RADIO PROPAGATION FUNDAMENTALS 



023 



50 



60 



LU 



o 

 < 



CL 



in 70 



LU 

 LU 



tr 



'^ 80 



$ 



o 



90 



(DlOO 



lU 

 Q 



no 



120 



25 30 35 40 45 50 55 60 65 70 



FREQUENCY IN MEGACYCLES PER SECOND 



75 



80 



Fig. 18 — Median signal levels for ionospheric scatter transmission. 



patches of ionization in the E layer. It is suspected that meteors are 

 important in estabUshing and in maintaining this ionization but this 

 has not been clearly determined. 



In common with other types of transmission, the fast fading follows 

 a Rayleigh distribution. The distribution of hourly median values rela- 

 tive to the long term median (after the high signals resulting from 

 sporadic E transmission have been removed) is approximately a normal 

 probability law with a standard deviation of about 6 to 8 db. 



Ionospheric scatter transmission is suitable for several telegraph 

 channels but the useful bandwidth is limited by the severe selective 

 fading that is characteristic of all ionospheric transmission. 



VI. NOISE LEVELS 



The usefulness of a radio signal is limited by the "noise" in the re- 

 ceiver. This noise may be either unwanted external interference or the 

 first circuit noise in the receiver itself. 



Atmospheric static is ordinarily controlling at frecjuencies below a 

 few megacycles while set noise is the primary limitation at frequencies 

 above 200 to 500 me. In the 10- to 200-mc band the controlling factor 

 depends on the location, time of day, etc. and may he either atmospheric 

 static, man made noise, cosmic noise, or set noise. 



The theoretical minimum circuit noise caused by the thermal agitation 

 of the electrons at usual atmospheric temperatures is 204 db l)elow one 



