APPLICATION OP WIRE TRANSMISSION TO RADIO 



131 



telephone line and the balancing network, especially if tlie telephone 

 line is to be switched at a nearby central office, and this factor, to- 

 gether with the margin of safety which is required between the oper- 

 ating condition and the singing condition, prevents the radio channels 

 from being operated much better than the zero equivalent. This 

 whole matter of realizing in practice an adequate transmission equiva- 

 lent, will be appreciated to be an especially difficult problem in the 

 case of marine radio telephony, where the connection is switched 

 from one vessel to another at varying distances. 



It should be noted further, with reference to two-way operation, 

 that the difficulty of effecting simultaneous sending and receiving 

 at a station arises primarily from the large attenuation which must be 



TWO WAV RADIO CIRCUIT 



Fig. 5 



overcome and the resulting large ratio between the energies trans- 

 mitted into and received from the ether. The receiver must be pre- 

 vented from being overloaded by the home transmitter and this, 

 in general, requires that there be provided between the high frequency 

 side of the transmitter and that of the receiver, a transmission loss 

 comparable in size to that obtaining over the radio circuit itself. This 

 "separating" transmission loss is ordinarily provided (a) by fre- 

 quency-selecting circuits (tuned circuits and filters), the sending and 

 receiving transmissions being placed on different frequencies; (b) by 

 balance, as w'hen using the blind spot of a loop-antenna receiver, 

 and (c) by spatial separation between sending and receiving points, 

 where the large step-off loss is used to advantage. 



