TWO-WAY RADIO TELEPHONE CIRCUITS 371 



portions. In view of the relatively higher cost and greater length of 

 the radio links, the highest grade wire circuits available are usually 

 justified so that, in general, they should not be allowed to contribute 

 much transmission impairment. In general, the added delay intro- 

 duced by these wire lines is their most important effect. 



If the radio links are quite stable and fairly free from atmospheric 

 disturbance, the circuit may be operated like four-wire land telephone 

 circuits. That is, the total amplification in the circuit may be kept at 

 such a value that it never exceeds the total attenuation, and over-all 

 singing will not occur. Four-wire terminating sets or hybrid sets 

 placed at the ends of the circuit, as shown in Fig. 1, are adequate to 

 prevent singing and minimize echo effects in four-wire circuits which 

 have over-all transmission times less than about 0.02 second, provided 

 the net loss from switchboard to switchboard does not become lower 

 than about 5 db. 



If the radio links or wire lines are long, the circuit will produce an- 

 noying echoes exactly as a four-wire cable circuit will, due to delay or 

 instability, or both. Also, as in the case of four-wire cable circuits, 

 echo effects may be reduced appreciably by voice-operated echo sup- 

 pressors which block the path of the delayed echoes while the other 

 path is transmitting speech. The possibilities and limitations of this 

 type of device are discussed elsewhere.^ 



When the radio channels are more noisy and/or less stable, the 

 transmission may be greatly improved through more efficient use of the 

 radio links. The noise may be minimized by bringing the speech waves 

 of all talkers, strong or weak, to the same "electrical volume" or 

 strength at the input to the radio transmitter. Thus, practically 

 full modulation may be maintained on the radio transmitter at all 

 times and the ratio of the desired signal to the radio noise kept a 

 maximum. Large changes in gain between the two-wire line and the 

 radio transmitter are necessary to accomplish this result. These 

 changes are made by technical operators who make the adjustments 

 with variable loss devices. An indication of the volume is obtained 

 through the aid of electrical meters called "volume indicators." 

 In practice, the over-all transmission of a long radio circuit may be 

 varied by the technical operators from a 30-db loss to a 30-db gain 

 within a few minutes. 



In short-wave circuits, the phenomenon known as "fading" in- 

 troduces an effect which is of great importance in two-way operation. 

 Where fading results in variations of the entire transmitted band of 

 frequencies, automatic gain control at the radio receiver is effective in 



2 A. B. Clark and R. C. Mathes, "Echo suppressors for long telephone circuits," 

 Jour. A. I.E. E., 44, 618-626; June, 1925. 



