Practical_Antenna_Handbook_0071639586
C h a p t e r 3 : A n t e n n a B a s i c s 101 Resultant A Resultant B Resultant C Resultant D Resultant E Figure 3.9 Development of standing wave from traveling wave. The half-wave center-fed dipole is simply one specific length of the arbitrary-length antenna shown in Fig. 3.9. In fact, if we assume that the curves of Fig. 3.9 are describing voltage waveforms, the vertical dotted line in the figure can be thought of as corresponding to one end of a center-fed l/2 dipole, and everything to the right of the dotted line can be discarded when discussing conditions on the dipole. The most important points to note for a l/2 center-fed dipole are then:
102 P a r t I I : F u n d a m e n t a l s • There is always a voltage maximum at the end of the wire on each side of the dipole, and it is the only voltage maximum on either side. • Although not shown in Fig. 3.9, there is always a current maximum at the source, and it is the only current maximum anywhere along the antenna. Standing Waves of Voltage and Current Just to fully clarify the time-varying nature of standing voltage and current waveforms on a l/2 dipole, let’s examine, with the help of Figs. 3.10 and 3.11, what happens in a Time T 16 T 0 T 8 T 8 T 2 T 14 T3 T 13 T 4 T 12 T 5 T 11 T 6 T 10 A Y X B Voltage distribution A Voltage 0 T 0 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 T 11 T 12 T 13 T 14 T 15 T 16 Voltage at A Voltage at Y B Figure 3.10 Standing waves of voltage at two points on an antenna.
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102 P a r t I I : F u n d a m e n t a l s<br />
• There is always a voltage maximum at the end of the wire on each side of the<br />
dipole, and it is the only voltage maximum on either side.<br />
• Although not shown in Fig. 3.9, there is always a current maximum at the<br />
source, and it is the only current maximum anywhere along the antenna.<br />
Standing Waves of Voltage and Current<br />
Just to fully clarify the time-varying nature of standing voltage and current waveforms<br />
on a l/2 dipole, let’s examine, with the help of Figs. 3.10 and 3.11, what happens in a<br />
Time<br />
T 16 T 0<br />
T 8 T 8<br />
T 2 T 14<br />
T3 T 13<br />
T 4 T 12<br />
T 5 T 11<br />
T 6 T 10<br />
A Y X B<br />
Voltage distribution<br />
A<br />
Voltage<br />
0<br />
T 0 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 T 11 T 12 T 13 T 14 T 15 T 16<br />
Voltage at A<br />
Voltage at Y<br />
B<br />
Figure 3.10 Standing waves of voltage at two points on an antenna.
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