A SINGLE SIDEBAND MUSA RECEIVING SYSTEM 309 



Separate transmission lines lead from each antenna to a building 

 placed a little to one side of the rear of the ninth antenna. Two 

 receivers, only one of which is shown in the figure, are connected in 

 parallel to each transmission line. Each receiver is designed to receive 

 five specific frequencies, ranging from 4,810 kc. to 18,620 kc, assigned 

 to the corresponding transmitter in England. 



After passing through selective input circuits the signals from each 

 antenna are demodulated by a common oscillator to a band adjacent 

 to a carrier frequency of 2,900 kc. The signals, after going through 

 two stages of intermediate frequency amplification are then applied 

 to the inputs of four phase shifter systems in parallel. In each of 

 these phase shifter systems the signals from the sixteen antennas are 

 combined so as to give reception from a particular vertical angle. 

 This angle can be varied by a mechanical movement of a phase shifter 

 drive shaft. Three of the phase shifter system outputs are used for a 

 three-branch angular-diversity system in which the signals arriving 

 over three separate paths are separately received and then either 

 combined or individually selected for connection to the line, while the 

 fourth branch is used for monitoring to determine where the phase 

 shifters of the three diversity branches should be set in order to 

 receive the best signals. From each phase shifter group the circuit 

 continues through the first intermediate frequency filter and two 

 further stages of amplification to the second demodulator where the 

 2,900 kc. carrier frequency is shifted to 100 kc. The carriers and 

 sidebands of each diversity branch are then amplified separately and 

 again combined in the final demodulators to give three distinct voice 

 frequency outputs for each sideband. These three outputs are either 

 combined after inserting variable delay in two of the branches or, 

 optionally, the branch having the greatest signal at any instant is 

 connected to the line. Both of these operations are performed 

 automatically. The output of the monitoring phase shifter group is 

 also heterodyned to 100 kc. and after amplifying the carrier only it is 

 rectified and applied to an automatic system for adjusting the phase 

 shifters of the three diversity branches. 



A general view of the receivers is shown in Fig. 2. The principal 

 parts of the two musa receivers occupy three rows of bays each about 

 25 feet long and 11| feet high. The row shown on the right contains 

 the input circuits and first demodulators for both receivers. The 

 middle row contains the remaining equipment for one receiver and 

 the left row that for the second receiver. In addition there are five 

 bays of rectifiers and power control equipment located in a fourth row 

 which is not shcnvn. 



