MICROWAVE REPEATER RESEARCH 219 



channel branching filter was measured with a double detection measuring 

 set and is plotted in Figs. IV-8 and IV-9. 



These measured electrical characteristics serve as a check on the general 

 theoretical ideas concerning constant resistance hybrid branching networks. 

 They also indicate that when these ideas are embodied in the form illustrated 

 in Fig. IV-7 the result is a branching filter which can be used in currently 

 planned radio relay circuits. 



The circuit of Fig. IV-7 provides a satisfactory channel spHtting network. 

 It does not, however, provide consistent high off-frequency discrimination 

 between one of the channel output terminals and the other terminals of the 

 filter. When systems requirements* are such that extra discrimination or 

 special impedance behavior is required, this can be supplied by inserting 

 suitably designed reflecting filters in the branch fines. These added filters 

 will not interact reflectively with the branching filter. 



V. The Repeater AMPLiFiERf 



In a microwave repeater circuit, Fig. II-l, the signal is amplified at each 

 repeater to compensate for the transmission loss in the preceding radio 

 path. Since we cannot build perfect amplifiers, the signal will not appear 

 at the output terminals as a true replica of the input signal ; the circuit will 

 distort the shape of the signal and it will also add noise. Therefore, the 

 main objectives in amplifier work have been to keep the signal distortion and 

 the added noise within certain requirements. 



To be more specific, the repeater amplifier in a relay system must be 

 capable of supplying a maximum gain, G, as given by the equation II-l; 

 it must have a ratio of output power capacity to noise figure which will meet 

 the signal to noise ratio requirements of the system as given by equation 

 II-4; since distortionless transmission is desired, it must have an amplitude 

 characteristic as flat as possible and a phase characteristic as linear as pos- 

 sible over the essential range of frequencies of the signal it is desired to 

 transmit ;24 and it must be equipped with an automatic gain regulating cir- 

 cuit to hold the output power constant over the expected range of input 

 levels. 



The simplest relay amplifier would be one which amplifies the signal and 

 sends it on without a change in frequency. However, two major considera- 

 tions indicated that early repeater amplifiers could not be so simple. No 



* E.g., the converter may require a reflection in the input line at the image frequency, 

 See Section V and Fig. V-4. 



t Those parts of this section dealing with the general layout, the requirements, and the 

 over-all testing of the repeater amplifier were prepared by D. H. Ring, who together 

 with A. C. Beck did the work on this phase of the problem. 



24 Sallie Pero Mead, "Phase Distortion and Phase Distortion Correction", B. S. T. J., 

 vol. VII, No. 2, pp. 195-224, April 1928. 



