28 



ANTENNAS 



The radiation resistance, both for integral and 

 nonintegral numbers of hah' wa^-elengths, is plotted 

 in Figure 12. 



3.2.6 



Cophased Half-Wave Dipoles 



OS 



5= 



Bl 



1.0 



ilii 



m 



3.0 



m 



i\t 



5.0 



iSM^ 



6.0 



7.0 



LENGTH OF WIRE IN WAVELENGTHS 



Figure 12. Radiation resistance for linear antennas. 



In Table 1 the radiation resistances and the power 

 gains for integral half-wavelength antennas are 

 listed. 



The directivity and gain of linear antennas may be 

 increased considerably by the suppression of alter- 

 nate current loops, leaving therefore only loops in 

 which the currents are all cophased. The suppressed 

 loops are contained in either (1) quarter- wave stubs 

 or (2) short inductive elements, as indicated in 

 Figure 1.3. The suppressed loops are practically 

 nonradiative. 



The radiation field at distance d is the vector sum 

 of the fields from the n half-wave elements. The 

 contribution from each element lags that of the next 

 element above by an angle 



a = — cos 

 2 



— = TT cos d radians, (17) 



X 



Table 1. Comparison of alternate and cophased half-wave dipoles. 



^doublet /-fdoubletV _ ^2 doublet / Em,n \2 

 B„ V I„ ) ~ R„ V^doublet/' 



Ic = 



FiGURK 13. Cophased half-wave dipoles. 



