504 



BELL SYSTEM TECHNICAL JOURNAL 



also eases the intermediate-frequency selectivity requirement when the 

 side- to-side antenna loss is greater than the span loss (repeater gain). The 

 stagger between input and output frequencies in one direction is employed 

 to permit frequency selectivity to augment the back-to-back ratio in separat- 

 ing the high-power output from the low-power input. 



Figure 5 further describes the repeater in relation to the frequency plan 

 of Fig. 4, showing how the three kinds of antenna discrimination are em- 

 ployed. The back-to-back (BB) and side-to-side (SS) antenna ratios at- 

 tenuate the high-power transmitted band adjacent to a low-power received 



transmitters : 

 east-west(bb) 

 west-east (ss) 



1 , 



receivers: 



east-west 



-10 - 



a. 

 O-20 



-25 



3950 3960 



3970 3980 3990 

 RADIO FREQUENCY I 



-75 DB SUBJECT TO _. 

 DIFF. FADING 



TRANSMITTERS : 

 EAST-WEST(BB) 

 WEST-EAST (SS) 



JL 



1 TOTAL 



I-F AMPLIFIER 



1 R-F AMPLIFIER 

 g. MODULATOR 



■TOTAL OF 



4000 MC BRANCH 



OF 1 REPEATER 



4020 4030 4040 4050 



N MEGACYCLES PER SECOND 



WEST-EAST (FB) 



(by OPPOSITE 



ANTENNA) 



Fig. 5 — Discrimination of I.F. and R.F. circuits in television type repeater, 



ANTENNA RATIOS'. 

 BB= BACK TO BACK 

 SS= SIDE TO SIDE 

 FB= FRONT TO BACK 



band to a level comparable with the receiving level. The front-to-back 

 ratio (FB) separates bands at the same frequency (which are always similar 

 in power level except for the disparity produced by differential fading of the 

 signals received from the two directions). 



For systems in which the signal is highly susceptible to adjacent band 

 crossfire and phase distortion, the signal band must be confined to a certain 

 fraction of the 40-mc band spacing in order to be carried through many such 

 repeaters. With signals of more rugged characteristics, frequency occu- 

 pancy can be reduced because a larger segment of the pass-band shown in 

 Fig. 5 can be utilized by the signal and/or because the bands can be more 

 closely spaced. With quantized systems and particularly with binary PCM 



