RADIO PROPAGATION FUNDAMENTALS 625 



for latitudes of 40° while in the Arctic region the noise may be 15 to 

 25 db lower. The corresponding values for other bandwidths can be 

 obtained by adding 10 db for each 10-fold increase in bandwidth. More 

 complete estimates of atmospheric noise on a world wide basis are given 

 in the National Bureau of Standards Bulletin 462.^9 These noise data 

 are based on measurements with a time constant of 100 to 200 millisec- 

 onds. Noise peaks, as measured on a cathode ray tube, may be consid- 

 erably higher. 



The man made noise shown on Fig. 19 is caused primarily by opera- 

 tion of electric switches, ignition noise, etc., and may be a controlling 

 factor at frequencies below 200 to 400 mc. Since radio transmission in this 

 frequency range is primarily tropospheric (ground wave), man made 

 noise can be relatively unimportant beyond 10 to 20 miles from the 

 source. In rural areas, the controlling factor can be either set noise or 

 cosmic noise. 



Cosmic and solar noise is a thermal type interference of extra-terres- 

 tial origin. ^^ Its practical importance as a limitation on communication 

 circuits seems to be in the 20- to 80-mc range. Cosmic noise has been 

 found at much higher frequencies but its magnitude is not significantly 

 above set noise. On the other hand, noise from the sun increases as the 

 frequency increases and may become the controlling noise source when 

 high gain antennas are used. The rapidly expanding science of radio 

 astronomy is investigating the variations in both time and frequency 

 of these extra-terrestial sources of radio energy. 



References 



1. H. T. Friis, A Note on a Simple Transmission Formula Proc. I.R.E., 34, pp. 



254-256, May, 1946. 



2. K. Bullington, Radio Propagation at Frequencies Above 30 IMegacjcles, Proc. 



I.R.E., 35, pp. 1122-1136; Oct., 1947. 



3. K. Bullington, Reflection Coefficients of Irregular Terrain, Proc. I.R.E., 



42, pp. 1258-1262; Aug., 1954. 



4. W. M. Sharpless, Measurements of the Angle of Arrival of Microwaves, Proc. 



I.R.E., 34, pp. 837-845, Nov., 1946. 



5. A. B. Crawford and W. M. Sharpless, Further Observations of the Angle of 



Arrival of Microwaves, Proc. I.R.E., 34, pp. 845-848, Nov., 1946. 



6. A. B. Crawford and W. C. Jakes, Selective Fading of Microwaves, B.S.T.J., 



31, pp. 68-90; January, 1952. 



7. R. L. Kavlor, A Statistical Studv of Selective Fading of Super High Frequency 



Radio Signals, B.S.T.J., 32, pp. 1187-1202, Sept., 1953. 



8. H. E. Bussev, Alicrowave Attenuation Statistics Estimated from Rainfall 



and WaterVapor Statistics, Proc. I.R.E., 38, pp. 781-785, July, 1950. 



9. J. C. Schelleng, C. R. Burrows, E. B. Ferrell, Ultra-Short Wave Propagation, 



B.S.T.J., 12, pp. 125-161, April, 19.33. 

 10. MIT Radiation Laboratory Series, L. N. Ridenour, Editor-in-Chief, Volume 

 13, Propagation of Short Radio Waves, D. E. Kerr, Editor, 1951, jNIcGraw- 

 Hill. 



