A Single Sideband Musa Receiving System for Commercial Operation on Transatlantic Radio Telephone Circuits

INTRODUCTION the operation which of short-wave r a d i o is caused by the telephone circuits f a d i n g is at the receiving N observed combination a n t e n n a of waves which arrive at different vertical angles a n d which h a v e traveled from the t r a n s m i t t e r over p a t h s of different lengths. This fading m a y be mitigated in by increasing the directivity of the plane so as to favor 2 receiving a n t e n n a the vertical the waves a r r i v i n g o v e r o n e p a t h t o t h e e x c l u s i o n o f t h e others. 1 , It is n o t *Proc. I.R.E., April 1940. 1 E. Bruce, "Developments in Short Wave Directive Antennas," Proc. I.R.E., Vol. 19, pp. 1406-1433, August 1931. 1 E. Bruce and A. C. Beck, "Experiments with Directivity Steering for Fading Reduction," Proc. I.R.E., Vol. 23, pp. 357-371, April 1935. 306 A SINGLE SIDEBAND MUSA RECEIVING SYSTEM 307 possible, however, to increase this directivity to any great extent with an ordinary antenna system before it is found that the signal arrives outside the angular range of the antenna an appreciable part of the time. To overcome this difficulty Friis and Feldman experimented with a receiving system consisting of a number of antennas, each having moderate directivity and each connected by a separate transmission line to a receiver where the outputs are phased by a variable phase shifting system in such a manner as to give a system of high, variable directivity. A system of this kind, which they called a " m u s a " system from the initial letters of "multiple unit steerable antenna," was built and found to give under most transmission conditions an improvement in the grade of circuit which could be obtained.