By Joseph H. Reisert
Antenna polarization is a very important consideration when
choosing and installing an antenna. Most communications systems
use either vertical, horizontal or circular polarization. Knowing
the difference between polarizations and how to maximize their
benefit is very important to the antenna user.
A Polarization Review
An antenna is a transducer that converts radio
frequency electric current to electromagnetic
waves that are then radiated into space. The
electric field or "E" plane determines
the polarization or orientation of the radio
wave. In general, most antennas radiate either
linear or circular polarization.
A linear polarized antenna radiates wholly in
one plane containing the direction of propagation.
In a circular polarized antenna, the plane of polarization
rotates in a circle making one complete revolution
during one period of the wave. If the rotation
is clockwise looking in the direction of propagation,
the sense is called right-hand-circular (RHC).
If the rotation is counterclockwise, the sense
is called left-hand-circular (LHC).
An antenna is said to be vertically polarized
(linear) when its electric field is perpendicular
to the Earth's surface. An example of a vertical
antenna is a broadcast tower for AM radio or the "whip" antenna
on an automobile.
Horizontally polarized (linear) antennas have
their electric field parallel to the Earth's surface.
Television transmissions in the USA use horizontal
A circular polarized wave radiates energy in both
the horizontal and vertical planes and all planes
in between. The difference, if any, between the
maximum and the minimum peaks as the antenna is
rotated through all angles, is called the axial
ratio or ellipticity and is usually specified in
decibels (dB). If the axial ratio is near 0 dB,
the antenna is said to be circular polarized. If
the axial ratio is greater than 1-2 dB, the polarization
is often referred to as elliptical.
Polarization is an important design consideration.
The polarization of each antenna in a system
should be properly aligned. Maximum signal strength
between stations occurs when both stations are
using identical polarization.
When choosing an antenna, it is an important consideration
as to whether the polarization is linear or elliptical.
If the polarization is linear, is it vertical or
horizontal? If circular, is it RHC or LHC?
On line-of-sight (LOS) paths, it is most important
that the polarization of the antennas at both ends
of the path use the same polarization. In a linearly
polarized system, a misalignment of polarization
of 45 degrees will degrade the signal up to 3 dB
and if misaligned 90 degrees the attenuation can
be 20 dB or more. Likewise, in a circular polarized
system, both antennas must have the same sense.
If not, an additional loss of 20 dB or more will
Also note that linearly polarized antennas will
work with circularly polarized antennas and vice
versa. However, there will be up to a 3 dB loss
in signal strength. In weak signal situations,
this loss of signal may impair communications.
Cross polarization is another consideration. It
happens when unwanted radiation is present from
a polarization which is different from the polarization
in which the antenna was intended to radiate. For
example, a vertical antenna may radiate some horizontal
polarization and vice versa. However, this is seldom
a problem unless there is noise or strong signals
Vertical polarization is most often used when it
is desired to radiate a radio signal in all directions
such as widely distributed mobile units. Vertical
polarization also works well in the suburbs or
out in the country, especially where hills are
present. As a result, nowadays most two-way Earth
to Earth communications in the frequency range
above 30 MHz use vertical polarization.
Horizontal polarization is used to broadcast television
in the USA. Some say that horizontal polarization
was originally chosen because there was an advantage
to not have TV reception interfered with by vertically
polarized stations such as mobile radio. Also,
man made radio noise is predominantly vertically
polarized and the use of horizontal polarization
would provide some discrimination against interference
In the early days of FM radio in the 88-108 MHz
spectrum, the radio stations broadcasted horizontal
polarization. However, in the 1960's, FM radios
became popular in automobiles which used vertical
polarized receiving whip antennas. As a result,
the FCC modified Part 73 of the rules and regulations
to allow FM stations to broadcast RHC or elliptical
polarization to improve reception to vertical receiving
antennas as long as the horizontal component was
Circular polarization is most often use on satellite
communications. This is particularly desired since
the polarization of a linear polarized radio wave
may be rotated as the signal passes through any
anomalies (such as Faraday rotation) in the ionosphere.
Furthermore, due to the position of the Earth with
respect to the satellite, geometric differences
may vary especially if the satellite appears to
move with respect to the fixed Earth bound station.
Circular polarization will keep the signal constant
regardless of these anomalies.
What is Available
As stated earlier, for best performance, it is
desirable to use an antenna with the same polarization
on both ends of a communications path. If a system
is already in place, all that is required is
to find out what polarization is presently being
used and match it.
Most base station antenna providers will supply
either vertical or horizontal polarized antennas.
They are the most economical types. Furthermore,
vertically polarized antennas seem to be the most
popular for two way communications, as stated above,
while horizontal polarization is most predominant
in broadcast communications such as TV and FM.
Circularly polarized antennas are normally more
costly than linear polarized types since true circular
polarization is difficult to attain. An example
of a true circularly polarized antenna is the helix.
However, the most common circularly polarized
antenna uses crossed Yagis for "near circular" or
elliptical polarization. Elliptical polarization
can be generated by placing two identical linear
polarized Yagis at right angles (90 degree phase
differential) to each other and then feeding them
with equal power and a phasing network. A well
made antenna of this type will have a typical axial
ratio of +/-1 to 3 dB. In special applications,
crossed Yagi antennas can be configured to accept
either RHC or LHC by a selection relay.
If your antenna is to be located on an existing
tower or building with other antennas in the
vicinity, try to separate the antennas as far
as possible from each other. In the UHF range,
increasing separation even a few extra feet may
significantly improve performance from problems
such as desensitization.
When setting up your own exclusive communications
link, it may be wise to first test the link with
vertical and then horizontal polarization to see
which yields the best performance (if any). If
there are any reflections in the area, especially
from structures or towers, one polarization may
outperform the other. Furthermore, if there are
other RF signals in an area, using a polarization
opposite the predominant high level signals will
give some isolation as discussed earlier.
On another note, when radio waves strike a smooth
reflective surface, they may incur a 180 degree
phase shift, a phenomenon known as specular or
mirror image reflection. The reflected signal may
then destructively or constructively affect the
direct LOS signal. Circular polarization has been
used to an advantage in these situations since
the reflected wave would have a different sense
than the direct wave and block the fading from
Even if the polarizations are matched, other factors
may affect the strength of the signal. The most
common are long and short term fading. Long term
fading results from changes in the weather (such
as barometric pressure or precipitation) or when
a mobile station moves behind hills or buildings.
Short term fading is often referred to as "multipath" fading
since it results from reflected signals interfering
with the LOS signal.
Some of these fading phenomenon can be decreased
by the use of diversity reception. This type of
system usually employs dual antennas and receivers
with some kind of "voting" system to
choose the busiest signal. However, for best results,
the antennas should be at least 20 wavelengths
apart so that the signals are no longer correlated.
This would be 20-25 feet at 880 MHz, quite a structural
Nowadays we are inundated with mobile radios and
cellular telephones. The polarization on handheld
units is often random depending on how they are
held by the user. This has led to new studies which
have found that polarization diversity can be an
advantage. The most important break through in
this area is that the antennas at the base station
do not have to be separated physically as described
above. They can be collocated as long as they are
orthogonal and well isolated from each other. Only
time will tell if these systems are truly cost
Polarization in an important parameter and consideration
when selecting an antenna. It helps to have a
good grasp of all the aspects of this subject.
Hopefully the information contained in this paper
will answer some of your questions. Other antenna
considerations are discussed in reference 1.
Ref. 1 "Antenna
Selection and Specification Made Easy" by Joseph H. Reisert. Astron Antenna
Co. technical application found on this website.
Wireless Technologies, Inc. and the author retain the rights
to all intellectual
This information should be used as a guideline
only to help you in the appropriate selection of an antenna.
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