Electronics

Microwaves are the frequencies of choice for many forms of communications especially telephone and television signals that are transmitted long distances through overland methods, broadcast stations, and satellites. Microwave electronics are also used for radar.

Microwaves are within the frequency of 3 GHz to about 300 GHz; because of their high frequency spectrum, microwaves can carry large numbers of channels. They also have short wavelengths from 10 cm to 0.1 cm; wavelength dictates the size of antenna that can be used to transmit that particular wavelength, so the small antennae for microwave communications are very practical. They do require repeater stations to make long-distance links.

Electronic devices like capacitors, inductors, oscillators, and amplifiers were not usable with microwaves because their high frequency and the speeds of electrons are not compatible. This complication of component size was studied in detail in the 1930s. Finally, it was found that the velocity of the electrons could be modulated to the advantage of microwave applications. The modulating device, the klystron, was a tube that amplified the microwave signal in a resonating cavity. The klystron could amplify only a narrow range of microwave frequencies, but the traveling-wire tube (invented in 1934)—a similar velocity modulator—could amplify a wider frequency band using a wire helix instead of a resonating cavity.

High-powered and high-pulsed microwave use especially for radar required another device, the magnetron. The magnetron was perfected in 1939 and was a tube with multiple resonating cavities. While these devices were successful for their specialized uses, they were expensive and bulky (like other vacuum tubes); they have been replaced completely by semiconductors and integrated circuits with equally sophisticated and specialized solutions for handling the high frequencies of microwaves that fit much smaller spaces and can be mass produced economically.

Microwave electronics have also required adaptations of other parts of transmission systems. Conventional wires can not carry microwaves because of the energy they give off; instead, coaxial cables can carry microwaves up to 5 GHz in frequency because their self-shielding conductors prevent radiating energy. Waveguides are used for higher-frequency microwave transmission; waveguides are hollow metal tubes with a refractive interface that reflects energy back. Microstrips are an alternative to waveguides that connect microwave components and work by separating two conductors with dielectric material. Microstrips (also called striplines) can be manufactured using integrated circuit (IC) technology and are compatible with the small size of ICs.

Masers were first developed in 1954. The Maser (Microwave Amplification by Stimulated Emission of Radiation) can be used for amplifying and oscillating microwaves in signals from satellites, atomic clocks, spacecraft, and radio. Masers focus molecules in an excited energy state into a resonant microwave cavity which then emits them as stimulated emission of radiation through the microwave output.