Two-Way Radio Telephone Circuits

ADIO telephone circuits are now in regular use between New York and London, New York and Buenos Aires, San Francisco and Honolulu, and many other points. At each end of such a circuit there are a transmitting radio station and a receiving radio station, usually geographically separated. The radio provides two one-way transmission paths. T h e circuit is completed by means of one-way wire lines which connect the radio sending and receiving stations to a common point. At this common point some rather intricate apparatus is called for in order to permit switching of the circuit to the wire telephone plants at the two terminals. This paper explains why this intricate apparatus is necessary even for the comparatively simple case of short-wave radio circuits which use different frequency bands to transmit in opposite directions. It also describes the latest form of this terminal apparatus in which provision is made for certain switching of privacy apparatus by means of which an important saving is made in the amount of privacy apparatus required. The original form of this apparatus is described in an earlier paper. TRANSMISSION PATHS Radio telephone circuits may employ the same frequency band for transmission in the two directions or they may employ separate bands. The present long-wave telephone circuit between New York and London is of the first type, while most existing short-wave circuits are of the second. A short-wave circuit, using separate frequency bands, is shown in its simplest form in Fig.