STANDING- WAVE ANTENNAS 



25 



at the end, center-fed and end-fed antennas are 

 distinguished. Center-fed hnear antennas are also 

 called dipole antennas. 



Typical current amplitude distributions are illus- 

 trated in Figure 5. The amplitude is always zero 

 at the open end while the amount at the input point 

 depends on the position of the mput connection. For 

 thin \\ares, compared with the length, the distribu- 

 tion of amplitudes is approximately sinusoidal. 



3.2.2 



Half- Wave Antennas 



Figure 6 illustrates two types of half-wave dipole 

 or center-fed antennas and one end-fed antenna, 

 together with their lumped-circuit analogues. The 



CURRENT- FED OR 

 CENTER -FED DIPOLE 



HIGH 2 



TAGE-f 

 END FED 



-INTERMEDIATE I- 

 DIPOLE VOLTAGE-FED OR 



SCHEMATIC CIRCUIT 



B C 



LUMPED-CIRCUIT ANALOGUE 



T 



1 



J 



SERIES RESONANCE 



PARALLEL RESONANCE 



Figure 6. Three methods of exciting half-wave an- 

 tennas and their analogues in lumped-constant resonant 

 circuits. 



input current required varies -with the position of 

 the input point. The voltage distribution in general 

 has a maximum at the points of current zero and 

 has a minimum where the current is maximum. 



*^ * Half- Wave Dipole 



The half- wave dipole, shown in A and B of 

 Figure 6 and in Figure 7, is the type most frequently 

 used in the 100 to 3,000 mc range. In this range the 

 length X/2 lies between 1.5 and 0.05 meters. In this 

 section it is assumed that the current distribution 

 is sinusoidal. 



1. Radiation field. The radiation field at point P, 

 Figure 7, where d > > X, is obtained by dividing 

 the half-wave current distribution into an infinite 



Figure 7. Half-wave dipole. 



number of infinitesimal doublets, using equation (2) 

 in Chapter 2 a-nd taking into account the differences 

 in phase at P introduced by the differences m the 

 distances which the radiation from the various 

 doublets must travel. The net result, using d in 

 place of r, is 



60/i cos [(Tr/2) • cos e)] 



E, 



sin 6 



volts per meter, (3) 



H. 



E„ 



= — — amperes per meter. 



120 TT 



(4) 



The normal part of the field, Eg (difference), pre- 

 scribes the antenna pattern factor (measured in 

 relative field strength) and is plotted in Figure 11. 

 The corresponding pattern for a doublet is Eg -^ sin B, 

 which is a circle in polar coordinates. These patterns 

 are circularly symmetric about the antenna axis. 

 Squaring the radial lengths in the above patterns 

 gives the pattern in terms of relative power per unit 

 area in the same angular direction. 



The radial component of radiated power per square 

 meter (Poynting's vector) is given by 



Wr = EeH^ = 



E- 

 1207r 



30/£ 



wd- 



cos { — cos 6 I 

 \2 / 



sin 6 



watts per square meter 

 In the equatorial plane, 



P 607i 



iifl = — — . 



(5) 



(6) 



