The Bell System Technical Journal 



Vol.XXVII April, 1948 No. 2 



Microwave Repeater Research 



By H. T. FRIIS 



Introduction 



TT was some 80 years ago that Maxwell and Hertz demonstrated that 

 -*- free space is a good transmission medium for electromagnetic waves. 

 Since this fundamental contribution, the radio art has advanced tremen- 

 dously and a decade ago it had progressed to the point where it was possible to 

 construct equipment suitable for quantitative propagation studies of micro- 

 waves. Such studies were made and they indicated that normal propagation 

 over "line-of-sight" paths of signals of 10 to 20 centimeters wavelength 

 was characterized by free space attenuation and freedom from atmospheric 

 interference. These results, together with the facts that in this wavelength 

 range wide bands of frequencies are available and it is possible to design 

 small antennas having high directivity, encouraged us to start more com- 

 prehensive research work on microwave repeater circuits. This paper gives 

 the present status of the work which was interrupted by our war efforts and 

 resumed at the end of the war with the construction of an experimental 

 New York-Boston system as an initial objective. 



The first section will describe our propagation studies. It will be followed 

 by sections on repeater circuit planning, antennas, radio frequency channel 

 filters, the construction and testing of the repeater amplifier, and a conclud- 

 ing section on the whole repeater. 



I. Propagation Studies* 



Vhat portion of the radio frequency spectrum represented by wave- 

 lengths shorter than about five meters has long been considered as the proper 

 domain for point-to-point communication links, local broadcasting, and 

 mobile radio communication. Since these ultra-short waves are not re- 

 flected by the ionosphere, their effective range is not much greater than the 

 horizon distance and it therefore becomes possible for a number of stations, 

 properly separated, to operate in the same frequency band; for the same 

 reason, atmospheric interference is not an important factor in this wave- 



* This section was prepared by A. B. Crawford who, with W. M. Sharpless, is at present 

 engaged in microwave propagation studies. 



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