Antenna Measurement Theory
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Introduction to
Antenna Measurement
1. Basic Concepts
1.1 ELECTRO MAGNETIC WAVES
The radiation field from a transmitting antenna is
characterized by the complex Poynting vector E x H* in
which E is the electric field and H is the magnetic field.
Close to the antenna the Poynting vector is imaginary
(reactive) and (E,H) decay more rapidly than 1/r, while
further away it is real (radiating) and (E,H) deca y as 1/r .
These two types of fields dominate in different regions in
space aro und the ant enn a. Bas ed o n t his c h arac ter ization
of the Poynting vector, we can identify three major
regions (Figure 1).
1.1.1. Reactive Field
This region is the s pace im m ediately surroun ding the antenna. T he extent of this regio n
is 0 < r < λ/2π, where l is the wavelength. In this space the Poynting vector is
predominantly reactive (non-radiating), has all three components in spherical
coordinates (r,θ,φ) and decays more rapidly than 1/r.
1.1.2 Radiating Near-Field
Beyond the immediate neighborhood of the reactive field the radiating field begins to
dominate. The ex ten t of this r egio n is λ/2π < r < 2D
2
/λ, where D is the l ar gest dimens io n
of the antenn a. T his r egion c an be div ided in to two s ubregi ons. F or λ/ 2π < r < D
2
/4λ the
fields d ecay more r apidl y than 1/r and the r adiation p attern (relat ive angul ar distributio n
of the field) is dependent on r. For D
2
/4λ < r < 2D
2
/λ the fields decay as 1/r, but the
radiation pattern is dependent on r. The radiation pattern is equal to the Fourier
transf orm of the aper ture dis tribution with a phas e err or of mor e than 22.5°. T he phase
error is dep en dent on r (for r →∞ t he phas e er ror is e qua l to zer o). Th is r egio n is often
referred to as the Fresnel zone, a terminology borrowed from optics.
Figure 1: Radiating Regions
Reprinted with the permission of ORBIT/FR Inc.
Antenna Measurement Theory
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1.1.3 Radiating Far-Field
Beyond the radiating Near-Field region r > 2D
2
/λ or r > 10l (c riteri on f or s m all antennas )
the Poynting vector is real (only radiating fields) and has only two components in
spherical coordinates (θ,φ). The fields decay as 1/r and the radiation pattern is
independent of r. The radiation pattern in this region is approximated by the Fourier
transf orm of the apertur e d i str ibution with a phase er r o r of les s than 22.5°. T his r egio n is
often referred as the Fraunhofer zone, a terminology borrowed from optics.
1.2 ANTENNA PARAM ET ERS
1.2.1 Antenna
The antenna is a device which transforms guided electromagnetic signals into
electromagnetic waves propagating in free space. It can be used for reception and
transmission.
1.2.2 Polarization
Polarization is the property of the electric field vector that
defines variation in direction and magnitude with time. If
we observe the field in a plane perpendicular to the
direction of propagation at a fixed location in space, the
end point of the arrow representing the instantaneous
electric field magnitude traces a curve. In the general
case, this cur ve is an e ll ipse (Figure 2) . The ellips e c a n be
characterized by the axial ratio (AR), the ratio of the two
major axes and its tilt angle t. Polarization may be
classified as linear, circular or elliptical according to the
shape of the curve. Linear and circular polarization are
special cases of elliptical polarization, when the ellipse
becomes a straight line or circle, respectively. Clockwise
rotation of the electric field vector is designated as right-
hand polarization (RH) and counterclockwise rotation is
left-hand polarization (LH), for an observer looking in the
direction of propagation.
Figure 2: Elliptical
Polarization
Reprinted with the permission of ORBIT/FR Inc.