A form of radio transmission that uses ultra-high frequencies, developed out of experiments with radar (radio detecting and ranging) during the period preceding World War II. Developed by Harald T. Friis and his associates at AT&T Bell Telephone Laboratories, microwave systems are point-to-point radio systems operating in the GigaHertz (GHz) frequency range. The wavelength is in the millimeter range, which is to say that each electromagnetic cycle or waveform is in the range of a millimeter, which gives rise to the term microwave. As such high-frequency signals are especially susceptible to attenuation due to interaction with the physical matter in the atmosphere, terrestrial microwave radio beams must be tightly focused and must be amplified or repeated frequently. Microwave is a Line-of-Sight (LOS) technology as such high-frequency radio waves will not pass through solid objects of any significance. See also frequency, frequency spectrum, Friis, LOS, point-to-point, radar, radio, and wavelength.