FREQUENCY ECONOMY IN MOBILE RADIO BANDS 



51 



Table IX — ^Significant RF Band versus Antenna 

 Spacing 



tivity can be reduced to one-third and possibly to one-fourth of the 

 above values by the use of bulky and expensive filters. 



The critical frequency band that needs to be considered in deter- 

 mining the usefulness of any given channel can be obtained by combining 

 the information given in the two preceding tables with the results shown 

 in Table IX. For example, if it be desired to work mobile receivers un- 

 restricted to within 500 feet of two or more unwanted transmitters, all 

 frequency assignments within ±3 mc in the 150-mc band (or within 

 ±7 mc in the 450-mc band) must be carefully chosen if intermodulation 

 interference is to be avoided. 



When the zb3-mc band is divided into 100 potential channel assign- 

 ments of 60 kc each and when the channels assigned to a given area are 

 chosen at random, 7 channels working 50 per cent of the time (or 37 

 channels working 10 per cent of the time) mil, on the average, cause 

 third order intermodulation interference about 10 per cent of the time 

 on each channel within the band. The interference is expected to be 

 above the minimum usable signal level in all receivers located less than 

 about a mile from the unwanted transmitters. Even if the operating fre- 

 quencies are selected carefully instead of at random, no more than 11 

 channels out of 100 can be found that are free of third order intermodula- 

 tion when used simultaneously in the same general area. These results 

 are discussed more completely in a companion paper. ^ When the num- 

 ber of potential channel assignments is greater or less than 100, the 

 corresponding number of usable channels limited by third order modula- 

 tion alone is shoAvn in Table X. The numbers of usable channels shoA\ii 

 above are further reduced when fifth and higher order intermodulation 

 products are considered. 



A reduction in the nominal channel spacing from 60 kc to 20 kc means 

 a three-fold increase in the potential channel assignments, but Table X 

 shows that the number of usable channels increases much more slowly. 



5 Babcock, W. C, Intermodulation Interference in Radio Systems. Page 63 of 



this issue. 



