Page 55 - Antennas for Base Stations in Wireless Communications

28 Chapter One

1.4 System Calibration

Gain and Polarization Calibrations of Standard Antennas This calibration ser-
vice is offered primarily for determining the absolute on-axis gain and
polarization of standard gain horns, which, in turn, are used as reference
standards in determining the gain and polarization of other antennas
by the gain comparison technique. The antennas need not be identical.
This method is the most accurate technique known for absolute gain and
polarization measurements. For gain measurements, the uncertainties
are typically 0.10–0.15 dB. Uncertainties of 0.05 dB/dB for polarization
axial ratio measurements are typical.
Near-Field Scanning Techniques With this technique, gain, pattern, and
polarization parameters are calculated from near-field amplitude and
phase measurements taken over a surface close to the test antenna.
The absolute gain can be determined to within about 0.2 dB, the polar-
ization axial ratio to within about 0.10 dB/dB, and sidelobe levels down
to −50 dB or −60 dB. The exact uncertainties in these parameters will
depend on such factors as the frequency, type, and size of antenna.
Calibrated probes are normally required for these measurements. In
order to achieve accurate results with the planar, cylindrical, or spheri-
cal near-field method, the transmitting or receiving properties of the
probe must be known. With this information, the measured data can
be corrected for the nonideal pattern and polarization properties of the
probe. Probes are characterized by a three-step process: (1) The on-axis
gain and polarization properties are measured using the technique
described; (2) the far-field amplitude and phase patterns are measured
for two nominally orthogonal polarizations of the incident field; and
(3) the on-axis and pattern data are combined to obtain the probe cor-
rection coefficients at the desired lattice points for the measurement
surface specified. 5, 31

1.5 Remarks
In this chapter, we briefly presented the fundamental parameters of
the antenna so beginning engineers can quickly grasp the meaning of
antenna parameters. We also gave examples of different printed and
simple antennas and their advantages and disadvantages. In addition,
we touched on the measurement techniques for antenna parameters. We
indicated the significance of the calibration of these systems to provide
reliable measurements. The reader who needs more detail should refer
to the references provided.