International Conference on 100 Years of Radio -- 5-7 September 1995
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by John S. Belrose VE2CV
International Conference on 100 Years of Radio -- 5-7 September 1995
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4. The First Transatlantic Experiment
On 12 December 1901 signals from a high power spark transmitter located at Poldhu were reported to have been received by Marconi and his assistant George Kemp, at a receiving station on Signal Hill, near St. John's, Newfoundland. The signals had traveled a distance of 3500 kilometres. Even at the time of the experiment there were those who said, indeed there are some who still say, that he misled himself and the world into believing that atmospheric noise crackling was in fact the Morse code letter 'S'.
A little later, in February 1902, when Marconi returned to England on the SS Philadelphia, using a tuned ship-borne antenna, he received signals using his filings coherer from the same sender up to distances of 1120 km by day and 2500 km by night. Even these distances are rather remarkable considering the receiving apparatus he used.
We discuss here in detail that first transatlantic experiment.
The Poldhu Station
Marconi's ambition at the turn of the century to demonstrate long-distance wireless communication, and develop a profitable long-distance wireless telegraph service, led to his pragmatic proposal in 1900 to send a wireless signal across the Atlantic. He conceived a plan to erect two super-stations, one on each side of the Atlantic, for two-way wireless communications, to bridge the two continents together in direct opposition to the cable company (Anglo-American Telegraph Company). For the eastern terminal, he leased land overlooking Poldhu cove in southwestern Cornwall, England. For the western terminal the sand dunes on the northern end of Cape Cod, MA at South Wellfleet, was chosen.
The aerial systems comprised 20 masts, each 61 metres high, arranged in a circle 61 m in diameter. The ring of masts supported a conical aerial system of 400 wires, each insulated at the top and connected at the bottom, thus forming an inverted cone. Vyvyan [1933], the Marconi engineer who worked on the 1901 experiment, when shown the plan, did not think the design sound. Each mast was stayed to the next one, and only to ground in a radial direction, to and away from the centre of the mast system. He was overruled, construction went ahead, and both aerial systems were completed in early 1901.
However, before testing could begin catastrophe struck, the Poldhu aerial collapsed in a storm on 17 September, and the South Wellfleet aerial suffered the same fate on 26 November, 1901.
At Poldhu Marconi quickly erected two masts and put up an aerial of 54 wires, spaced 1 metre apart, and suspended from a triadic stay stretched between these masts at a height of 45.7 m. The aerial wires were arranged fan shaped, presumably insulated at the top, as was his conical wire aerial, and connected together at the lower end, see Fig. 1. This photograph has been published and republished, and clearly one can see only 12 wires -- but the view generally held is that the aerial system as described above by Vyvyan [1933) is right, that is there were 54 wires, and the photograph has been retouched.
The antenna was driven by the curious two stage spark transmitter, previously discussed. There were many problems in getting it to work at the high power levels desired [see Thackeray, 1992]. Our principal concern here is the frequency generated by the Poldhu station. The oscillation frequency is determined by the natural resonant response of the antenna system, which includes the inductance of the secondary of the antenna transformer T2, since in effect the antenna system is a base-loaded monopole (see Fig. 2).
The primary of this transformer consisted of 2 7/20 wires in parallel, the secondary consisted of 7 or 9 wires of 7/20 wire in series. Fleming's sketch indicates 9 wires; Entwisle [1922] said there were 7 wires. The inductance values for this transformer have long been debated, since the original transformer is lost, there are no drawings, and reports about them differ [Thackeray, 1992]. G. Garratt made a copy of Ls (the secondary of this transformer) and measured its inductance to be 6 x 10-6 H [see Ratcliffe 1974].
While the inductance Ls changes the resonant frequency, the exact value does not change the conclusion reached in our study. For Ls equal to 6 x 10-6 H, we have modelled Marconi's Poldhu antenna, assuming the fan comprised 12 wires. According to the antenna analysis code MININEC, the resonant frequency of the antenna system was 850 kHz.
A number of scientists and engineers interested in the actual frequency or frequencies radiated by this first high power transmitter at Poldhu have discussed the possibility that the aerial transformer was overcoupled, resulting in a double-humped frequency/amplitude response. We do know that Fleming tuned the primary oscillatory circuit by varying the discharge capacitor C2 to maximize the aerial current. Since our best estimate for the component values (C2 = 0.037 muF and Lp = 8 x 10-7 H) would result in a resonant frequency of 925 kHz, it seems logical to conclude that the overall system response would result in a single peak centred on the resonant frequency of the aerial system, viz. about 850 kHz.
Historians have also speculated that the transmitter might also have radiated a high-frequency signal as well, since an HF signal would have been more suitable for transatlantic communications (to be discussed), see for example Ratcliffe [1974]. If Marconi had used a thin wire transmitting antenna at Poldhu, this antenna would indeed have radiated efficiently at odd harmonics of the fundamental resonant frequency. But for our model the antenna is inductive for all frequencies greater then the fundamental resonant frequency response of the antenna system. One must conclude therefore that the Poldhu spark-transmitter system radiated efficiently only on the fundamental oscillation frequency of the tuned antenna system -- about 850 kHz.
Marconi himself has been evasive concerning the frequency of his Poldhu transmitter. Fleming in a lecture he gave in 1903 said that the wavelength was a 1000 feet or more, say, one-fifth to one-quarter of a mile (820 kHz is the generally quoted frequency). Marconi remained silent on this wavelength, but in 1908 in a lecture to the Royal Institution he quotes the wavelength as 1200 feet, see Bondyopadhyay [1993].
Reception on Signal Hill
For his transatlantic experiment, Marconi decided to set up receiving equipment in Newfoundland. In December 1901 he set sail for St. John's, with a small stock of kites and balloons to keep a single wire aloft in stormy weather.
A site was chosen on Signal Hill, and apparatus was set up in an abandoned military hospital. A cable was sent to Poldhu, requesting that the Morse letter " S " be transmitted continuously from 3:00 to 7:00 PM local time.
On 12th December, 1901, under strong wind conditions, a kite was launched with a 155 m long wire. The wind carried it away. A second kite was launched with a 152.4 m wire attached. The kite bobbed and weaved in the sky, making it difficult for Marconi to adjust his new syntonic receiver which employed the Italian Navy coherer. "Difficult" I will accept, but how he determined the frequency of tuning for his receiver is a mystery to me. Whatever, because of this difficulty, Marconi decided to use his older untuned receiver. History has assumed that he substituted the metal filings coherer previously used with this receiver for the newly acquired Italian Navy coherer, but Marconi never really said he did [see Phillips, 1993]. He referred only to the use of three types of coherers.
Despite the crude equipment employed, and in our view the impossibility of hearing the signal, Marconi and his assistant George Kemp convinced themselves that they could hear on occasion the rhythm of three clicks more or less buried in the static, and clicks they would be if heard at all, because of the low spark rate. Marconi wrote in his laboratory notebook: Sigs at 12:30, 1:10 and 2:20 (local time). This notebook is in the Marconi Company archives and is the only proof today that the signal was received.
The Enigma
Today we know that signals (depending on frequency used) can indeed travel across the Atlantic, and far beyond. But in 1901, anyone who believed that they could, and did, believed so as an act of faith based on the integrity of one man -- Marconi.
If 850 kHz was indeed the frequency used, the tests took place at the worst time of day, because the entire path would have been daylight, and the daytime skywave would be heavily attenuated, even though it was a winter day, in sunspot minimum period, and there were no magnetic storms at the time, or for ten days before. The day-time absorption of an ionospherically-reflected signal is a maximum in the LF/MF band. Ratcliffe [1974] has deduced that, from a knowledge only of propagation conditions, reception on Signal Hill is consistent with the observed limiting ranges of reception on the ship only if the untuned landbased receiver was 10-100 times more sensitive than the tuned receiver on the ship.
It is therefore difficult to believe that signals could have been heard on Signal Hill, since the receiving equipment after all consisted of a long-wire antenna, coupled to an untuned receiver which had no means of amplification whatsoever, and the type of detector used was less sensitive and its performance unpredictable compared with Fessenden's barretter detector, or the galena crystal detector which evolved a few years later.