A Single Sideband Musa Receiving System for Commercial 

 Operation on Transatlantic Radio Telephone Circuits * 



By F. A. POLKINGHORN 



In the operation of short-wave radio telephone circuits selective 

 fading is observed which is a result of the combination at the 

 receiving antenna of waves which have arrived from the transmitter 

 over paths of different lengths. The poor quality resulting from 

 this fading may be mitigated by increasing the directivity of the 

 receiving antenna in the vertical plane so as to favor the waves 

 arriving at one angle to the exclusion of others. Friis and Feldman 

 have described an experimental system designed to accomplish this 

 end which they call a "Musa" receiving system. This system was 

 found under certain transmission conditions to give an improve- 

 ment in the grade of circuit which could be obtained. A com- 

 mercial installation of this type has now been constructed for use on 

 the single sideband circuits of the American Telephone and Tele- 

 graph Company from England. Two receivers have been provided 

 for the operation of four radio telephone circuits. 



The antenna system consists of a row of sixteen rhombic antennas 

 two miles long, each antenna connected by a separate transmission 

 line to receivers located near the center of the row of antennas. In 

 each receiver the signals from the antennas are combined in the 

 proper phase to permit simultaneous reception from three ad- 

 justable vertical angles. The three signals are then added through 

 delay equalizing circuits or discretely selected on the basis of 

 amplitude to obtain diversity reception. A fourth branch of the 

 receiver has its vertical angle of reception continuously varying and 

 is used to set automatically the angles of reception of the three 

 diversity branches. The delay equalization is also automatically 

 adjusted. A recorder is provided which continuously registers the 

 relative carrier field strength with variation of vertical angle of 

 reception, and the amount of delay equalization. 



Introduction 

 TN the operation of short-wave radio telephone circuits fading is 

 -*- observed which is caused by the combination at the receiving 

 antenna of waves which arrive at different vertical angles and which 

 have traveled from the transmitter over paths of different lengths. 

 This fading may be mitigated by increasing the directivity of the 

 receiving antenna in the vertical plane so as to favor the waves 

 arriving over one path to the exclusion of the others.^- ^ It is not 



*Proc. I. R.E., April 1940. 



' E. Bruce, "Developments in Short Wave Directive Antennas," Proc. I.R.E., 

 Vol. 19, pp. 1406-1433, August 1931. 



* E. Bruce and A. C. Beck, "Experiments with Directivity Steering for Fading 

 Reduction," Proc. I.R.E., Vol. 23, pp. 357-371, April 1935. 



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