BAND WIDTH AND TRANSMISSION PERFORMANCE 543 



regeneration in preventing accumulation of disturbances that are not much 

 affected by the distribution of fading. 



1\'. Frequency Occupancy Tables For Radio Relay 



The frequency space occupancy for a single two-way route is, according 

 to principles laid down in the introduction, a frequency block 2U times the 

 signal bandwidth.-^ Our problem, as stated in the introduction, is to ex- 

 amine the situations arising when a number of 1000-channel routes converge 

 toward a terminal city, assuming all of the routes to be of the same kind. 

 We will determine the number of times the above frequency blocks must 

 be repeated in the spectrum in order to keep interference within tolerable 

 bounds. The sum of these blocks then really defines the frequency occu- 

 pancy and determines the space which must be allocated or, conversely, 

 determines the number of routes a given allocation will accommodate. 

 We will use the tolerable ratios of similar system interference taken from 

 Tables IV and V, together with appropriate antenna directivity, to deter- 

 mine the number of these blocks. 



Antenna Characteristics 



The directional discrimination afforded by the antennas is obviously an 

 important factor in frequency economy. For our present study, we employ 

 an antenna having a directional pattern slightly superior to that of the 

 4000-mc shielded lens antenna in use on the New York-Boston radio relay 

 circuit. Figure 21 shows the assumed directional characteristic omitting 

 "nulls" between the minor lobes. Of importance also are the nearby dis- 

 crimination characteristics of the antennas as given in Fig. 4. 



The situations arising at a point where a number of routes converge (or 

 cross) or where a route is equipped with a spur connection are variations of 

 that occurring at a single repeater point. In fact, the situation in which 

 two routes converge from approximately opposite directions occurs at every 

 repeater point in a straight route, while a repeater point at which the route 

 bends sharply is like a terminal point at which two routes converge at a 

 small angle. 



The crosstalk in our assumed two-frequency long distance repeater system 

 has been estimated in Section I (under "The Radio Repeater") and was 

 found equivalent to a single source of similar system interference 44 db 

 down. A system which possesses just enough tolerance to withstand the 

 accumulated crosstalk on a long straight repeater system is not capable of 

 meeting another such system at an angle unless additional frequencies are 



2' In the case of verv tender systems, such as FDM, the factor 2U is replaced by 4U be- 

 cause a four-frequency plan is needed for a two-way repeater. U is the band spacing 

 factor discussed under "The Radio Repeater." 



