130 BELL SYSTEM TECHNICAL JOURNAL 



constant volume of transmission would become extremely difficult. 

 Similarly in radio transmission, the fluctuations in the ether attenu- 

 ation, particularly at short wave lengths where over long distances 

 we experience the well known "swinging" or fading efTects, render the 

 maintenance of a satisfactory volume of transmission a difficult 

 problem. As noted above these fluctuations, particularly as between 

 day and night transmission with very high frequencies, may be 

 enormous. 



It is of value to the radio engineer to have some idea of the over-all 

 circuit transmission equivalents which are necessary for satisfactory 

 telephone communication. In the wire telephone art, the maximum 

 equivalent between subscribers is ordinarily taken as about 30 miles 



of standard cable or logio ~ = 1.4. Under quiet conditions, con- 



-'2 



siderably larger transmission equivalents can be talked over. The 

 long distance toll lines themselves are usually designed for 

 transmission equivalents of 0.5 to 0.75 or 10 to 15 miles of standard 

 cable. These figures will serve as a general guide for the transmission 

 equivalents which radio telephone circuits should provide. Where a 

 radio circuit forms a link in a direct wire circuit as, for example, in 

 the case of Catalina Island, it is desirable to work the radio link as 

 close to a zero equivalent as possible, that is, to give out at the receiv- 

 ing end a volume nearly equal to that fed in at the transmitting end. 



Two-Way Operation 



When the two one-way radio channels are merged at their two 

 ends into a regular telephone circuit for connection to the wire net- 

 work, as illustrated in Fig. 5, then there is a limit in the transmission 

 equivalent which can be given over the radio part of the circuit. 



This limit will be appreciated by reference to Fig. 5. It is im- 

 posed by the tendency of the two one-way channels to form a round- 

 trip circuit by "feeding-back" from one to the other via the voice 

 frequency connecting circuit. If the total amplification around the 

 circuit including the voice-frequency line, exceeds the total losses in 

 the circuit, "singing" will result. Were no line balance provided at 

 the voice frequency terminals, then it would be impossible to operate 

 the circuit at a zero equivalent. By setting up a balancing circuit at 

 each end in the manner illustrated, a transmission loss is, in effect, 

 inserted between the sending and receiving sides of the voice circuit 

 which tends to prevent this sing-around action. Actually, there is a 

 limitation in the degree of balance which can be realized between the 



