This first special issue of IEEE Antennas and Propagation Magazine is dedicated to high-frequency(HF) radio systems and techniques. Up to the 1960s, most telecommunication between the continents was realized by large HF (3–30-MHz) radio systems built especially for that purpose. The ionosphere was used as a natural high-altitude reflector to obtain the large operation distances. Inventions such as adaptive beamforming, antenna diversity, and frequency diversity stem from the 1930s, an impressive achievement considering the technology of that time.
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