FREQUENCY ECONOMY IN MOBILE RADIO BANDS 



47 



Table III — Required Suppression versus Distance 

 Between Antennas 



160 db greater than the minimum signal that is useful in the receiver 

 (140 db below one watt), so ideally no appreciable interference would 

 result if the overall selectivity of the receiver and the suppression of 

 extra band radiation in the transmitter could be in excess of 160 db. 

 This amount of isolation is difficult to obtain by filtering. The inter- 

 action between transmitter and receiver of the same system is frequently 

 avoided by the use of ''push-to-talk" operation, but the potential inter- 

 ference between different systems requires the full 160 db (based on 100 

 watt transmitters). Fortunately, a substantial part of the desired isola- 

 tion can be obtained by modest geographical separation. The net re- 

 quirements for either receiver selectivity or transmitter filtering are 

 less than 160 db by the losses shown in Table I and are summarized in 

 Table III. 



Receiver selectivities of 90-100 db or more are feasible except on 

 nearby channels and possibly on certain image channels. Typical values 

 of the guard bands that are required between the edge of the desired 

 pass band and the frequency at which the desired attenuation to inter- 

 fering signals can be obtained are estimated in Table IV. 



Even if the guard band, shown in Table IV, required to provide ade- 

 quate selectivity in the receiver could be reduced to zero by providing 

 infinitely steep sides on the IF selectivity curve, there would still re- 

 main the guard band needed to avoid the extra band radiation from the 

 transmitter. The amount of suppression of extra band radiation needed 



Table IV — Guard Band versus IF Selectivity 



Desired IF Selectivity 



40 db 



60 



80 

 100 

 120 



Required Guard Band 



12 kc 



15 



20 



25 



30 



