Aerospace and Electronic Systems Magazine March 2017 - 22
Coltano: The Forgotten Story of Marconi's Early Powerful Intercontinental Station
Box 4
Coltano: An outstanding record-setting station.
1. It has been the first intercontinental radiotelegraphic station in Italy.
2. It was considered the "most powerful in the
world" (see [25], [26]).
3. On Nov. 19, 1911, the maiden transmission
from Coltano to Glace Bay (5360 km away) set
a new distance record for wireless communications [25].
4. Assuming a uniform radiation pattern, the distance between the furthest points at which the
signals could be received was 10720 km, i.e.,
more than one-fourth of the Earth circumference.
5. The area around Coltano covered by station's
signals was equivalent to about one-sixth of
the Earth surface.
6. The transmission from Coltano to Massawa
went through more than 2000 km of Sahara
and was the first across a desert. This proved
wrong the belief that ground wave propagation
is impossible over dry (poorly conductive) soil.
So, as early as 1919, the radio station was judged obsolete
by the IRN and a more powerful and efficient VLF station was
planned for transcontinental commercial service with North America. A renovation of the Coltano radio center was designed and
personally supervised by Prof. Giancarlo Vallauri5 (Figure 11), a
prominent scientist and director of IRN's Electrotechnical and Radiotelegraphic Institute, based in the nearby city of Leghorn (see
[27, p. 322]). Work started in 1920 and went on smoothly until
early 1923, when the Italian Government decided to entrust the
management of the new VLF station (referred to as the "new radio") to a private company from 1924. As a consequence, the IRN
suspended its commitment and, though the main transmit facility
was already operational, some planned upgrades were left unfinished.
The new radio, which is described in great detail in [9], consisted of a main building (labelled "6" in Figure 7 and also shown
in Figure 12), which hosted the transmit and control equipment,
and a huge curtain-type antenna ("9" in Figure 7), and some sub5
22
Giancarlo Vallauri (1882-1957) was an Italian admiral of the
IRN, a researcher in radio electronics and a university professor.
In 1916, he founded the Electrotechnical and Radiotelegraphic
Institute of the IRN, in Leghorn, and was in office as director
until 1926. From 1920 to 1923 he directed the design and construction of the new VLF radiotelegraphic station in Coltano.
Later, he was the dean of the Faculty of Engineering at the University of Pisa (1923-1926), the president of the Turin Politechnic, chairman of EIAR (the former Italian public broadcasting
company), and of CNR (the National Research Council). Vallauri was also a talented researcher. He achieved fundamental
results on ferromagnetism and derived the "Vallauri's equation"
for vacuum tubes.
Figure 11.
The new radio. Left: the two 60-meter pylons of the SW antenna (1930);
right: the station building (1923) in front of one of the 250-meter masts
of the VLF curtain antenna.
sidiary buildings for the accommodation of the IRN personnel in
service at the station and a garage (items labelled "2"-"5" in Figure
7). Following the evolution of radio technology, spark gap technology (like that used in the old Marconi's station) was abandoned in
favor of newer and more efficient solutions. So, the station was
equipped with a couple of modern VLF transmitters: i) a 250 kW
transmitter based on a Béthenod-Latour alternator, i.e., a rotating
machine which generated a sinusoidal current, to be employed for
producing a continuous wave in the VLF band, while fine frequency tuning was carried out via a rotation speed control; ii) a 350 kW
transmitter, featuring two static Poulsen converters that used an
electric arc to convert direct current into a radio frequency continuous wave. Any of the two transmitters, whose nominal frequency
was 18.750 kHz (16,000 m wavelength), could be selected to feed
a huge curtain antenna (labelled "9" in Figure 7), having a square
base with 420 m-long sides, supported by four masts; each one was
250-m high and 80-ton heavy (Figure 12a and b), with a sphericalshaped bottom end. The antenna's pavillon consisted of a net of
phosphorous-bronze wires 3.4 mm thick and spaced about 15 m
apart from each other. The net was reinforced with steel cables for
the outer wires, yielding a total weight of about 4 tons [11]. The
radiating pavillon net was linked to the transmitters in the station
building by a fan-shaped descent. The antenna was designed so as
to provide at least 35,000 meter-ampere,6 which was the minimum
strength of the transmitted signal for establishing the service with
North America. Assuming a radiation height of 165 m, the effective current intensity resulted 212 A. Therefore, in order to radiate
a signal with wavelength of 16,000 m, using a voltage of 70,000
V, the antenna was designed so as to yield also the required static
capacity which amounted to 25 nF.
The four masts were quickly erected in the summer of 1922.
The construction of this audacious structure in such a short time
represented a formidable challenge that amazes even today. The
assembling procedure consisted in raising and putting in place 5
m-high trunks, one by one, for each mast. This was done by using
a special platform that was placed on top of the incomplete mast,
6
This coefficient, which was once a popular measure of the
strength of a radio transmitter, is given by the product between
the radiation height of the antenna and the maximum antenna
current.
IEEE A&E SYSTEMS MAGAZINE
MARCH 2017
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