Antenna Measurement Theory
1
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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.