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by John S. Belrose
International Conference on 100 Years of Radio -- 5-7 September 1995
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| Figure 6: Curve showing the variaton of the intensity of transatlantic signals for the month of January, 1906. Unity corresponds to a just audible message. Such a curve is certainly one of the first reocrdings of LF propagation data. (Fessenden) |
5. The First Radio Propagation Experiments
There is no evidence that Marconi made any serious attempt to systematically investigate the charactericistics of the HF, MF and LF portions of the radio frequency spectrum when he began the downward frequency trend, in his struggle to achieve transatlantic wireless communications. He did not do this until 20 years later, in the early 1920's, when attracted to the HF band by amateur radio operators. The radio amateurs had been banished to the then-believed useless frequencies higher than 1500 kHz.
The first record showing qualitatively the variation of the intensity of transatlantic messages transmitted between Brant Rock, MA and Machrihanish, Scotland, at night, during the month of January 1906 is reproduced in Fig. 6 [Fessenden, 1908]. Nothing at all was received that month during daytime.
It was found (measurements made during 1906) that absorption at a given instant was a function of direction as well as distance, since on a given night the signals received by stations in one direction would be greatly weakened, while there would be less weakening of the signals received by stations lying in another direction; and a few hours or minutes later the reverse would be the case. It was also found that variations of absorption on transatlantic signals appeared to have a quite definite relation with variations of the geomagnetic field, i.e., the greater the absorption the greater the magnetic variation [Fessenden, 1908].
Experiments were made between Brant Rock and the West Indies, a distance of 2735 km, during the spring and summer of 1907. Frequencies in the band 50 kHz to 200 kHz were used. It was found that the absorption at 200 kHz was very much greater than at 80 kHz, and that messages could be successfully received over this path in daytime at the latter frequency. Antenna radiation efficiency was an important factor for frequencies less than 80 kHz. No messages were received in daytime with the higher frequency.
The fact that these experiments were made during summer, that the receiving station was in the Tropics (high noise levels), and the fact that the distance, 2735 km was practically the same as between Ireland and Newfoundland was reported by Fessenden [1907]. After publication of the above results, Marconi, in early October, 1907 abandoned his previously used frequencies, and immediately succeeded in operating between Glace Bay and Clifden, a distance of more than 3000 km, the frequency being about 70 kHz. The same messages were received at Brant Rock, MA, a distance of nearly 4825 km. A little later Marconi moved to an even lower frequency, 45 kHz.