Practical_Antenna_Handbook_0071639586

C h a p t e r 1 0 : W i r e A r r a y s 257
16
14
Antenna length (wavelengths)
12
10
8
6
4
2
0 1 2 3 4 5 6 7 8 9 10
Figure 10.5 Antenna length versus gain over dipole.
Gain relative to dipole (dB)
the forward and reflected waves sets up stationary (i.e., “standing”) currents and voltages
along the wire, similar to the process described in Chap. 3.
In contrast, a nonresonant longwire is terminated at the far end in a resistance equal
to its characteristic impedance. (See Fig. 10.6.) Thus, the outward-bound wave is absorbed
by the resistor, rather than being reflected. Such an antenna is called a traveling
wave antenna. Terminating resistor R 1 is selected to be equal to the characteristic impedance
Z 0 of the antenna (i.e., R = Z 0 ). When the wire is 20 to 30 ft above average ground,
Z 0 is about 500 to 600 Ω.
The radiation pattern for the terminated longwire is a unidirectional version of the
multilobed pattern found on the unterminated longwires. The orientation of the lobes