Aerospace and Electronic Systems Magazine March 2017 - 20

Coltano: The Forgotten Story of Marconi's Early Powerful Intercontinental Station

Box 3
Marconi's bent antennas: directive or deceptive?

Figure 7.

Sketch and plan of the Coltano radio center in 1923 [9]: Marconi's 1911
station (1) and two VLF bent antennas (7), (8); Vallauri's 1923 station
(6) and the VLF curtain antenna (9).

Since the early 1900s Marconi began
experimenting on antenna structures capable
of concentrating the radiation in a narrow
sector [11]. During an experimental campaign
carried out in 1904 in some coastal sites of
the British Islands [16], Marconi verified that,
for frequencies below 2 MHz, a bent antenna
like that in Figure 8, has a directional behavior
[17]. In 1905, he promptly patented this new
antenna [18]. The radiation pattern depicted
in Figure 8, from [17], was derived using a
60 m-long bent antenna at 600 kHz (500 m
wavelength). A couple of such bent antennas
were thus installed in Coltano (Figure 9): the
Colonial Aerial ("7" in Figure 7) was aimed
at Massawa (Eritrea) (Figure 9a), and the
Continental Aerial ("8" in Figure 7) was aimed
at Glace Bay (Canada) (Figure 9b). (The name,
"Continental Aerial," was assigned later in
the 20s, when this aerial was only used for
links toward West Europe.) It is worth noting,
however, that the pattern in Figure 8 was
measured 260 m (about half wavelength)
far from the transmitter, i.e., in the nearfield region (which was a concept not yet
understood at the time). Therefore the "eightshaped" directive pattern in Figure 8 is not
valid for long-distance communications. In this
connection, Professor Ambrose Fleming [19]
analytically confirmed the directivity feature
discovered by Marconi, but also remarked that
"theory points to the fact that at extremely
large distances the asymmetry [of the radiation
pattern w.r.t. the isotropic one] tends to vanish,
and that any bent oscillator has no asymmetry
of radiation [i.e., directive behavior] for very
large distances." [4].

Figure 8.

Bent antenna, like the type used in Marconi's station in Coltano, and
its radiation pattern diagram, plotted by measuring the received signal
strength around a circle with radius 260 m and centered on the transmitter, for a transmit frequency of 600 kHz approx. [17].

20

IEEE A&E SYSTEMS MAGAZINE

MARCH 2017



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