BAND WIDTH AND TRANSMISSION PERFORMANCE 545 



frequency were to be used on two or more routes, receiving directional dis- 

 crimination amounting to 75 db would have to be secured: 



1. Required interference ratio 44 db from Table IV FDM-FM 



2. Allowance for repeater crosstalk 1 (51 db down) 



3. Differential fading allowance^* 30 



75 db 



The repeater crosstalk is here taken to be equivalent to one source 51 db 

 down which is the value corresponding to no differential fading on adjacent 

 spans as calculated in Section I. It will be remembered that allowance was 

 made for a single differential fade of 30 db occurring somewhere along the 

 route. Here we assume that this differential fade may occur between two 

 of the converging paths and we demand that the receiving directional dis- 

 crimination shall protect the system against such an occurrence. In this 

 case the required directional discrimination turns out to be equal to the 

 75 db front-back ratio from which the 44 db figure was obtained. This is 

 manifestly impossible with the assumed directivity characteristic^* and the 

 angles involved. Therefore, different receiving frequencies are required on 

 each route. These same frequencies may be used for transmitting at the 

 junction, provided the disposition of terminals is such as to provide enough 

 directional discrimination and physical separation to permit operation at 

 the low received level in the face of the high transmitted level on the same 

 frequency. The interference path loss plus antenna discrimination must be, 

 for the case involving the longest span: 



1. Required interference ratio 44 db (FDM-FM, Table IV) 



2. Allowance for repeater crosstalk 1 



3. Free space span loss 60 



4. Fading allowance 25 



130 db 



We continue our discussion of the converging routes of Fig. 22 by assum 

 ing that: 



1. Conditions encountered elsewhere on the routes do not restrict the 

 freedom to switch the frequencies among the routes. 



2. The disposition of terminals at the junction is such that inter- terminal 

 interference is not a controlling factor. 



Under the above assumptions the directional discrimination of the terminal 



^^ This differential fading allowance corresponds to a fade of 25 db below free space on 

 one route and a 5 db increase over free space on the other. 



^' The use of perpendicular polarizations cannot, we assume, be counted on to give 

 further discrimination when the directional discrimination is already 40 or more decibels. 



