> > > >

1954-55 Theatre Catalog, 12th Edition, Page 384 (346)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 384
Page 384

1954-55 Theatre Catalog, 12th Edition, Page 384

FIGURE 2 illustrates a tvpical radio repeater station and lower that aid in TV transmission.

the adjacent link. The amplification points are called repeater stations. Separate antennas, designed to beam the radio waves along a narrow path, are used for receiving and transmitting at each repeater station. Thus intermediate stations ordinarily have four separate antennas. Antennas are mounted on towers of sumcient height to provide line of sight transmission; the height depending on the terrain encountered, higher towers being required in flat country. To the extent practicable, use is made of high natural elevations for the repeater stations in hilly or moune tainous country, thus reducing the

height of tower required. A typical repeater station and tower is shown in Figure 2.

The TD-2 system uses frequencies in the 3700 to 4200 megacycle range, transmitting on an FM basis. The 500 me available band is divided into 12 oneway channels, the mid-band frequencies of which are 40 me apart. Channels with adjacent frequency assignments transmit in opposite directions, thus producing six channels in each direction. In adjacent sections the same frequencies are used with directions of transmission reversed. Such a plan conserves frequency assignments and reduces crosstalk between channels to a workable minimum.

To guard against interruptions, one channel of a system in each direction is usually assigned as a protection

standby. A switching system has been designed to replace automatically the working channel with the standby chan

FIGUBE 4 is a typical coaxial cable repeater but used in the two Bell coaxial cable systems.

FIGURE 3 shows the cross-section of a coaxial cable needed for long range TV broadcasting.

nel whenever trouble is experienced. When the trouble is cleared, the original channel is automatically switched back into service.

Where economic and service conditions justify, radio relay systems of types other than the TD-2 have been used. Such a condition might exist where short systems are involved, where requirements are of short duration, or where portable units are required to meet short installation intervals.

Coaxial Cable Systems

A coaxial cable usually consists of eight coaxials in one cable sheath. Each coaxial is used as a one-way broad band channel and in the usual case two coaxials are paired together (one transmitting in each direction) to provide a two-way coaxial broad band channel. The coaxial consists of a copper tube with a copper wire located at its center. This wire is suspended throughout the length of the cable by insulators spaced about an inch apart. This type of structure confines the electromagnetic field largely within the tube and there is small susceptibility to interference from outside sources of energy at frequencies above 50 kc. The copper tube in the type of system used in most cases for television is three-eighths of an inch in diameter.

There are two types of coaxial systems in use. in the Bell System at the present time. The original system is known as "LI" and the later as ithl." The "LI" has amplifiers, used to come pensate for attenuation in the coaxial, located approximately eight miles apart and will transmit frequencies up to about 3.] megacycles. The "L3" system llsvs amplifiers spaced approximately four miles apart and will transmit frequencies up to about eight mogacycles. This latter system will provide over three times as many circuits as the former, using present standard

FIGURE 5 shows a switching panel capable oi handling 20 incoming and 24 outgoing channels.

1954-55 Theatre Catalog, 12th Edition, Page 384