3,

THE BELL SYSTEM’S TRANSCONTINENTAL FM RELAY NETWORK,
COMPOSED OF 107 STATIONS OPERATING AT 4,000 MC.,
BEGAN SERVICE AUGUST 17th

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THE Bell System’s coast-to-coast radio relay system was opened officially on August 17 with a ceremony at the New York headquarters. Built at a cost of over $40 million, the network has been under construction for three years. Its route takes it through many major cities between New York and San Francisco.

The cross-country network is made up of 107 stations spanning 2,992 miles. Operating frequencies are in the SHF band, from 3,700 to 4,200 mc. Eventually, 6 channels in each direction will be available, each capable of carrying a television program or several hundred telephone conversations

Repeater stations are spaced from 19 to 50 miles apart, depending on terrain. with an average separation of slightly more than 28 miles. Each station provides signal amplification of over one million, yet has an actual power emission of less than 1 watt to the antenna for each channel.

Various types of towers are used, according to the requirements of location, topography, weather, and height required. Structures of both steel and concrete are employed varying height from 2.5 to 415 ft. When a concrete tower is used, it houses the associated equipment. On the ground floor, an emergency gasoline-driven generator is installed. Second and third floors are filled with batteries and power apparatus. Amplifying, control, and test equipment are located on the top floor.

Most towers are normally unattended. However, an alarm system is provided for signalling to a control point in case of faulty operation.

Towers are situated in a weaving pattern along the route, rather than in straight line, in order to prevent pickup from stations other than the preceeding one. To this end, also, two relay frequencies separated by 40 mc. are provided, and are employed alternately by succeeding transmitters. Thus, interference from skip transmissions is virtually eliminated.

Some of the more interesting tower installations include the station at Creston, Wyoming, where best results were obtained by placing the antennas facing west at a different Ievel than those facing east. At Cisco-Butte, California, the relay station is located at 40 feet above ground, high in the Sierra, Nevada Mountains. A 200-ft. steel tower at Salt Lake City, overlooks the City and Great Salt Lake in the distance. The station at Mt. Rose, Nevada, is the highest station in the chain, at an elevation of 10,075 ft.

Three distinct types of equipment are employed in the system, installed according to the function of the repeater station concerned. The basic unit of equipment is the transmitter-receiver bay, consisting of a transmitter and a receiver mounted in a single 9-ft. rack. At repeater stations each 2-way channel requires two bays, one for transmitting in each direction. Only one bay, however, is required at terminal stations for each 2-way channel.

These bays are alike in most respects. Different connections are employed, of course, at the terminal stations. Also, at terminals and main repeater stations, seperate temperature-controlled microwave generators are used as beat oscillators for transmitter and receiver sections of the repeater bays, while a single microwave generator and a 40 mc oscillator are used as a transmitter and receiver beat oscillator in auxiliary repeater stations. This is because at terminals and main repeater stations the received and converted signals may be patched, or filtered subcarriers intended for that station may be removed, while the only function of an auxiliary repeater station is that of amplification. This is made clear in the following section.

The incoming signal is fed to the 6 channel networks, which are tuned to accept the proper portions of the total signal and feed them to the, corresponding repeater bays.

Assume that the signal for one channel is centered on 3,730 mc. A microwave generator provides a signal of 3,840 mc. which is fed to a shifter converter. This converter stage is also supplied with a 40-mc. signal from a shift generator. The output of the converter is fed to a filter which selects the difference frequency for beating with the incoming 3,730-mc. channel signal. An IF of 70 mc. is formed and fed to an IF preamplifier, then to a delay equalizer. Equalization is necessary because of the wide bandwidth.

After equalization, the signal is brought to the main IF amplifier and then to the modulator, where it beats directly with the 3,840-mc. signal. The difference frequency of 3,770 mc. is 40 mc removed from the incoming signal, as was mentioned earlier. At the following repeater station it is shifted back to 8,780 mc.

From the modulator the signal goes to a power amplifier, consisting of three 461A triodes in a grounded-grid cavity assembly. The output signal of about ½ watt is combined with those of the other five channels and fed to the antenna.

An AGC circuit is included in the main IF amplifier section. The AGC is flexible enough to vary the IF gain from 40 to 65 db. A power monitor circuit sends an instant alarm to an alarm center should the output power drop below a fixed level.

The microwave generator need not be temperature-controlled because it is the determining factor in the output frequency. Effectively, it is subtracted and then added to the incoming frequency.

The 40-mc. shift generator does affect the final frequency, but it operates at such a comparatively low frequency that it can be made sufficiently stable without temperature control.

The first service of the relay system was in providing the equivalent of 46,000 miles of telephone circuits. That figure will be increased to 500,000 miles within a few weeks. There is ample reserve for the time when circuit requirements increase still further.

Coast-to-coast television service will begin on September 30 with one west-bound channel. A west-to-east channel will go into operation about one month later.