1046 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



the main stations. From an operating standpoint, main stations differ from 

 auxiliary stations primarily in that each channel is terminated in IF switch- 

 ing circuits. This permits the removal of a channel for maintenance, or the 

 replacement of a section which has failed by a spare circuit, by patching or 

 remote control of the IF switching circuits. An alarm center may also be 

 identified in Fig. 4 as an attended office to which a maximum of twelve 

 repeater stations are connected by wire or radio for the purpose of reporting 

 abnormal conditions that exist. Maintenance personnel are dispatched to 

 unattended stations from this point. Not all TD-2 units are repaired and 

 maintained at the radio stations. Maintenance centers are established along 

 the route to service these units which require more elaborate test facilities 

 than are provided at the stations. This requires the furnishing of certain 

 spare units at the individual repeater stations, 



B. Route Selection and Towers 



The interconnecting of two or more communication centers by a radio 

 relay system presents many new problems in plant engineering. The selection 

 of hundreds of mountain top sites to obtain line-of-sight transmission be- 

 tween stations, sites which are accessible to roads and power lines, sites 

 which permit reasonable tower heights and which are an economic balance 

 of these and other factors was a new challenge to the plant engineering 

 force.^ In brief, these were accompKshed first by a detailed study of topo- 

 graphical" and road maps, inspection of sites selected and finally the measure- 

 ment of the transmission loss of the path. 



The construction of towers several hundred feet high also involved new 

 thinking by the building engineers.^ The type of structure used on the 

 New York-Chicago section of the TD-2 System was somewhat influenced 

 by the availability of materials during 1948 and 1949. Concrete structures 

 were used for this section of the system as shown in Fig. 5 with steel towers 

 appearing on the Omaha to San Francisco section. Where steel towers are 

 used, conventional type single-story buildings house the radio and associ- 

 ated equipment as shown in Fig. 6. Double antenna decks are provided on 

 towers where branching radio routes are required. 



III. TD-2 Radio Equipment 



A. Repeater — General 



The design of the TD-2 microwave repeater follows in principle that of its 

 predecessor for the NewYork-Boston system.^ Rapid advancement in the 

 development of microwave vacuum tubes and other repeater components 

 during the period from 1945 to 1947 led to a general improvement of re- 

 peater components for the TD-2 System. The realization in late 1947 of a 



