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Antenna Measurement Theory
1
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Introduction to
Antenna Measurement
1. Basic Concepts
1.1 ELECTROMAGNETIC 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) decay as 1/r .
These two types of fields dominate in different regions in
space around the antenna. Based on this characterization
of the Poynting vector, we can
identify three major
regions (Figure 1).
1.1.1. Reactive Field
This region is the space immediately surrounding the antenna. The extent of this region
is 0 < r <
λ
/2
π
, where l is the wavelength. In
this space the Poynting vector is
predominantly reactive (nonradiating), has all three components in spherical
coordinates (r,
θ,φ
) and decays more rapidly than 1/r.
1.1.2 Radiating NearField
Beyond the immediate neighborhood of the reactive field the radiating field begins to
dominate. The extent of this region is
λ/
2
π
< r < 2D
2
/
λ
, where D is the largest dimension
of the antenna. This region can be divided into two subregions. For
λ/
2
π
< r < D
2
/4
λ
the
fields decay more rapidly than 1/r and the radiation pattern (relative angular distribution
of the field) is dependent on r. For D
2
/4
λ
< 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
transform of the
aperture distribution with a phase error of more than 22.5°. The phase
error is dependent on r (for r
→∞
the phase error is equal to zero). This region is often
referred to as the Fresnel zone, a terminology borrowed from optics.
Figure 1: Radiating Regions
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1.1.3 Radiating FarField
Beyond the radiating NearField region r > 2D
2
/
λ
or r > 10l (criterion for small 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
transform of the aperture distribution with a phase error of less than 22.5°. This region is
often referred as the Fraunhofer zone, a terminology borrowed from optics.
1.2 ANTENNA PARAMETERS
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
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 Spring '10
 ThoLeNgoc
 Electromagnet

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