018 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



V. IONOSPHERIC TRANSMISSION 



111 addition to the troposphoric or ground wave transmission discussed 

 in the preceding sections, useful radio energy at frequencies below about 

 25 to 100 mc may be returned to the earth by reflection from the iono- 

 sphere, which consists of several ionized layers located 50 to 200 miles 

 above the earth. The relatively high density of ions and free electrons in 

 this region provides an effective index of refraction of less than one, and 

 the resulting transmission path is similar to that in the well known optical 

 phenomenon of total internal reflection. The mechanism is generally 

 spoken of as reflection from certain virtual heights.^^ Polarization is not 

 maintained in ionospheric transmission and the choice depends on the 

 antenna design that is most efficient at the desired elevation angles. 



Regular lonosphei'ic Transmission 



The ionosphere consists of three or more distinct layers. This does not 

 mean that the space between layers is free of ionization but rather that 

 the curve of ion density versus height has several distinct peaks. The 

 E, F\, and F2 layers are present during the daj'^time but the Fi and Fo 

 combine to form a single layer at night. A lower laj^er called the D layer 

 is also present during the day, but its principal effect is to absorb rather 

 than reflect. 



Information about the nature of the inosphere has been obtained 

 by transmitting pulsed radio signals directly overhead and bj- record- 

 ing the signal intensity and the time delay of the echoes returned from 

 these layers. At night all frequencies below the critical frequency jc are 

 returned to earth with an average signal intensity that is about 3 to 6 

 db below the free space signal that would be expected for the round 

 trip distance. At frequencies higher than the critical frequency the signal 

 intensity is very weak or undetectable. Tj^pical values of the critical 

 freciuency for Washington, D. C, are shown in Fig. 16. 



During the da\'time, the critical frequency is increased 2 to 3 times 

 over the corresponding nighttime value. This apparent increase in the 

 useful frequency range for ionospheric transmission is largely offset by 

 the heavy daytime absorption which reaches a maximum in the 1 to 2- 

 mc range. This absorption is caused by interaction between the free elec- 

 trons and the earth's magnetic field. The absence of appi-eciable absorp- 

 tion at night indicates that most of the free electrons disappear when 

 the sun goes down. Charged particles traveling in a magnetic field have 

 a resonant or gyromagnctic frequency, and for electrons in the earth's 

 magnetic field, of about 0.5 gauss, this resonance occurs at about 1.4 



