Whale song and microwave ovens both rely on waveguides
A layer of ocean called the SOFAR channel traps sound so well that whale calls can travel enormous distances through it. Engineers build the same kind of trap on purpose. A waveguide keeps a wave from spreading out and fading, whether it carries microwaves to your popcorn, light down a glass fibre or radar pulses to an antenna.
Left alone, a wave spreads in every direction and weakens according to the inverse square law. Confine it and it can travel far. The idea is old in practice: sound carried along a taut wire, through a hollow pipe or a stethoscope. The classic engineering version is a hollow metal pipe, usually rectangular or round, carrying high-frequency radio waves. Glass fibres do the same for light, and horns and ducts shape sound in instruments and loudspeakers. Every guide has a cutoff: waves longer than a size-dependent limit will not pass, which is why an optical fibre cannot carry microwaves.
J. J. Thomson proposed the first guiding structure in 1893, Oliver Lodge tested it the next year, and Lord Rayleigh worked out the mathematics for a metal cylinder in 1897. That same year Jagadish Chandra Bose described to London's Royal Institution his millimetre-wave research done in Kolkata. But early radio used long wavelengths that would need absurdly wide pipes, so the subject went quiet and Rayleigh's work was forgotten. In the 1930s George Southworth at Bell Labs revived it, stumbling in some experiments because he did not know about the cutoff effect Rayleigh had already described.
The Second World War changed everything. The cavity magnetron, built in 1940 by John Randall and Harry Boot at Birmingham, made microwave radar practical, and Allied laboratories such as MIT's Radiation Laboratory poured effort into waveguide theory. Germany, by contrast, largely neglected microwaves, believing them useless for electronic warfare, and even let academics publish freely on the subject.
After the war waveguide dominated microwave engineering, but it was bulky, costly and poor at covering wide frequency ranges. Coaxial cable and later printed planar circuits displaced it in most bands. It still links dish antennas to their electronics, feeds magnetron power into microwave ovens, and underpins guided wave testing, a way of inspecting materials without damaging them.
Source: Waveguide
