Wide-band high frequency signals from Poldhu? The propagating spectrum and terminal equipment revisited

In 1901 Marconi attempted to transmit radio signals across the Atlantic Ocean. The most powerful spark transmitter ever built was operated at Poldhu in Cornwall. While on Signal Hill in St. John´s, Newfoundland, at about midday, Marconi listened for Poldhu with the very simplest untuned receiver. It is therefore hardly surprising that his claim to have heard the famous Morse code “S” signal was greeted with profound skepticism. The present authors discuss the equipment and propagation aspects and show that the Poldhu transmissions probably centered on 475 and 540 kHz, the power summed over these spectral peaks being about 1 kW. Total energy radiated was approximately 1.8 kW, with a substantial proportion at frequencies above 2 MHz. A summation of the predicted high frequency sky-wave propagation has given an average received power at St. John´s of 50 pW, with an upper decile of 150 pW. The authors also determined the probable median noise power in the receiver to be 200 nW, with a lower decile of 50 nW, and resultant signal to noise ratios of -36 and -30 dB. If 20 dB gain is attributed to the peak/average power ratio, and 10 dB aural discrimination gain is allowed, the median and lower decile effective signal-noise ratios would be -6 and 0 dB, and +5 dB at a 1% probability. The authors therefore argue that in December 1901, Marconi is likely to have received high frequency wide band signals, spurious components of the spark transmitter output, propagated across the Atlantic by sky waves near the maximum usable frequency