TD-2 MICROWAVE RADIO RELAY SYSTEM 



1059 



channel 2 at this station and both a regular and a spare circuit are now 

 available at succeeding stations. 



H. Automatic IF Switching* 



At present IF switching is handled on a manual basis by attendants at 

 the main stations or on a remote control basis over the order wire facilities. 

 This type of switching is satisfactory for maintenance purposes but ob- 

 viously is not fast enough to avoid a circuit interruption in replacing a 

 circuit which has failed. The reliabihty of wire circuits will be difficult to 

 meet in a long radio relay system without standby facilities because of 

 vacuum tube failures and fading. Work is now under way to develop auto- 

 matic IF switching facihties which will detect instantaneously a circuit 



AUXILIARY 

 REPEATERS 



AUXILIARY 

 REPEATERS 



Fig. 15 — IF switching and distributing amplifier. Interconnection diagram. 



failure or an increase in noise on a radio channel and switch in a spare circuit 

 for the poor section without circuit interruption. Fading data indicate that 

 nrost deep fades which go beyond the range of the AGC circuit are of the 

 selective type. Thus switching to a spare channel will provide frequency 

 diversity advantages. With automatic switching it is believed the TD-2 

 System circuit outage time will not exceed that of wire circuits. 



I. Television Monitoring 



Visual monitoring facilities are provided at terminal and main repeater 

 stations for observing circuit performance. Auxiliary repeater stations may 

 also be so equipped in special cases. At transmitting or receiving terminals, 

 monitoring connections are bridged to the video cables which run to operat- 



* Prepared by T. R. D. Collins. 



