Page 35 - Antennas for Base Stations in Wireless Communications

8 Chapter One

circular polarizations. However, circular polarization can be generated
from a linearly polarized antenna by feeding the antenna by two ports
with equal magnitude and with a 90° phase difference between them.
From the linear field components in the far zone, the circular polariza-
tion can be presented as
φ
φ
E ( , ) + jE ( , )
θ
θ
E ( , ) = φ θ (1.7)
φ
θ
c
2
E ( , ) − jE ( , )
φ
θ
θ
φ
E ( , ) = φ θ (1.8)
θ
φ
x
2
where E c is the copolar of the circular polarization, in this case, the left-
hand CP, and E x is the cross-polarization, or the right-hand CP.
In practice, regardless of confusing terminology, matching linearly
polarized antennas is important, or the received signal strength is
greatly reduced. So, horizontal polarization should be used with hori-
zontal antennas and vertical with vertical. Intermediate matching will
cause the loss of some signal strength, but not as much as a complete
mismatch. Transmitters mounted on vehicles with large motional free-
dom commonly use circularly polarized antennas so there will never
be a complete mismatch with signals from other sources. In the case of
radar, these sources are often reflections from rain drops.
In order to transfer maximum power between a transmit and receive
antenna, both antennas must have the same spatial orientation, the
same polarization sense, and the same axial ratio. When the antennas
are not aligned or do not have the same polarization, power transfer
between the two antennas will be reduced. This reduction in power
transfer will reduce the overall system efficiency and performance as
well. When transmit and receive antennas are both linearly polarized,
physical antenna misalignment will result in a polarization mismatch
loss, which can be determined using the following formula:
Loss (dB) = 20 log (cosf) (1.9)
where f is the difference in alignment angle between the two antennas.
For 15°, the loss is approximately 0.3 dB; for 30°, the loss is 1.25 dB; for
45°, the loss is 3 dB; and for 90°, the loss is infinite.
In short, the greater the mismatch in polarization between a transmit-
ting and receiving antenna, the greater the apparent loss will be. You
can use the polarization effect to your advantage on a point-to-point link.
Use a monitoring tool to observe interference from adjacent networks,
and rotate one antenna until you see the lowest received signal. Then
bring your link online and orient the other end to match polarization.