30 



ANTENNAS 



regions of the curves correspond to antenna half- 

 lengths near X/4 and near X/'2. The former repre- 

 sents a center-fed half -wave antenna, whereas the 

 latter represents a pair of end-fed half-wave antennas 

 excited in phase. The half-kmgth of the antenna was 

 used in plotting, because in these terms the reactance 

 curves resemble those for an open-ended trans- 

 mission line. 



In the regions of principal interest the reactance 

 curves are nearly straight lines whose slopes depend 

 on the diameter of the antennas expressed in wave- 

 lengths. The slopes of the reactance curves decrease 

 as the antenna diameter increases. This feature is 

 important in radar antennas which need to be in- 

 sensitive to small changes in frequencj'. The ciu'ves 



than when the half-length approximates X/4, as it 

 does for a single center-fed antenna. The values for 

 an antenna whose half-length is X/4 is not readable 

 on the curve, but the component representing radia- 

 tion ranges from 73 ohms for infinitely thin antennas, 

 through 64 ohms for a diameter of 0.0001 X, 55 ohms 

 for a diameter of 0.01 X, to less than 50 ohms for 

 certain large-diameter radar antennas. The change 

 is mainly due to a decrease in the resonant length of 

 the thicker antennas. 



A feature of Figure 15 \\hich is not easily readable 

 is that the lengths at which the reactance is zero are 

 less than X/2 and X. The amount by which an an- 

 tenna with zero reactance is shorter than these 

 lengths depends on the antenna diameter. For very 



10 



z 

 I 

 o 



ui 

 o 



z 

 < 



« 

 (A 



tu 



C 



10,000 

 9000 

 8000 

 7000 

 6000 

 5000 



4000 



3000 



2000 



1000 



X/4 



X/2 



3X/4 



5X/4 



HALF-LENGTH OF ANTENNA 



Figure 1G. Resistanfe at ininit of a eenter-fed antenna of arbitrary length. 



show that antennas of large diameter present less 

 than a specified amount of reactance, say one ohm, 

 over a greater range of antenna length than slender 

 antennas do. In terms of frequencj^, this means 

 that a given length of antenna has less than one-ohm 

 reactance over a wider range of frequency when the 

 antenna has a large diameter than when it has a 

 small diameter. Radar antennas are commonly 

 made of tubing and frequently have diameters in 

 excess of X/20. 



Figure 16 shows that the input resistance also 

 depends on antenna diameter. This dependence is 

 more pronounced when the half-length is about X/2 



slentler antennas the shortening is slight, but for 

 large-diameter antennas or for special shapes as 

 shown in Figure 17, a resonant length may be as 



COAXIAL LINE 



FiGURp; 17. Xon-cylindrical half-wave antenna. 



much as 20 per cent shorter than X/2. Special shapes, 

 such as the one shown in Figure 17, ha^•e the ad- 



