Early Electronic Television
This is part of an article that appeared in the Sept. 5, 1936 issue of L'illustration magazine, published in Paris. It describes options for using coaxial cable and microwave for interconnecting television stations. Many thanks to Jerome Halphen for translating it into English.
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
Frequency in KC
No. of Radio Stations
No. of Television Stations
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).