Page 49 - Antennas for Base Stations in Wireless Communications

22 Chapter One
+
−
the ratio between the reflected to incident waves as G = V /V . Then, the
impedance at the reference plane can be computed as Z = Z c (1+Γ)/(1−Γ),
where Z c is the characteristic impedance of the cable connecting to the
antenna. Consequently, before starting to measure the DUT, the net-
work analyzer has to be calibrated using the slandered techniques of
standard short, open, and matching loads. One of the most important
parameters that calibration establishes is the reference plane, which is
critical for phase information. This is a single port measurement.
In some cases, two port measurements are needed to measure the
reflection coefficients for the two ports and the mutual coupling between
them. These are referred to as the S-parameter measurements. This
measurement determines the mutual coupling between two antennas
or, for some antennas, determines the isolation between two ports of a
dually polarized antenna.

1.3.2 Measurement Setups
for Far-Zone Fields
For far-field measurements, the distance between the transmitter and
receiver has to be large enough to be sure that the transmitter is in the
far zone of the antenna under test. To perform these measurements
indoors, you have to provide an environment that ensures the antenna
does not interact with the surroundings and operates within the envi-
ronment as if in free space. To achieve this, an anechoic chamber is
used with its walls covered with proper absorbing materials that reduce
or eliminate the reflections from the walls. The absorbing materials
have a certain bandwidth or, in other words, a certain lower frequency
bound. The lower frequency is reduced as the absorbing material size
is increased.

1.3.2.1 Far-Field System To measure the far-zone field, the transmit-
ting and receiving antennas are put into an anechoic chamber, using
a spacing that satisfies each other’s far-zone requirement. Figure 1.13
shows an anechoic chamber instrumentation block diagram. This dis-
tance guarantees the wave impinging on the receiving antenna can be
approximated as a plane wave. Generally, the far-field distance, d, is
considered to be

d = 2 D 2 (1.26)
λ
where D is the antenna diameter and l is the wavelength of the radio
wave. Separating the antenna under test (AUT) and the instrumenta-
tion antenna by this distance reduces the phase variation across the
AUT enough to obtain a reasonably good antenna pattern.