Practical_Antenna_Handbook_0071639586

182 p a r t I I I : h i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
The Dipole Radiation Pattern
In Chap. 3 we developed the pattern of a very short (relative to a wavelength) dipole
in free space and compared it to that of a totally fictitious reference antenna called the
isotropic radiator. Compared to the latter, which is truly omnidirectional (“all directions”),
the short dipole generates a greater radiated signal broadside to the axis of
the wire and has little or no radiation off its ends. We describe the short dipole in part
by saying it has 1.5-dB gain relative to an isotropic radiator in its favored direction,
but we simultaneously understand that the gain we’re referring to came at the expense
of radiated signal in other directions. Thus, gain in an antenna is a direct consequence
of its directivity.
In fact, much of the time what we call gain (G) is actually directivity (D). Specifically,
G = ξD (6.7)
where x is the efficiency of the antenna. For dipoles and similar devices having lengths
of l/4 or longer, efficiency is usually close to 100 percent (x = 1.0), and we can be a little
careless with our use of the word gain. But for some antenna types that we cover later
in this book, x is much lower. A Beverage wire, for instance, has very low efficiency and
terrible gain, yet can have excellent directivity.
Always keep in mind that directivity and gain are specified in three dimensions. Too
many times, people oversimplify the topic by publishing only the azimuthal (compass
heading) view of the radiation pattern. In other words, the reader is given a pattern as
viewed from directly above the antenna that shows the directivity in the horizontal
plane. But a signal does not propagate away from an antenna in an infinitely thin sheet,
as such presentations seem to imply; rather, it radiates to varying degrees at all elevation
levels above ground. Thus, proper evaluation of an antenna takes into consideration
both horizontal and vertical plane patterns.
Figure 6.4 shows the radiation pattern of a dipole antenna in free space as seen from
different perspectives. In the horizontal plane (Fig. 6.4A), when viewed from above, the
pattern is a figure eight that exhibits bidirectional gain broadside to the axis of the antenna.
What appear to be two main “lobes” contain the bulk of the radiated RF energy,
with little or no power off the ends of the antenna axis. This pattern is the classical dipole
pattern that is published in most antenna books.
Also shown, however, is the vertical plane pattern for a dipole antenna in free space
(Fig. 6.4B). Note that when sliced this way the radiation pattern appears circular. When
the two patterns are combined in the round, you can see the three-dimensional
Âdoughnut-shaped pattern (Fig. 6.4C) that helps us visualize the true pattern of an unobstructed
dipole in free space.
When a dipole is installed “close” to the earth’s surface or other ground beneath it,
as is the case for most HF antennas, the antenna pattern is distorted from that of Fig. 6.4.
Two effects must be taken into consideration:
• Radiated energy from the antenna that strikes the ground directly below the
antenna and is reflected back up to the antenna induces additional currents and
voltages in the antenna. The phase of these relative to the original transmitted wave
is a function of the height of the antenna above the reflecting plane. This changes the