Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 11
A. Wave amplitude
at point A versus time t.
A
1 s
t
make the analogy to radio waves more realistic, the wave must be generated in a continuous
fashion, as shown in Fig. 2.2A: A ball is dipped up and down at a cyclic rate,
successively reinforcing new wave crests on each dip. New waves continue to be generated
at point A as long as the ball continues to oscillate up and down. The result is a
continuous wave train spreading out through the water.
Perhaps the most important point to note about the water-wave analogy is that
careful observation of individual water molecules (if that were possible) a small distance
from the ball would show little or no net horizontal movement or displacement of
any specific molecule sideways across the surface of the water. Nonetheless, a clearly
visible wavefront travels outward from the point where the ball was dropped. Similarly,
the motion of a radio wave does not require that electrons travel with it. To the
contrary, we shall see in our discussion of basic antennas that maximum radiation occurs
at right angles to the direction of travel of electrons along a wire antenna!
There are two fundamental properties of all waves that are important to our study
of radio waves: frequency (f) and wavelength (l).The frequency is the number of oscillations
(or cycles) in the wave’s amplitude per unit of time. Since the fundamental unit of
time is universally accepted to be the second, frequency is often expressed as the number
of cycles per second (cps). If the time between successive positive peaks at any single
point in space is 0.5 s, as suggested by Fig. 2.2A, exactly two complete amplitude cycles
of the wave occur in 1 s, so the frequency of the wave created by the oscillating ball is
2 cps. Note that we can make the measurement from consecutive observable points
elsewhere on the waveform, such as negative peaks, zero crossings, etc.
At one time, frequencies (along with the frequencies of other electrical, mechanical,
and acoustical waves) were expressed
in cycles per second
but, in honor of Heinrich Hertz,
the unit of frequency was renamed
the hertz (Hz) many
years ago. Since the units are
equivalent (1 Hz = 1 cps), the
wave in Fig. 2.2 has a frequency
of 2 Hz.
Because the frequencies best
suited for efficient radio propagation
are so high, they are usually
expressed in kilohertz (kHz
= 1000s of Hz), megahertz (MHz
= 1,000,000s of Hz), or gigahertz
(GHz = 1,000,000,000s of Hz).
Thus, the frequency of WBZ in
Boston, operating near the mid-
B. Wave amplitude at time t
versus distance.
A
B
d
Figure 2.2 A bobbing ball on a
string demonstrates continuous
wave generation. (A) Wave amplitude
at point A versus time t. (B) Wave
amplitude at time t versus
distance d.