52 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



Table X — Number of Usable Channels versus Number of 

 Potential Channels 



Thus far, only the intermodulation interference generated in the re- 

 ceivers has been considered. Intermodulation also occurs at the same 

 frequencies in the transmitters, but it usually can be made less impor- 

 tant than the corresponding interference in the receivers. Ideally, the 

 intermodulation products generated in the transmitters should not be 

 stronger than 140 db below one watt (about 1 microvolt at the input to 

 the receiver) which requires about 75 db RF filtering in each transmitter 

 output. This ideal requirement is based on 100 watt transmitters with 

 both the transmitters and receiver working on the same antenna. In prac- 

 tice, the RF filtering requirement is less than 75 db because of physical 

 separation between transmitters and receivers, and typical values based 

 on free space transmission are shown in Table XI. 



A comparison of the filter requirements on 100 watt transmitters with 

 the corresponding receiver selectivity requirements given in Table VII 

 shows that the receiver requirements are greater as long as the effective 



Table XI — RF Transmitter Filtering versus Antenna 



Separation 



* Common antenna. 



