Page 140 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes Advantages of Highly Directional Antennas

A highly directional antenna improves the signal reception because it is pointed at the origin of
the signal. Multi- and omni-directional antennas pick up all signals from all directions, resulting
in too many incoming signals and a weaker signal from the direction of choice. Highly directional
antennas also “ignore” signals coming from places other than the one they are directed to, which
cuts down the interference with a chosen signal. Another advantage these antennas offer is the
ability to change the focus of the receiver to another direction. For example, when switching
channels on a TV set, the grid aerial, or antenna, is moved around to get the best reception or
signal. To get the best signal from any highly directional antenna, it is recommended to mount it
as high as possible on a given building. This cuts down the chances of signals being blocked by
tall buildings and landscape features such as trees, hills and mountains.

8.2.8 Polarization

Polarization (or Polarisation for our British friends) is one of the fundamental characteristics of
any antenna. First we’ll need to understand polarization of plane waves, then we’ll walk through
the main types of antenna polarization.

Now that we are aware of the polarization of plane-wave EM fields, antenna polarization is
straightforward to define.
The polarization of an antenna is the polarization of the radiated fields produced by an antenna,
evaluated in the far field. Hence, antennas are often classified as “Linearly Polarized” or a “Right
Hand Circularly Polarized Antenna”.
This simple concept is important for antenna to antenna communication. First, a horizontally
polarized antenna will not communicate with a vertically polarized antenna. Due to the
reciprocity theorem, antennas transmit and receive in exactly the same manner. Hence, a
vertically polarized antenna transmits and receives vertically polarized fields. Consequently, if
a horizontally polarized antenna is trying to communicate with a vertically polarized antenna,
there will be no reception.
In general, for two linearly polarized antennas that are rotated from each other by an angle f, the
power loss due to this polarization mismatch will be described by the Polarization Loss Factor
(PLF):
PLF = cos f
2
Hence, if both antennas have the same polarization, the angle between their radiated E-fields
is zero and there is no power loss due to polarization mismatch. If one antenna is vertically
polarized and the other is horizontally polarized, the angle is 90 degrees and no power will be
transferred.
As a side note, this explains why moving the cell phone on your head to a different angle can
sometimes increase reception. Cell phone antennas are often linearly polarized, so rotating the
phone can often match the polarization of the phone and thus increase reception.
Circular polarization is a desirable characteristic for many antennas. Two antennas that are both
circularly polarized do not suffer signal loss due to polarization mismatch. Antennas used in GPS
systems are Right Hand Circularly Polarized.
Suppose now that a linearly polarized antenna is trying to receive a circularly polarized wave.
Equivalently, suppose a circularly polarized antenna is trying to receive a linearly polarized
wave. What is the resulting Polarization Loss Factor?
Recall that circular polarization is really two orthongal linear polarized waves 90 degrees out of
phase. Hence, a linearly polarized (LP) antenna will simply pick up the in-phase component of

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