TRANSOCEANIC TELEPHONE SERVICE 263 



telegraph, and telephone wire systems at the station are shielded or 

 segregated. 



At both the transmitting and receiving stations at least three antenna 

 systems are supplied for each circuit, one antenna for each of the three 

 frequencies normally employed. The design and arrangement of 

 these are dictated by the requirements flowing from their uses. The 

 purpose of the transmitting antenna is to concentrate as much power 

 as possible in one direction. The purpose of the receiving antenna is to 

 increase reception from the desired direction and to cut down reception 

 at all other angles. In the former the forward-looking portion of the 

 characteristic is of greatest importance, while in the latter the rearward 

 characteristics need greatest refinement. 



Transmission Performance 



In short-wave telephone systems the width of the sidebands is so 

 small a percentage of the frequency of transmission that tuning charac- 

 teristics of the antennas and high-frequency circuits are relatively 

 broad and impose little constriction on the transmission-frequency 

 characteristic. A flat speech band is easy to obtain over the range 

 of approximately 250 to 3000 cycles employed for these commercial 

 circuits. This relieves the short-wave circuits from many of the 

 problems of obtaining sufficient band width which are troublesome in 

 designing long-wave systems. 



Short-wave transmission is subject to one frailty which particularly 

 hampers its use for telephony. This is fading. Where fading is of 

 the ordinary type, consisting of waxing and waning of the entire trans- 

 mitted band of frequencies, automatic gain control at the receiving 

 station is of value and is employed in the transoceanic circuits under 

 discussion. The amplification in the receiver is controlled by the 

 strength of the incoming carrier and is varied inversely with this 

 strength so as to result in substantially constant signal output. Ob- 

 viously this control can be effective only to the extent that the signal 

 seldom falls low enough to be overwhelmed by radio noise. 



When fading is of the selective type, that is, the different frequencies 

 in the transmitted band do not fade simultaneously, the automatic gain 

 control system is handicapped by the fact that the carrier or control 

 signal is no longer representative of the entire signal band. 



Selective fading is believed to result from the existence of more than 

 one radio path or route by which signals travel from transmitter to 

 receiver. These paths are of different lengths and thus have different 

 times of transmission. Wave interference between the components 

 arriving over the various paths may cause fading when the path lengths 

 change even slightly. 



