The current dilemma : will
Radiovision networks use cables or microwaves ?
The future appears certain and clear cut,
where reporters using Zworykin or Farnsworth cameras will show us
in Paris a bullfight taking place in Seville, or show us with
sight and sound an event happening in the provinces; but already
today a new problem faces the engineers : the transmission of
pictures and sound coming from remote locations. A quite
unexpected dilemma presents itself : will television be
transmitted by cables or using the airwaves ? With or without
wires ?
By a curious twist of fate
and a return to the past, the cable is revealing itself as
television's most precious ally. The "ultra-short" wave
of less than 10 meters wavelength now used by all transmitters,
has a common behavior with light in the sense that it cannot go
through obstacles, the most formidable one being the earth's
curvature. Locating the transmitting antennas at a height of 300
meters, such as the Eiffel tower or 200 meters in the case of
Baird at the Crystal Palace only gives a range of 50 to 60 km,
which is the limit of the horizon
for such heights. Furthermore, the high frequency waves have
already so much trouble reaching out to these distances, that it
is quite useless to try raising the transmitting antenna at a
greater height as the power gain could not keep up with the
further away horizon.
Then there is the problem of
connecting the antenna to the broadcast studio. A cable link
cannot be avoided, and its role will be to "telegraph"
the camera's signals to the transmission site, i.e. as close as
possible to the transmitter. A special wire (known as a feeder)
will anyway be necessary to connect the transmitter located at the
foot of the tower with the antenna 300 m above it. The cable carrying
the high frequency modulation immediately experiences,
multiplied a hundredfold, the difficulties which were encountered
by the telegraph cable when it was decided to transform it into a
voice carrying telephone cable. The same difficulties were also
encountered when the telephone cable carrying only 3000 Hz voice
signals had to become a high quality studio to transmitter link
capable of carrying 12000 Hz music
signals. It is already a feat to have been able to carry visual
modulation signals at a frequency of 500,000 Hz (an average value with respect to the 1
million Hz normally required by the 240 line television signal) between
the cities of London and Birmingham (80 Km) and Berlin and Leipzig (180
Km). One could be enthusiastic about such wonders if we didn't know that
it is only the very beginning. It will be necessary to "pass through
the cable" the frequencies of well over the one million Hz delivered
by the electronic cameras. This will be achieved.
Calculating the structure of
such cables is incredibly difficult and can only be mastered by using the
most abstract theories of electromagnetic behavior. Their
"dielectric" (inside insulator), their shape (diameter of
concentric layers), the nature of the metals used and the diameter of the
strands are determined with great precision. These conductors don't carry
electric current but they "channelize waves".
Predicting the future by a
few years :
The Post Office ministry has
established since a few years an ultra-short wavelength service (4 m)
between the city of Nice and Corsica for current needs. It was foreseen
to extend the service all the way to Paris by using elevated locations
having line of sight visibility from station to station. This
resurrection under a modern form of the antique Chappe telegraph let us
imagine that one could add to the current ultra short wave service (1 m
wavelength) a microwave television service using 10 cm wavelengths. Only
173 relay stations would be necessary to cover France entirely.
One could think of these
special cables as carriers of materialized "Hertzian rays".
Twenty years from now, they may bring to your desktop the face of your
phone caller. These cables will probably not constitute a continuous
"network". They will connect you to a neighborhood
"exchange", which will be a transmitting site, as today these
cables connect the television studio to the transmitter. We are far from
having exploited all the possibilities of the electromagnetic spectrum
with wavelengths under 10 meters. The following chart shows what rich
resources wavelengths of around 10 cm offer to television.
Wavelength in CM
100,000
10,000
1,000
100
10
1 |
Frequency in KC
300
3000
30,000
300,000
3,000,000
30,000,000 |
No. of Radio Stations
27
270
2,700
27,000
270,000
2,700,000 |
No. of Television Stations
0
2
20
200
2,000
20,000 |
This chart shows the
progression in frequency as the
wavelength diminishes. An
estimation is given of the number of broadcasting stations which could be
accommodated in each of the intervals.
These "microwaves",
directed by mirror reflectors are used for telephone service between
France and England in the Pas-de-Calais. In a similar fashion,
"ultra-short" 4 m waves are used to link the Mont Agel near
Nice with Monte-Cinto in Corsica, or Saint-Inglebert (France) to Lympne
(UK). The only difficulty is to produce enough power, but the constant
progress of the electronic art authorizes many hopes. We may see reborn
the towers of the antiquated Chappe telegraph, which from hilltop to
hilltop will relay, select and distribute "microhertzian" beams
of radio waves anywhere in the territory. This is a plausible projection
of what the future "vision telephone" central exchanges may
look like. In a drawing inspired by a study made by a Post Office
engineer, Mr Loeb, we show what a microwave link between Paris and
Ajaccio (Corsica) could look like.
This project is not some
dream by Jules Verne, but a very precise plan containing all the
essential facts to make possible the goal to see and be heard at a
distance using the same Hertzian link. The "central exchanges"
of such a network would be relay towers located on elevated locations of
the territory. A pylon tower of 300 m height costs 600.000 Francs (the
Eiffel tower is a luxury). With a useful range of 60 km, the transmitter
covers a circular area of 10.000 square km. Factoring in the partial
overlap of the coverage zone, necessary for inter-site transmission of
signals, a total of 183 towers would be sufficient to cover with
ultra-short waves or microwaves the 550.000 square km of France. These
are the conclusions amiably given to me by Mr Loeb.
An additional advantage of
broadcasting television signals with microwave beams is that the signal
becomes impervious to atmospheric disturbances to which our eye is much
more sensitive than the ear. The daily press and l'Illustration have
recently published wire photos transmitted by radio from Djibouti
(Africa). The pictures were marred with white or black spots. The radio
waves gather these parasitic signals as they bounces back and forth
between the ground and the ionized Heaviside layer, in a constant state
of flux. In television transmissions, the eye as well as the photographic
plate are sensitive to parasitic degradations which the auditory system
can't hear. Like ultra-short waves and microwaves, television signals
transmitted through cables are immune to parasitics, but both
technologies await progress in amplification of very high frequencies to
reach their full potential. For the time being, cables are ahead of radio
waves for carrying television signals over long distances.

A view showing the parabolic
microwave projectors used between Saint-Inglebert (Pas-de-Calais)
and Lympne (Great Britain).
The 12 cm wavelength
microwaves are shaped into a parallel beam by the 3 m diameter
metal reflectors. The transmitting or receiving antenna is placed
at the focal point of the reflector. The antenna's size is about
the same as the microwave's wavelength (12 cm). |