Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 473
Antenna Impedance
Impedance is a measure of device or system opposition to the flow of alternating current
(e.g., RF); in the general case it is complex; that is, it includes both resistive and reactive
components. The reactive components can be either capacitive or inductive, or a
combination of both. Impedance can be expressed in either of two notations.
The magnitude can be obtained from
2 2
Z = R + ( X – X )
(20.21)
Alternatively, in complex plane notation, where pure reactances lie along the imaginary
axis,
Z = R ± jX
(20.22)
Of these, Eq. (20.22) is more useful in RF applications because it provides both magnitude
and phase information. This is especially important when dealing with the antenna’s
near-field performance because energy transfer in the near field is highly
reactive, consisting predominantly of large circulating amounts of electric and magnetic
energy (close to the radiator) being exchanged with the radiating antenna throughout
the course of each and every cycle.
As discussed in earlier chapters dealing with antennas for lower frequencies, the
resistive part of an antenna’s impedance consists of two elements: ohmic losses R W and
radiation resistance R RAD . The ohmic losses are due to heating of the antenna conductor
elements by RF current passing through, as when current passes through any conductor.
Efficiency x is then:
L
c
RRAD
ξ =
R + R
RAD
Ω
(20.23)
A major goal of the antenna designer—for transmitting antennas, at least—is to
maximize x by minimizing R W and by developing an antenna design that results in a
value for R RAD that is as large as is practical.
Dipole Antenna Elements
The dipole can be modeled as either a single radiator fed at the center (Fig. 20.20A) or as
a pair of radiators fed back to back (Fig. 20.20B). As discussed in Chaps. 3 and 6 in particular,
by definition the polarization of an electromagnetic field is the direction of the
electrical field vector. Since the dipole’s far E-field is parallel to the radiating element, a
horizontal dipole produces a horizontally polarized signal, while a vertical element
produces a vertically polarized signal.
A microwave dipole is shown in Fig. 20.21. The antenna radiating element consists
of a short conductor at the end of a section of waveguide. Although most low-frequency
dipoles are a half-wavelength, microwave dipoles might be exactly a half-wavelength,
less than a half-wavelength, or greater than a half-wavelength, depending upon the application.
For example, because most microwave dipoles are used to illuminate a reflector
of some sort, the length of the dipole depends upon the exact illumination function
required for proper operation of the reflector. Most, however, will be a half-wavelength.