A SINGLE SIDEBAND MUSA RECEIVING SYSTEM 311 



transmission over two paths is t there are alternate maxima and minima 

 in the frequency spectrum caused by these two components which are 

 separated by 1/2/. Continuous small changes in the lengths of the 

 paths cause these maxima and minima to wander back and forth 

 through the spectrum. By separating the waves arriving at distinctly 

 different angles the musa receiver succeeds, for the most part, in 

 separating those waves which have greatly different transmission 

 times and thus widens the frequency interval between a maximum and 

 an adjacent minimum. As the interval increases the fading appears 

 less selective. The signal appearing to arrive at any one angle, 

 however, is in reality composed of a bundle of waves, the components 

 of which have traveled over slightly different paths and which might 

 be expected to be nearly alike in amplitude and transmission time but 

 not in phase. As a consequence it is to be expected that the general 

 fading on a single-angle musa receiver will be greater than on an 

 ordinary receiver and it is essential that some form of diversity be 

 used to insure a satisfactory output amplitude at all times. Sudden 

 shifts in the received angle of signals will also give general fading which 

 will be greater the greater the angular discrimination of the musa 

 system. 



A musa receiver differs from an ordinary receiver in that there are 

 a number of separate antenna branches, the outputs of which must 

 be added in the proper phase over an appreciable band of frequencies. 



When delay equalization is used between the various diversity 

 branches, these branches must also have equal phase shifts if the 

 audio-frequency outputs are to add properly. In both antenna and 

 diversity branches the problem of keeping equal phase shifts is com- 

 plicated by the action of the automatic volume control system which 

 changes the operating condition of the vacuum tubes over a wide range. 

 In designing these receivers a nominal overall value of non-uniformity 

 of ±10° was taken as acceptable and an effort made to keep the 

 phase uniformity of individual elements to within one or two degrees 

 wherever possible. 



Within the receiving station all radio-frequency wiring is made 

 with a flexible coaxial cable having rubber insulation. The various 

 panels composing the receivers are placed on the racks with a view 

 to operation and maintenance rather than ease of wiring and conse- 

 quently long leads between panels are frequently necessary. For this 

 purpose the circuit impedance is dropped to 70 ohms and at a number 

 of points brought out to jack panels to facilitate testing. 



Coaxial jacks are used which fit into the usual jack strips. Nor- 

 malling jacks are not available and consequently it is necessary to 



