Although one of Marconi's first spectacular demonstrations of communication by radio waves used a frequency around 1000 MHz, in 1896, his work on LF, MF and HF bands prevented him from experimenting further with VHF and above until 1928. Returning to Italy to assume the Presidency of the Italian Royal Academy, he took his research staff and their work with him. The Italian government requested him to experiment with communications in the low VHF region (just above 30 MHz) between Sardinia (Golfo Aranci) and the Italian mainland at Fiumicino. Using an effective radiated power of about 40 kW, Marconi demonstrated that communication was possible over a path of 270 km, about 8 times the optical range. Among these early observations was the fact that signal strengths in the winter were some 20 dB less than in summer.
Marconi first presumed that the radio waves were being refracted within the troposphere, saying in 1930:
From measurements effected recently it would seem that along the route between Sardinia and the Italian mainland this wave is refracted and contained within a space lying between the surface of the earth and a layer situated somewhat lower than the Heaviside layer.
At this time, theory was based on Watson's solution of diffraction around a smooth spherical earth (1919), which predicted an exponential decrease in signal strengths beyond the optical horizon of about 1.2 dB per mile (at 500 MHz). Thus, beyond line of sight propagation at VHF and above was considered to be a freak phenomenon, rather like optical mirages.
Further pioneering work at VHF and higher frequencies was carried out by Uda (Japan, 1930s), Pistor (Germany, 1930), and Clavier & Gallant (who established the first 2-way communication link across the Channel in 1933, using a frequency of 1500 MHz). However, it was still technically very difficult to construct transmitters and receivers capable of working reliably and accurately at these higher frequencies.
In 1932, Marconi and his team of scientists and engineers started a series of experiments designed to extend the range of communication at VHF and above. Using the steam yacht Elettra and stations on the Italian mainland, they demonstrated repeatedly that signals could be received reliably to about 3 or 4 times the optical range, although fading became deep and relatively rapid once beyond line of sight. They established that the angle of arrival of these signals was tangential to the horizon and in the direction of the transmitter, the basic geometry of tropo modes of propagation.
Unfortunately, attempts in 1932 to achieve a path of 400 km (between Rocca di Papa, Rome, and the Elettra in Venice Harbour) failed. In the autumn of 1933, though, Marconi achieved a total distance of 258 km (9 times optical) in spite of two groups of high hills in the path. This was probably the first evidence of tropospheric propagation which could not be explained by simple diffraction and refraction. This was reported to the Royal Academy of Italy on 14th August 1933, and an account has been given by G A Isted, one of Marconi's team at the time (see below).
BBC research also started in 1932, with the establishment of an experimental transmitter using 38.7 MHz at Broadcasting House, London, which was assessed at distances of up to 100 miles. The inauguration of BBC television transmissions in 1936 brought regular reports of signal reception up to 200 miles away, and the BBC Research Department conducted tests over short periods at up to 500 miles distant.
The massive effort devoted to the development of radar systems by the USA and UK during the Second World War brought the next fruits. Although much of this work focussed on ducting tropo, which was beautifully demonstrated in many of the early radar experiments, interest in non-ducting modes was also fostered. The most accessible account of this work is the compilation by Kerr, which is still to be found in some bookshops (including Foyle's, Charing Cross Road, London). The contribution of amateurs, particularly Ross A Hull in the period 1934-1937, has been widely acknowledged.
Tropo propagation beyond line of sight became commercially important in 1949, when the USA licensing authority had to freeze the introduction of new TV stations because co-channel interference proved to be much greater than had been expected from the smooth sphere theory. A rapid growth in research resulted in the Booker-Gordon theory (1950), and later signal contributions by many others including Tatarskii. The BBC conducted further experiments between 1946 and 1957, using frequencies around 90 MHz, and others between 45 MHz and 560 MHz, and these are described by Rowden et al.
Eventually, tropo scatter links were used for commercial and military communications: early systems were installed in 1953, and they were popular during the 1960s and 70s, before satellites took over. Amateurs took advantage of the availability of VHF equipment following the Second World War, and from the 1950s onwards have regularly worked well beyond line of sight on VHF and above, particularly using single sideband (SSB, mode J3E) and Morse (CW, mode A1A).
So the next time that someone suggests that VHF propagation is only 'line of sight plus a third', tell them that they are 70 years out of date, and that the BBC knows they are wrong too!
Bibliography:
Bullington K (1955) Characteristics of beyond-the-horizon radio transmission. Proceedings of the IRE Oct 1955, 1175-1180.
Isted G A (1958) Guglielmo Marconi and communication beyond the horizon: a short historical note. IEE Symposium on Long-Distance Propagation above 30 Mc/s, 28 Jan 1958. Pp. 79-83.
Kerr D E (1951)(editor) Propagation of Short Radio Waves. Reprinted by Peter Peregrinus, 1987, ISBN 0-86341-099-5.
Rowden R A, Tagholm L F & Stark J W (1958) A survey of tropospheric wave propagation measurements by the BBC, 1946-1957. IEE Symposium on Long-Distance Propagation above 30 Mc/s, 28 Jan 1958. Pp. 84-90.
- and the books cited on the NDT Inner Sanctum page.
Last updated 27 Dec 1998
Howard Oakley
Mail howard@quercus.demon.co.uk