INTERMODULATION INTERFERENCE IN RADIO SYSTEMS 69 



transmitting and 5 for receiving. (7" = 0.50 = R) In this case it is the 

 2A -{- B — 2C type of product that requires the use of such a wide 

 band. 



2. Fig. 2a shows that there is practical certainty of interference in a 

 band of 500 adjacent channels when 30 operating channels, picked at 

 random from that band, are fully used. In this case it is the A + B + 

 C — D — E type of product that requires the use of such a wide band. 



3. If the same total traffic as was assumed in (1) is handled by a greater 

 number of operating channels, the number of available consecutive channels 

 required for the same probability of interference remains the same. 

 Thus, Fig. 2b shows that a band in excess of 500 consecutive channels is 

 still required to limit the chance of interference to ten per cent when the 

 traffic is distributed among 20 randomly selected operating channels; 

 {T = 0.25 = R) similarly Fig. 2c shows that the required number of 

 available consecutive channels remains the same when the traffic is dis- 

 tributed among 40 operating channels. {T = 0.125 = R). 



CHANNEL SELECTION FOR THE ELIMINATION OF INTERMODULATION IN- 

 TERFERENCE 



Discounting the effect of selectivity in the radio equipment, it was 

 shown in the preceding section that only a very limited number of 

 channels can operate together without some degree of mutual inter- 

 ference when these channels are picked at random from a very consid- 

 erable number of available channels. This is of course extravagant of 

 frequency space. In this section, it is proposed to determine whether 

 frequency space can be conserved by carefully selecting the operating 

 channels in such fashion that the various tjrpes of intermodulation 

 products that are formed ^vill all fall on other than operating channels. 

 This is readily accomplished in the case of 3rd order products by select- 

 ing the operating channels in such fashion that the frequency difference 

 between any pair of these channels is unlike that between any other 

 pair of channels. Many other rules inherently more complicated and 

 more cumbersome to apply than the one stated above must be obeyed 

 if 5th order as well as 3rd order products are to be controlled in this 

 way. Table IV presents p operating channels selected from a band of n 

 adjacent channels (numbered sequentially in order of ascending fre- 

 quency) in such fashion as to avoid 3rd order interference within the 

 system. Considerable effort has been spent in selecting these channels 

 to insure that the number n associated with each value of p is the lowest 



