FREQUENCY ECONOMY IN MOBILE RADIO BANDS 



49 



Table VI — Channel Spacing Required for Unrestricted 

 Operation of Two FM Channels in Same Area 



VERSUS Antenna Separation 



The above table shows that if interference of the types so far con- 

 sidered is to be kept below the minimum usable signal at all distances 

 greater than about 500 feet from undesired transmitters, the channel 

 spacing needs to be at least 62 to 75 kc in the 150 mc band and 74 to 105 

 kc in the 450 mc band. The channel spacings for AM are equal to the 

 minimum shown above, while the higher figure is for FM with ±5 kc 

 swing (a modulation bandwidth of =b8 kc). 



Since the above channel spacings are considerably greater than the 

 necessary IF bandwidth, it should be possible to use intermediate chan- 

 nels in adjacent non-overlapping areas. This geographical limitation 

 does not appreciably decrease the overall efficiency in the use of fre- 

 quency space as long as the needs for mobile channels are more or less 

 uniformly distributed within a large region, but it becomes important 

 where a large percentage of the available channels are needed in the same 

 metropolitan area. 



In a later section it is shown that channel spacings less than the values 

 given in Table VI are feasible in the same area providing sufficient co- 

 ordination is achieved in both geographical spacings and operating 

 methods. 



The estimated channel spacings shown in Table VI do not take into 

 account the effect of intermodulation interference which is discussed in 

 the following section. Intermodulation interference may limit the num- 

 ber of usable one-way channels to only 1 or 2 per megacycle instead of 

 the above 6 to 20 per megacycle, unless further restrictions are placed on 

 the selection of frequencies and on the method of operation. 



limitations imposed by insufficient rf filtering 



When a strong unwanted signal on a frequency within the RF bandwidth 

 is present at the input to a receiver, overloading occurs and the receiver 



