridor, or sound funnelled through a speaking tube. At microwave frequencies a hollow guide can carry far more power with far less loss than any cable, which is why radar dishes, satellite feeds and microwave ovens use them.

The catch is that a waveguide only works above a cutoff frequency set by its cross-sectional size: below that frequency the wave simply cannot propagate and dies away. This makes a waveguide inherently a high-pass structure, and it forces the dimensions to scale with wavelength — a 10 GHz guide is a few centimetres across, while a guide for FM radio would be metres wide and absurd. Inside, energy travels in distinct field patterns called modes (TE and TM), each with its own cutoff.