Skywave
Most long-distance shortwave (high frequency) radio communication – between 3 and 30 MHz – is a result of skywave propagation.
Since the early 1920s amateur radio operators (or "hams"), limited to lower transmitter power than broadcast stations, have taken advantage of skywave for long-distance (or "DX") communication.
When high-frequency signals enter the ionosphere at a low angle they are bent back towards the Earth by the ionized layer.
[1] If the peak ionization is strong enough for the chosen frequency, a wave will exit the bottom of the layer earthwards – as if obliquely reflected from a mirror.
At some frequencies, generally in the lower shortwave region, the high angle skywaves will be reflected directly back towards the ground.
When the wave returns to ground it is spread out over a wide area, allowing communications within several hundred miles of the transmitting antenna.
NVIS enables local plus regional communications, even from low-lying valleys, to a large area, for example, an entire state or small country.
For example, using the F layer during the night, to best reach a receiver 500 miles away, an antenna should be chosen that has a strong lobe at 40 degrees elevation.
Frequencies below approximately 10 MHz (wavelengths longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), propagate most efficiently by skywave at night.
This, and the difficulties of generating and detecting higher frequencies, made discovery of shortwave propagation difficult for commercial services.
On December 12, 1901, he sent a message around 2,200 miles (3,500 km) from his transmission station in Cornwall, England, to St. John's, Newfoundland (now part of Canada).
Skepticism from the scientific community and his wired telegraph competitors drove Marconi to continue experimenting with wireless transmissions and associated business ventures over the next few decades.
[10] In June and July 1923, Guglielmo Marconi's land-to-ship transmissions were completed during nights on 97 meters from Poldhu Wireless Station, Cornwall, to his yacht Ellette in the Cape Verde Islands.
Marconi, in July 1924, entered into contracts with the British General Post Office (GPO) to install high speed shortwave telegraphy circuits from London to Australia, India, South Africa and Canada as the main element of the Imperial Wireless Chain.
Far more spectrum is available for long-distance communication in the shortwave bands than in the long wave bands; and shortwave transmitters, receivers and antennas were orders of magnitude less expensive than the multi-hundred kilowatt transmitters and monstrous antennas needed for long wave.
It recommended and received Government approval for all overseas cable and wireless resources of the Empire to be merged into one system controlled by a newly formed company in 1929, Imperial and International Communications Ltd.
