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 "musa" 

 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.^ Accord- 

 ingly it was decided that a commercial system should be built for use 

 on the circuits of the American Telephone and Telegraph Company 

 from England. A corresponding system of modified design has been 

 built by the British General Post Office.^ The purpose of this paper 

 is to review a few of the principles upon which a musa receiver operates, 

 to describe the equipment which has been built in this country, and to 

 discuss some of its operating characteristics. 



The transmissions which are to be received are of the so-called twin 

 single-sideband reduced-carrier type described by Oswald ^ and con- 

 sist of two sidebands, representing two distinct speech channels, on 

 opposite sides of a carrier which is 16 to 26 db below the maximum 

 sideband amplitude. Under normal conditions one of the sidebands 

 is adjacent to the carrier while the second is spaced by the width of 

 one sideband from the carrier. A single sideband receiver for this 

 type of transmission has been described by Roetken ® and many of 

 the features discussed by him were developed for use also in the musa 

 receivers. These features include highly stable oscillators, crystal 

 filters and automatic tuning circuits. 



Outline Description of Receivers 



A block schematic of one channel of the commercial musa system 

 is shown in Fig. 1. The sixteen rhombic antennas are placed in a line 

 two miles long in the direction of the English transmitting station. 



* H. T. Friis and C. B. Feldman, "A Multiple Unit Steerable Antenna for Short- 

 Wave Reception," Proc. I.R.E., Vol. 25, pp. 841-917, July 1937 ; B.S.T.J., Vol. XVI, 

 No. 3, pp. 337-419, July 1937. 



* A. J. Gill, Wireless Section, Chairman's Address, Jour. I.E.E., Vol. 84, No. 506, 

 pp. 248-260, February 1939. 



^ A. A. Oswald, "A Short-Wave Single Sideband Radio Telephone System," Proc 

 I.R.E., Vol. 26, No. 12, pp. 1431-54, December 1938. 



* A. A. Roetken, "A Single Sideband Receiver for Short-Wave Telephone Service," 

 Froc. I.R.E., Vol. 26, No. 12, pp. 1455-65, December 1938. 



