MadSci Network: Engineering

Re: How does circular polarity on satellites work?

Date: Fri Mar 2 22:15:44 2001
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Engineering
ID: 982362501.Eg



Circular polarization (CP) has been used since the
1950s for microwave radar, tracking and communications
applications. Weather radars use CP because it passes
through rain with less loss than linear polarization
(LP). Aircraft and low earth orbit (LEO) and medium
earth orbit (MEO), satellite tracking, control (T&C)
and communications (SATCOM) systems use CP transmitters
because receivers have difficulty in receiving signals
from (LP) antennas that change orientation relative to
the ground station as the vehicle moves and rotates
through space. Geostationary earth orbit (GEO), direct
broadcast satellites (DBS) and MEO Global Positioning
Satellites (GPS) have been using CP since the late
1970s to increase channel capacity.

In nature electromagnetic (EM) waves have an infinite
number (states or degrees) of polarization. Most
natural EM waves have elliptical polarization (EP).
The elongation and tilt angle of the ellipse can range
from linear to circular. CP and LP are special forms of
the more general EP form.

It is difficult to discuss CP without 3D or isometric
drawings; however, I will try to present word
pictures. Lets assume that we are discussing a
terrestrial link so that I can use common antenna
orientations to discuss your question. Once the CP link
is understood, you can then orient the antenna toward
any direction in space.

If I am transmitting with a simple dipole antenna,
similar to a rabbit ear antenna on portable TV sets,
when the transmitter dipole is parallel with the
horizon, the sinusoidal electric field of my EM waves
will alternate in direction between heading right and
left as the waves travel toward the receiver located on
the horizon. This is horizontal linear polarization
. I will call this horizontal dimension the X
axis. If my transmitter dipole is vertical to the
horizon, the sinusoidal electric fields of the EM wave
will alternate between facing upward and downward as
the waves travel toward the horizon. These EM waves
will have vertical linear polarization (VLP). I will
call the vertical dimension the Y axis. The direction
in which the EM waves are traveling toward the horizon
I will call the Z axis. It might help if you sketch the
X, Y, and Z axis in a simple isometric drawing as I
procede with this discussion.

When a receiver dipole antenna is parallel to the
transmitter dipole antenna, the receiver antenna will
receive the maximum signal. So if my transmitter
antenna is HLP (parallel to the X axis) my receiver
antenna should also be HLP. If I slowly rotate the
receiver antenna about the Z axis, the received signal
will drop. When the receiver antenna is rotated half
way (45 degrees)between HLP and VLP it will receive
about one half of the available HLP signal power. When
the receiver antenna is rotated 90 degrees and becomes
parallel to the Y axis (VLP) it will receive no signal
from the HLP signal. We call this a null signal and the
null is very sharp and sensitive to antenna rotation
about the Z axis.

Linear Polarization Diplexing

All of the terrestrial microwave links used by the
telephone companies since the 1940s and the first
several generations of GEO satellite links have used
linear polarization diversity to double the channel
capacity in a given microwave frequency band. The
transmitter will transmit one group of channels using
HLP and will transmit a second set of channels on the
same frequency using VLP. Special antennas with two
feed ports have been developed to operate with both HLP
and VHP (more about this later). Thus if I rotate my LP
receiver antenna precisely to HLP I will receive one
set of channels and null out the second set of channels
using VLP. If I rotate my receiver antenna precisely to
VLP and null out the HLP channels, I will receive the
second set of channels. If the antenna is not precisely
polarized with the transmitter polarization of the
desired channel group, signals from the other
polarization will leak into my receiver and cause
interference (cross talk).

Polarization diplexed receiver antennas have been
developed to receive both HLP and VLP delivering the
signals to two different transmission lines (ports).
While linear polarization dipletxing is suitable for
fixed antenna locations and GEO satellites it is very
difficult to follow moving antennas on aircraft and LEO
and MEO spacecraft and maintain the nulls in the cross
polarized channels. For this reason CP receiver
antennas began to be used in the early space program.

Circular Polarization

Iíll begin by describing how CP is used and its
advantages and will end with a discussion about how CP
waves are generated and received.

While a LP wave electric field has an amplitude that
varies sinusoidal in time, a CP wave has a constant
amplitude electric vector that rotates through 360
degrees for each cycle of the radio frequency. Looking
from the transmitter toward the receiver, along the Z
axis, if the electric field rotates clockwise like an
aircraft propeller (an air screw) as it travels toward
the receiver, we say that the wave has right hand
circular polarization (RCP)
. If the electric field
rotates counter clockwise (anticlockwise) as it travels
toward the horizon, the wave is called left hand
circular polarization (LCP)

Important Note: The convention that radio engineers use
for naming CP is the opposite of the convention used in
classical optics. Classical optics views signals coming
toward the receiver which results in a reversal of
radio RCP and LCP definitions. However, it will be
important to remember that the radio receiver also
senses the wave as the opposite CP from the
transmitters view. Having worked with both microwave
links and laser links, the difference in polarization
definitions in the microwave and optical literature can
be very confusing.

If I transmit RCP or LCP from a spacecraft as the
Global Positioning Satellites (GPS) do, my LP receiving
antenna will be insensitive to polarization.
No matter what the orientation of my LP receiver
antenna is, I will receive a signal, because the
rotating electric field of the CP wave aligns itself
with my antenna twice during each cycle of the
microwave frequency. This is great for moving
transmitters and moving receivers with whip antennas
(monopoles) used on automobiles, cell phones etc.

However, there is one problem with this scenario; when
the CP electric field rotates cross wise to my receiver
antenna, I loose one half of the available signal power
(- 3dB). However, if I use a CP receiver antenna I can
capture all of the available signal power, if my
antenna has the correct CP (RCP for a LCP transmitter
or LCP for an RCP transmitter).

Circular Polarization Diplexing

In the 1980s DBS GEO satellites began to use CP
diplexing to double the number of channels available in
a given microwave frequency band. CP also helps reduce
rain fade which can be a problem at 12 GHz during heavy
rain. I am most familiar with the DirecTV system which
has 500 MHz of bandwidth allocated to it near 12 GHz.
One half of the TV channels are transmitted in RCP and
the other half are transmitted in LCP. The first
generation of receivers had a RCP/LCP switch in the
antenna low noise amplifier. Thus only one of the 2
sets of channels was sent down the coaxial cable
transmission line to the set top box and the other set
of channels was not available at the set top box for
other TV sets to view. Later model receivers had both
an RCP and an LCP down converter at the antenna and two
coaxial cables run from the antenna to the set top box
giving both sets of TV channels and 1000 MHz of
bandwidth.Even later receivers frequency multiplexed
both sets of channels on one coaxial cable to the set
top box.

How do we generate RCP and LCP?

There are many different types of antennas that operate
with CP. The helical antenna and helical antenna arrays
were used during the early years of satellite tracking.
The helical antenna is too complex to discuss here and
so Iíll discuss a more common form of CP antenna.

A CP wave can be broken down into two parallel sine
waves, one in HLP and one in VLP. The two sine waves
are then shifted 90 electrical degrees apart in phase
and added orthogonnaly (at right angles) in space at
the antenna feed. To demonstrate this, I would plot the
X and Y axes at the transmitter with the Z axis headed
into the paper. I then make the rotation angle of the
CP vector about the Z axis start at the Y axis which
will be zero degrees rotation. The VLP signal will
start at a phase of 90 degrees (a cosine wave)and the
HLP signal will start at zero degrees in phase. We will
assume that the peak amplitude of each sine wave is 1.0
volt. We then find the vector sum of the X amplitude
VLP) and the Y amplitude (HLP)

Signal_______VLP (Y)_+___HLP (X)______=CP (vector sum)
Phase______Amplitude____Amplitude_____Amplitude/rotation angle

0____________1.0_______0.0_________1.0 / 0 degrees
45___________0.707_____0.707_______1.0 / 45
90___________0.0_______1.0_________1.0 / 90
135_________-0.707_____0.707_______1.0 / 135
180_________-1.0_______0.0_________1.0 / 180
225_________-0.707____-0.707_______1.0 / 225
270__________0.0______-1.0_________1.0 / 270
315__________0.707____-0.707_______1.0 / 315
360__________1.0_______0.0_________1.0 / 360

Notice that the CP vector rotates clockwise with a
constant amplitude of one volt as the wave leaves the
antenna. To convert the antenna to LCP simply reverse
the sine of the X axis points, this is a 180 degree
phase shift. This puts the X channel 90 degrees ahead
of the Y channel.

A typical circuit CP feed circuit divide the
transmitter power into two feed lines and make one feed
line 90 electrical degrees longer than the other. Then
excite a parabolic antenna dish with a conical feed
horn feed by a HLP wave guide feed (or a monopole) on
one side and a VLP waveguide feed (or a monopol) on the
other side. This same antenna could also be used as a
CP receiver antenna.

Best regards, Your Mad Scientist
Adrian Popa

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