First Wireless Contact

By: Joe, VO1NA
(Originally published in the December, 1996 issue of 'The Canadian Amateur')

Amateurs are generally credited with discovering short wave propagation, but the original (and self-proclaimed) amateur Marconi may have unknowingly discovered it by accident long before we did. This article commemorates the 95th anniversary of Marconi in Newfoundland.

December, the 12th month, has a special significance in the radio history of Newfoundland. It was on the 12th hour of the 12th day that Marconi received the first transatlantic radio signal on Signal Hill in 1901. Many accounts of the event, including Marconi's lectures to the Royal Institution, have been published. Interpreting these accounts in retrospect can shed some insight as to wavelength of the signal which crossed the Atlantic. It may have been much shorter than originally thought.

Contemporary references to the 1901 Poldhu transmitter have specified the wavelength as 1200 feet (366 metres); others have placed it between 2000 and 3000 metres. Since the transmitting aerial was relatively small at these wavelengths, it was unlikely to radiate these long waves efficiently unless an elaborate grounding system was used. In addition, since the signals were heard during the daytime when D layer absorption is substantial, it is unlikely that there was sufficient propagation of these waves across the Atlantic to be detected with the receiving systems Marconi was using atop Signal Hill. This point and the subsequent difficulties Marconi experienced in establishing a transatlantic long wave circuit from Glace Bay have been cited by authorities who remain skeptical that Marconi heard the Poldhu signal in Newfoundland.

In addressing the Royal Institution, Marconi stated he could hear the Poldhu signals using untuned detectors, but not with a tuned receiver. Marconi attributed this to 'the varying capacity of the aerial wire' as the kite supporting it moved about in the wind. Two months later, he determined that the maximum distance over the Atlantic that the Poldhu transmitter could be detected with a tuned long wave receiver during the daytime was about 700 miles. This suggests that long waves were not detected in Newfoundland on December of 1901 and indeed, nothing was heard on the long wave receiver.

What was heard was detected on an untuned receiver, which, unlike the tuned circuit, would not reject shortwaves. It is not only possible, but quite likely that Marconi's transatlantic signal comprised these shortwaves. Such signals could be detected by an untuned coherer circuit, be reflected by the ionosphere and could cross the Atlantic during daytime, whereas the longer waves would be absorbed. Finally, shortwaves would be generated by a spark transmitter as harmonics and efficiently radiated by the Poldhu aerial,even if the ground system had been inefficient. No doubt, more evidence will emerge in support of this hypothesis.

To estimate the wavelength would entail a detailed analysis of the facts and would be speculative at best. Since 1901 corresponded to a sunspot minimum, a wavelength shorter than 20 metres would be unlikely. D-layer absorption would probably limit the wave to a maximum of 60 metres. This range could be narrowed by computer modelling of the ionospheric propagation of the conditions at the time.

It would be interesting to perform an authentic replication of the experiment and to establish the radiation characteristics and frequency response of the Poldhu transmitter and antenna. Unfortunately, this would be impractical since spark transmissions are no longer permitted and an untuned receiver would be swamped by existing man made RF.

On the other hand, a wavelength and power level could be selected on the basis of what was likely transmitted by Poldhu and received by Marconi in 1901. This signal could be transmitted again from Poldhu and attempts could be made to receive the signal at the same site using exactly the same apparatus Marconi used, but with one minor modification. Since the coherers which were used to detect the signals in 1901 are known to behave as diode detectors, it may be possible to design a direct conversion coherer receiver. This could be used to receive CW signals transmitted from Poldhu without the aid of amplification; an idea that is being investigated by the Technical Committee of the Society of Newfoundland Radio Amateurs (SONRA).

SONRA operates the Club station, VO1AA in Cabot Tower on Signal Hill, less than 100 metres from where Marconi made history in 1901. SONRA will be commemorating the 101st anniversary of the event in December by conducting a 24 hour operation on the 12th. Contacts will be attempted with Hobart on the other side of the world, and of course Poldhu, England. Guest operators are welcome to use VO1AA.

Note: Dr. John Belrose, VE2CV, has advised me that a spark transmitter cannot radiate harmonics. The term 'spurious radiation' should replace 'harmonics' in the fourth paragraph.