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 103
small region somewhere along either side of the dipole over the course of one complete
cycle of RF energy from the transmitter.
In Fig. 3.10A, standing voltage waveforms occurring at equal time intervals over
the course of one cycle of RF energy are brought together on one axis, AB, corresponding
to the total length of a half-wave antenna. As before, we assume that RF energy to
drive the antenna is being injected at the center (point X).
Figure 3.10B plots the standing voltage waveform as a function of time throughout
one cycle of RF energy for two different points (A and Y) on the dipole. Note that the
standing voltages seen at these two points are in phase—that is, their amplitudes go up
and down simultaneously even though the peak amplitudes of the two points are, in
general, different. This curve is valid for any pair of points or any number of points on
the dipole.
T 4
T 4
T 2
T 6
T 1
T 7
T
Time 0
T 8
T 16
A
Y
X
B
T 9
T 15
T 10
T 14
T 12
T 12
A
Current distribution
Current
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
Current at X
Current at Y
B
Figure 3.11 Standing waves of current at two points on an antenna.