c. Propagation at Frequencies of 3 - 30 mc 



In the high frequency (HF) band, attenuation of the surface wave is 

 more rapid than at lower frequencies, while the sky wave is attenuated less be- 

 cause of the higher frequency. As the frequency is increased, the refractive 

 index tends toward unity and the skip distance increases . Thus, for a given dis 

 tance between transmitter and receiver there is an optimum frequency. The 

 frequency used is also determined by the state of the ionosphere . Statistical 

 data is available which helps determine the maximum utilizable frequency for 

 operation between two given points . 



For long-distance communication and navigation, the sky wave may 

 require more than one hop to cover the required distance . In this way it is pos - 

 sible for the radio wave to complete one or more trips around the earth, thus 

 giving rise to signal interference. However, transmitted frequencies in this 

 range are generally less subject to atmospheric and local interference than the 

 medium wave field. Fading is a problem in this high-frequency range and is 

 often minimized by such means as automatic sensitivity control and diversity 

 reception. 



d . Propagation at Frequencies Greater than 30 mc 



Rf energy, when radiated at the VHF and higher frequencies, behaves 

 in the same manner as light rays . The surface wave follows an almost optically 

 straight path from the transmitting antenna to the receiving antenna, with only 

 small atmospheric refraction. Because of the high frequency, the index of re- 

 fraction approaches unity, and wave fronts entering the ionosphere will not be 

 bent sufficiently to return to earth . For this reason all communications at fre- 

 quencies above about 30 mc operate within line -of -sight range . 



Because of the inhomogeneities in the earth's atmosphere, some re- 

 fraction does take place, and a direct wave can be directed to distances beyond 

 the line -of -sight range . The probable range of transmission R is given by the 

 expression: 



R = k [h^ -H fh^ in miles (3) 



Antennas at the transmitter of height hj^ and the receiver h2 are generally 

 placed as high as possible, and a refraction factor k of 1 .1 - 1 .6 is used to ac- 

 count for the increased range due to atmospheric refractivity . 



Atmospheric ducts which may make even line -of -sight transmission 

 within this frequency range impossible are sometimes formed. The ducts result 



22 



arthur ai.UittlcIlnr. 



