The wireless telegraphy

At Signal Hill on December 12, 1901,Guglielmo Marconiand his assistant, George Kemp, confirmed the reception of the first transatlantic radio signals. With a telephone receiver and a wire antenna kept aloft by a kite, they heard Morse code for the letter "S" transmitted from Poldhu, Cornwall. Their experiments showed that radio signals extended far beyond the horizon, giving radio a new global dimension for communication in the twentieth century.

On 12 December 1901,Guglielmo Marconiand his assistant, George Kemp, heard the faint clicks ofMorse codefor the letter "s" transmitted without wires across the Atlantic Ocean. This achievement, the first reception of transatlantic radio signals, led to considerable advances in both science and technology. It demonstrated that radio transmission was not bounded by the horizon, thus promptingArthur KennellyandOliver Heavisideto suggest, shortly thereafter, the existence of a layer of ionized air in the upper atmosphere (theKennelly-Heaviside layer, now called the ionosphere). Marconi's experiment also gave the new technology of "wireless telegraphy" a global dimension that eventually made radio one of the major forms of communication in the twentieth century.

In 1901, Marconi built a powerful wireless station at Poldhu, Cornwall, (corresponding IEEE Milestone) in preparation for a transatlantic test. The spark-gap transmitter fed a mammoth antenna array -- four hundred wires suspended from 20 masts, each 200 feet tall, placed in a circle. A similar station was set up on the American side of the Atlantic at South Wellfleet, Cape Cod.

Then a series of disasters struck. On 17 September a ferocious gale hit the Poldhu station, destroying the elaborate antenna system. A temporary one was put in its place a week later, but tests showed that it was too inefficient to reach the Cape Cod station. Consequently, before leaving England for North America, Marconi decided to set up his equipment at St. John's, Newfoundland, which was much closer to Poldhu. The decision proved academic in any case, because on 26 November, the day before Marconi's scheduled departure, the Cape Cod antenna blew down in a hurricane.

Landing at St. John's on 6 December, Marconi and his assistants set up their experimental apparatus on a table in the Signal Hill barracks near the harbor. Meanwhile, an improved antenna: had been installed at the Poldhu station, whose operators had instructions to send Morse code for the letter "s" from 3 to 7 pm (GMT) starting on 11 December. Marconi tested the winds on the 10th by sending aloft a kite trailing a wire antenna, but the kite broke loose. At the prearranged time on the 11th, Marconi and his assistants sent up a balloon, but heard nothing from their receiver. They next dispensed with the tuned receiver and tried a more sensitive detector, but the balloon broke loose. On the 12th, a strong gale still blew and carried away the first kite they sent up. The second kite, which trailed 500 feet of antenna wire, stayed up long enough for Marconi and Kemp to hear the transatlantic signals through a telephone earpiece connected to the receiver. Marconi's diary for that date has the simple entry, "Sigs. at 12:30, 1:10 and 2:20. 11 more signals were confirmed on the next day, Friday the 13th, but none on Saturday. On Monday the 16th, Marconi released the news to the press and then began packing for a new location because the Anglo-American Telegraph Company threatened legal action for violating its communication monopoly in Newfoundland.

Marconi's announcement met with enthusiastic acclaim, but also with some skepticism. After all, the only witness was George Kemp, hardly an impartial observer, and the signals were too weak to operate an automatic recorder. Two months later, though, Marconi received transatlantic signals of sufficient strength from Poldhu to operate a Morse inker in the presence of witnesses. (Although later knowledge of radio-wave propagation indicates that the Signal Hill reception occurred under inopportune conditions, recent historians have suggested that Marconi picked up a high-frequency harmonic on his un-tuned receiver.) In January 1902, between the time of the Signal Hill reception and the later verification, theAmerican Institute of Electrical Engineersheld theirannual dinner meeting in honor of Marconi. In attendance were such electrical engineering notables asAlexander Graham Bell,Charles Proteus Steinmetz, andMichael Pupin.Thomas Edison, who sent his regrets, called Marconi "the young man who had the monumental audacity to attempt, and succeed in, jumping an electrical wave clear across the Atlantic Ocean."

In the VLF band the wavelength are so long they are comparable with the height of the lowest ionospheric layer (about 50km). The ionosphere and the surface of the earth act as a waveguide allowing VLF signals to propagate worldwide.

Propagation at LF is mainly by means of a surface wave. A vertically polarized wave induces a current in the earth and this produces a component of electric field parallel to the ground. This causes the wave front to tilt forwards and follow the curvature of the earth. The induced earth currents take power from the signal and cause attenuation which places an upper limit on the distance the surface wave will propagate. This band is used for sound broadcasting for distances upto about 1500km.

MF signals are also propagated by surface waves. However, the amount of diffraction is proportional to wave length ,so the strength of the surface wave decrease with increasing frequency. Hence, MF signals do not propagate as far as LF signals by surface wave. Sky wave propagation increases the range particularly at night when ionospheric absorption is a minimum.

At HF the transmissions sky wave reflected from the ionosphere since there is virtually no surface wave at frequencies. The ionosphere is formed by ultra- violent and X-ray emission from the sun ionising the molecules in the upper atmosphere. There are several layers where the ionisation density is concentrated; in order of height these are designated the D,E,F1 and F2 layers. The ionisation causes refractive bending which may result in the wave returning to earth .It is possible to cover distances up to 4000 km with a single "hop" and worldwide coverage with multiple hops. Propagation via sky wave is very variable and is affected by variations of radiation from the sun. This variation exhibits daily, seasonal and an approximately 11 years cycle.

The sky wave mechanism does not operate at frequencies much above 30 MHz. Hence, radio communications at VHF and greater frequencies depends on direct free space propagation. If the wave travelled in a straight line then distances would be limited to true "line of sight" range. However, the radio refractive index of the troposphere varies with height due to changes in temperature, pressure and humidity. This causes a slight downward bending of the wave as it propagates. This enables propagation beyond the optical horizon. The refractive bending may be allowing the earth's radius is larger than its actual value. This is convenient is route planning because the wave may now be assumed to be propagating in a straight line.The amount of ray bending, and hence the Required increase in earth's radius, depends on the atmospheric conditions over the route. The average or "Standard atmosphere"requires the earth radius to be increased by a factor of 4/3.the true radius is 6400km and so the effective Earth's radius is: 4/3*6400km=8500km

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