ABSTRACT
Propagation of electromagnetic radiation within the HF frequency band over very long distances is only possible due to a region of electrically charged atoms and molecules that exists some 60 km to 1,000 km above the surface of the earth. Reflection of HF signals by the ionosphere involves excitation of free electrons there by the electric field of the radio signal striking them. Due to the constantly changing nature of the ionosphere, shortwave broadcasts inevitably suffer from fading—which can be slow or rapid. By international agreement, the radio spectrum is broken into various blocks: low frequency at 30–300 kHz; medium wave at 300 kHz–3 MHz; very high frequency at 30–300 MHz, etc. The high-frequency (HF or shortwave) international broadcasting portion of radio spectrum encompasses frequencies between 5.9 MHz and 26.1 MHz. The transfer of RF power levels to transmitting antennas also requires physically larger transmission lines than the typically employed in medium-wave broadcasting.
