326 



BKI.L SYSTEM TEC/I. \/CAL JULR.XAL 



server is looking into one-half of a cut-away section of the total configuration. 

 In the complete configuration, the individual lines of electric force (solid 

 lines) and magnetic force (dotted lines) form closed loops, thereby pro- 

 ducing in each half-wave interval a packet of energy. The stream of projected 

 energy from an antenna is, according to this view, a series of these packets 

 one behind the other moving along the major axis of transmission. At the 

 transmitter each packet may have lateral dimensions that are only slightly 

 greater than the corresponding dimensions of the radiating antenna; but, 

 since the packet has curvature and since propagation is radial, the packet 

 spreads as it progresses so that at the distant receiver it may be very large 

 indeed. 



p"=o 



Fig. 6.3-5. Highly idealized representation of a wave-packet radiated by a typical micro- 

 wave source. One half of the total packet is assumed to be cut awa\'. 



Around the edge of each packet there is a region where the relationship 

 between the vectors E, H, and v is rather involved. For example, in the vicin- 

 ity of point 1 in Fig. 6.3-5, there is a substantial component of E but at this 

 point the vector // is zero and accordingly the Poynting vector P' at that 

 point is also zero. (See Equation 6.1-4.) In a similar way there may be in the 

 vicinity of point 2 a substantial component of magnetic force H; but, since 

 at this point the electric force is substantially zero, we conclude that the 

 Poynting vector P" is again zero and again no power is propagated." 



" The ])eculiar edge effects noted may be regarded as a result of a kind of wave inter- 

 ference not unlike that ])rc\-ailing in the regions of minimum E and // in the case of stand- 

 ing waves as discussed in Section 6.3. A similar kind of wave interference is cited in Section 

 6.5 to account for regions of low E and // in transmission along a waveguide-. 



