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BELL SYSTEM TECHNICAL JOURNAL 



is curved, as shown in cross section in Fig. 6.3-2, the component Poynting 

 vectors which specify the directions in which energy is flowing will be slightly 

 divergent. As a result, only a portion of the total wavepower will proceed 

 in the preferred direction. It follows that, for best directivity, the emitted 

 wave front should be substantially plane, and the lines of force should be as 

 nearly straight as possible. There is shown in Fig. 6.3-3 a series of configura- 



Direction of propagation 



Fig. 6.3-2. Cross section of electromagnetic waves radiated from the flared end of a trans- 

 mission line. Lines of electric force lie in the plane of the illustration; lines of magnetic 

 force are perjiendicular to the illustration while the flow of ]:)owcr is along the divergent 

 arrows P. 



tions based partly on speculation and partly on deductions from Huygens' 

 principle. They illustrate in a rough way how, by increasing the aperture 

 between the two wires of the elementary radiator, we may make the indi- 

 vidual coiTiponent Poynting vectors more nearly parallel. ^° 



'" Figure 6.3-3 has been greatly oversimplified. Experiment shows that, to achieve the 

 result desired, the angle between the two wires of Fig. 6.3-3 must be smaller for larger 

 apertures than for small apertures. 



