High Frequency Broadcasting (HF) (also known as Shortwave Broadcasting), is a radio service licensed by the Federal Communications Commission (FCC) to operate between 5,950 kHz and 26,100 kHz. This is an international broadcast service where transmissions are intended to be received by the general public in foreign countries. The rules applicable to this service are located in Part 73 Subpart F of the FCC Rules. Unlike other broadcasting services, HF broadcasters are authorized frequencies on a seasonal basis. Currently, two seasons exist: a Summer season and a Winter season. The adjustment of frequencies between seasons results mainly from changes in propagation conditions, altered programming needs, and objectionable interference situations.
Due to the nature of the shortwave propagation, these broadcast signals propagate very long distances and are subject to interference from HF stations around the world, as a result, the HF frequency bands are extremely congested (particularly the lower frequency ranges). The worldwide demand for usable HF frequencies far exceeds the capacity of the allocated frequency bands. To remedy this overcrowding the FCC has authorized the use, on a non-interference basis, frequencies outside the allocated bands. In addition, the FCC regularly participates in international frequency coordination meetings in an effort to maximize its broadcasters access to clear channels and minimize any potential harmful interference to and/or from foreign HF broadcasters.