302 
RESISTANCE IN OHMS 
PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
[biameTer | 
aweren {07300 1[| | 
5\/4 
HALF-LENGTH OF ANTENNA 
Figure 16. Resistance at input of a center-fed antenna of arbitrary length. 
as the antenna diameter increases. This feature is 
important in radar antennas which need to be in- 
sensitive to small changes in frequency. The curves 
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 frequency, 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 4/20. 
Figure 16 shows that the input resistance also 
depends on antenna diameter. This dependence is 
more pronounced when the half-length is about \/2 
than when the half-length approximates 4/4, as it 
does for a single center-fed antenna. The values for 
an antenna whose half-length is \/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 , 55 ohms 
for a diameter of 0.01 \, 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 which is not easily readable 
is that the lengths at which the reactance is zero are 
less than 4/2 and \. The amount by which an an- 
tenna, with zero reactance is shorter than these 
COAXIAL LINE 
Wrq@urE 17. Non-cylindrical half-wave antenna. 
lengths depends on the antenna diameter. For very 
slender 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 
much as 20 per cent shorter than \/2. Special shapes, 
such as the one shown in Figure 17, have the ad- 
AXIS 
DIRECTION OF vaste RADIATION 
fay 
- 
- 
2 
Qa 
2 
Ficure 18. Standing-wave V antenna (@= 36° 
for n = 4 half-wavelengths). 
