PROPAGATION OF RADIO WAVES 



understand this general mechanism, be able to eval- 

 uate the factors which plaj^ contributory roles, and, 

 for a given amount of power emitted from a given 

 transmitter, be able to compute the strength of the 

 radiation field at anj'- point in space or to locate all 

 the points in space where a given field strength 



OCCVU'S. 



The problem divides naturally into two parts, (1) 

 the one-way transmission or communication problem, 

 and (2) the two-way transmission or radar problem. 

 In the former the prime requisite is to calculate the 

 amount by which the wave and its field strength are 

 attenuated in passing from the transmitter to a 

 receiver and yet permit a field at the receiver suffi- 



'•^•^ Atmospheric Layers 



The atmosphere from one point of view is com- 

 posed of two layers, the troposphere and the strato- 

 sphere. The former is a layer adjacent to the earth 

 which extends upward approximately 10 km, in 

 which the temperature decreases about 6.5 C per 

 kilometer with increasing altitude to a value, at the 

 upper boundary, of about — 50 C. Above this is 

 the stratosphere in which the temperature remains 

 approximately constant at — 50 C. 



The ionosphere, as its name implies, is a layer (or 

 series of layers) composed of ions and free electrons 

 lying at an elevation of approximately 100 km. See 





Figure 1. Transmission along and reflection from the ionosphere occurs primarily with frequencies below 30 me. 

 At higher frequencies useful transmission is primarily concerned with the nearly horizontal rays in the troposphere; 

 higher angle radiation passes thi'ough the ionosphere and is lost. 



cient at least to produce the minimimii detectable 

 signal. In the latter problem the attenuation must 

 be calculable for the two-way journey from trans- 

 mitter to the target and back to the receiver, which 

 frequently uses the same antenna as the transmitter. 

 In this type of problem, due accoimt must also be 

 taken of the reflecting and reradiating properties of 

 the target. 



Knowledge of these factors is indispensable for the 

 design, installation, and successful operation of both 

 communic ition and radar systems. 



Figure 1. These layei-s play an important role in the 

 transmission of waves at frequencies below 30 mc 

 and are responsible for transmission over very long 

 distances. At the higher frequencies, which are the 

 concern of this volume, the portion of the waves 

 which penetrate the ionosphere is not useful for 

 transmission. 



From this it follows that propagation at the higher 

 frequencies (above 30 mc), to be useful, must occur 

 entirely in the troposphere. This volume therefore 

 is concerned only with tropospheric propagation. 



