198 BELL SYSTEM TECHNICAL JOURNAL 



short-wave region. Although fading becomes worse as the wavelength is 

 decreased, the advantages of increased antenna gain and directivity possible 

 at the shorter wavelengths suggest the use of a wavelength just above the 

 region where rain attenuation becomes objectionable; i.e. above about 5 

 centimeters. 



The use of two antennas, operated in space diversity, should reduce the 

 effects of fading caused by multiple-path transmission. The use of space 

 diversity may be essential in those localities where strong ground reflections 

 are present. On the basis of the comparatively weak ground reflections 

 measured on the New York-Boston path it was decided to avoid the com- 

 plications that would result from the use of space diversity in this experi- 

 mental system. 



II. Repeater Circuit Planning 



The diagram in Fig. II- 1 shows schematically a repeater circuit. At the 

 input terminal toward the left the signal, 5, is fed to the terminal's trans- 

 mitting antenna. The radiated signal is propagated as discussed in Section 

 I and produces the signal power Si in the output of the receiving antenna 

 of repeater 1. The signal is then amplified Gi times and radiated toward 

 repeater 2 and this process is repeated until the signal finally appears in the 

 output terminal towards the right. In each repeater the signal is gain- 

 controlled automatically for the same level of output powers, i.e., S = Si = 

 S2 = " ' =Sr,. It is assumed that the signals are amplitude or frequency- 

 modulated C.W. carriers of substantially the same frequency in each link 

 and that the repeaters have linear amplifiers. The diagram shows a West- 

 to-East circuit only. A circuit for the opposite direction requires duplica- 

 tion of all the equipment with the exception of the antenna supporting 

 towers. 



Some simple formulas for the repeater gain and the signal-to-noise ratio 

 at the terminal will be given in this section, without going into any details 

 on propagation phenomena, antennas, amplifiers, etc. The formulas will 

 orient the reader in regard to the importance of quantities such as : 



d = Repeater separation ] 



X = Wavelength of signal [same units of length 



A — Effective area of each antenna J 



F = Noise figure of each repeater amplifier 



B = Bandwidth of circuit in cycles/sec. 



The free space attenuation (5x-iAx) of link ":*:" is^^ 



» H. T. Friis, "A Note on a Simple Transmission Formula", Ptoc. I. R. E., vol. 34, 

 No. 5, pp. 254-256; May 1946. 



