Marconi – demonstrating transatlantic radio communication
Many dates have been suggested as representing the ‘birth’ of
radio, among them the first microwave experiments of Hertz (1887)
with which the predictions of Maxwell were confirmed, and the
initial demonstrations by Marconi of his system for ‘wireless
telegraphy’ in 1895-96.
Certainly, by the turn of the century, it was clear that ‘wireless’
was a useful technology for short-range communication, particularly
for shipping when beyond semaphore range. Indeed, equipment was
coming to be installed quite widely for this purpose, and increaing
levels of interference had led to the controversial patenting by
Marconi of tuned transmission or reception. Until this time,
emissions from the spark transmitters used had simply spread across
a spectrum dictated only by the largely accidental characteristics
of the aerial system – this approach seems to be enjoying a
resurgence in the guise of Ultra Wide Band (UWB) technologies!
A particularly important date in the history of radio, however,
must be the 12th December 1901, on which transatlantic communication
was first achieved by radio. Having sucessfully demonstrated
communication over non-line-of-sight paths of more than a hundred
miles, Marconi gambled that truly long-distance communication would
be limited only by transmitter power. If such a result could be
demonstrated, the future of his fledgling company would be secure.
One of the shortest possible transatlantic paths was chosen for
the tests, between the small village of Poldhu, on the coast of
Cornwall in the UK, and St. John's, Newfoundland. The transmitter at
Poldhu was designed by Professor Ambrose Fleming of University
College, London, a prominant electrical engineer of the time. A
cascade of resonant circuits was employed, driven by a 25 kW
alternator.
Poldhu Transmitter
When the alternator circuit was keyed, C1 was charged via T1
until the voltage was sufficient to trigger the spark gap, and
generate a high-frequency oscillatory current flow in T2. The
secondary of this transformer allowed C2 to charge, in turn, until
it too is discharged via the second spark gap and oscillations at
the radiated frequency were coupled to the aerial systen via T3.
Perhaps surprisingly, the frequency at which the transmitter was
intended to operate remains unclear, with estimates ranging between
about 150 to 850 kHz. The uncertainty relects the lack of
understanding, at the time, of the way in which radio propagation
varies with frequency. The aerial system was intended to consist of
an inverted cone of wires, some 200' high, supported by twenty
wooden masts. This elaborate system fell victim to gales shortly
before the scheduled tests, and was replaced by a simpler
arrangement using the few surviving masts.
The receiving system at Newfoundland was extremely simple,
consisting of an untuned detector connected between a simple 400'
wire antenna (supported by balloon or kite) and earth.
Marconi in Newfoundland
Two types of receiver were used in the tests: the standard
arrangement for the time employed a ‘coherer’, a device in which
a tube of loosely-packed iron filings with a contact at each end was
coupled to the aerial system. The (very weak) high frequency voltage
due to the received signal caused the filings to stick together
slightly, reducing the electrical resistance of the tube and causing
an indicator (bell or pen recorder) to operate. The opportunity was
also taken to test a new and more sensitive form of detector,
effectively a solid-state rectifier. This device consisted of a
junction between dissimilar metals, and was connected to a telephone
earpiece, allowing far greater sensitivity than was possible with
the coherer. The arrangement would be familar today as the ‘crystal
radio set’ still popular as an educational toy.
The signal used in the tests was the morse letter ‘S’,
consisting of three ‘dots’. This rather uninteresting choice was
dictated by the liklihood of the transmitter breaking down should
the operator be so ambitious as to attempt to send a lengthy ‘dash’!
In Newfoundland, the Marconi team initially had further antenna
problems due to high winds, and lost an assortment of kites and
balloons. Finally, however, at mid-day on December 12, Marconi
recorded that signals had been heard, and confirmed by his
assistant.
Deteriorating weather, and the threat of legal action by the
transatlantic cable firm holding the local monopoly, ensured that
the test could not be continued. The results of the tests were,
however, given enormous publicity, and the success of the Marconi
company seemed assured. There was, nevertheless, considerable
scientific scepticism as to whether the signals had actually been
heard. Debate continues to this day as to what had been heard, and
how.
If the transmiter had been working at the assumed frequency, the
majority of the signal would have been absorbed in the (then
unknown) D-layer of the ionosphere – the same layer that protects
listeners to medium-wave (AM) radio stations from interference
during the day. One possible explanantion is that the transmitted
signal was so rich in harmonic energy that the untuned receiver was
actually responding to signals radiated at a much higher, shortwave,
frequency that escaped absorption by the D-layer, and was reflected
from higher in the ionophere to complete the Atlantic crossing.
Whatever the truth, and despite the scepticism, by the close of
the decade radio was in regular commercial use for both fixed
circuits and for maritime use.
contributed by Richard Rudd
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