High frequency (HF) radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters. Shortwave (2.310 - 25.820 MHz) overlaps and is slightly lower than HF.Since the ionosphere often reflects HF radio waves quite well (a phenomenon known as skywave), this range is extensively used for medium and long range terrestrial radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:Sunlight/darkness at site of transmission and receptionTransmitter/receiver proximity to terminatorSeasonSunspot cycleSolar activityPolar auroraMaximum usable frequencyLowest usable high frequencyFrequency of operation within the HF range
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