Practical_Antenna_Handbook_0071639586

176 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
uate electrical engineering texts on antennas use the elementary doublet as their starting
point for mathematical derivation of the characteristics of all dipole structures, including
half-wave dipoles. For clarity, the authors of this book eschew the use of doublet in describing
any “real” antennas and use the term elementary doublet only when referring to
the infinitesimally short hertzian dipole that gives antenna theorists such joy.
The half-wavelength dipole is an electrically “balanced” antenna consisting of a single
conductor one half-wavelength long. At HF and below, the antenna is usually installed
horizontally with respect to the earth’s surface or other local ground plane. The
resulting electric fields at distances greater than a few wavelengths from the antenna lie
parallel to the wire. Thus, by definition, the horizontal dipole produces a horizontally polarized
signal. At VHF and above, dipoles are commonly found in either orientation.
Although the transmitter output can be injected anywhere along the length of the
antenna, most commonly the wire is cut in the middle and a signal source injected
there, as shown in Fig. 6.1. This configuration is the ubiquitous center-fed halfwave dipole.
Because the velocity of propagation of an electromagnetic wave along a wire is
slightly less than in free space, the physical length of a half-wavelength on the antenna
is about 5 percent shorter than a half-wavelength in free space. A free-space half-wavelength
is found from
492
L (feet) =
(6.1)
F(MHz)
or, in metric terms,
150
L (meters) =
F(MHz)
(6.2)
where L = length of half-wavelength radiator, in feet
F = operating frequency, in megahertz
R
I
L
R
I
I
I Insulator
R Rope (etc)
75 Coaxial cable
Figure 6.1 Simple half-wave dipole antenna.