Page 53 - Antennas for Base Stations in Wireless Communications

26 Chapter One

q 0 q
30 30

60 60

90 90
Figure 1.16 Example of an axial ratio
pattern

that is able to get only the magnitude of the data. The two envelops are
presented by E 1 and E 2 as
φ
φ
E ( , ) = E ( , ) + E ( , )
θ
θ
φ
θ
x
1
c
(1.27)
,φ
E ( , ) = E ( , ) − E (θ, )
φ
θ
φ
θ
2
x
c
From E and E the axial ratio that is used as a measure of the quality
2
1
of the circular polarization is given as
θ
|AR ( , )|= 20 log E 1 ( , ) φ (1.28)
θ
φ
θ
E 2 ( , ) φ
Usually the AR is measured for the main beam and is given as
|AR | |AR ( , )| (1.29)
=
0
0
1.3.3 Measurement Systems
for Intermodulation
The intermodulation phenomenon exists in nonlinear devices working
in a high-power environment. Intermodulation is widely discussed for
the design of power amplifiers. Similarly, for a base station transmitting
antenna, which emits high power, the intermodulation exists due to the
metal/metal joint and material nonlinearity. The intermodulation is a kind
of interference that should be suppressed. Suppose two closely located
and f , are transmitted by the antenna; due
fundamental frequencies, f 1 2
to nonlinearity, the radiated field has components at the frequencies of
f , f , 2f , 2f , 3f , 3f , f + f , f − f , 2f − f , 2f − f , and so on. Since f and
2
1
1
1
1
2
1
2
1
1
2
2
1
2
2
f are close to each other, the 3rd order intermodulation components at
2
2f − f and 2f − f are very close to the fundamental frequencies and are
2
1
1
2
difficult to remove using filters, thus introducing interferences.
To measure the 3rd order intermodulation, the antenna under test
operates at a transmitting mode and the testing probe operates at a
receiving mode. The testing probe should have a very good linearity