The Bell System Technical Journal 



Vol XV April, 1936 No. 2 



The Reliability of Short- Wave Radio Telephone Circuits * 



By R. K. POTTER and A. C. PETERSON, Jr. 



From empirical measurements of noise-to-signal ratio made during the 

 routine operation of short-wave radio telephone circuits there is obtained a 

 general relation between percentage lost circuit time and transmission 

 improvement in decibels. In this relation "percentage lost circuit time" is 

 the percentage of time that the noise-to-signal ratio is considered unsatisfac- 

 tory. No attempt is made to define such a standard quantitatively. 



If, from past experience with a long-range, short-wave telephone, tele- 

 graph or broadcast circuit, it is known that the circuit is unsatisfactory a 

 certain percentage of the time, the above-mentioned relation may be used to 

 estimate the effect of transmission improvement upon this percentage of 

 unsatisfactory or lost time. For a given circuit the variation in percentage 

 lost circuit time, as the standard for the tolerable sers'ice is changed by a 

 given number of decibels, may also be estimated. 



There are included estimates of the relation between the number of lost 

 time inter^'als of various lengths and transmission improvement. 



Introduction 



WITHIN a comparatively few years short-wave radio telephone 

 circuits have become an important part of the international 

 communication network. These years have represented a wide variety 

 of experience ranging between the quiet and the disturbed extremes of 

 an eleven-year sunspot cycle. An attempt is made here to review some 

 of this transmission experience in a quantitative way and show certain 

 relations that may be useful in the engineering of short-wave circuits. 

 During a magnetically disturbed year, such as 1930, a low-power 

 short-wave transmitter with a simple antenna arrangement would have 

 provided very uncertain means for communication across the North 

 Atlantic. The percentage of time that such equipment could transmit 

 what according to lenient standards in terms of noise-to-signal ratio 

 are useful telephone signals, would have been very low. If the power 

 of the transmitter were increased or a directive antenna employed to 

 reduce the noise-to-signal ratio the percentage useful time would, as 

 based upon the same standards, be increased or conversely the per- 

 centage lost time decreased. Any improvement which will decrease 



*To be presented at joint meeting of U. R.S.I, and I.R.F., Washington, U. C, 

 May 1, 1936. 



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