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1994-04-18
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SUBJECT: Circular Polarized Antenna - part one
There has been a request for a series on 'CP' antennas. The term
'CP' eluded me at first as I was not familar with the abriviated
designator for circular polarization. At work, we just use the entire
words.
I'm going to begin this ten part series with the basics. After
researching CP designs with a few engineers and fellow hams, I found
that they knew very little about the subject. I also found I didn't
know quite as much as I thought I did about circular polarization. So
starting at the begining will help all out.
First, let's discuss the circular polarized wave. There seems to
be conflicting standards used by the world of physics and the IEEE.
I found this to be true in four reference manuals including the ARRL
Antenna Handbook. At least it's stated right up front but biased
according to which text you read. We will follow the IEEE/ARRL standard
in the following series for obvious reasons.
There are two types of circular polarization; right and left.
All of us agree up to this point. According to the ARRL Antenna Handbook,
the following statement:
'Polarization Sense is a critical factor, especially
in EME work or if the satellite uses a circular polarized antenna.
In physics, clockwise rotation of an approaching wave is called
"right circular polarization," but the IEEE standard uses the term
"clockwise circular polarization" for a receeding wave. Amateur
terminology follows the IEEE standard, calling clockwise
polarization for a receeding wave as right-hand.'
Confused yet? I read it five times before it sunk in. Physics uses
'right', IEEE uses 'clockwise', the ARRL follows the IEEE and calls
it 'right-hand'. Just remember the 'right-hand' or 'clockwise' is
for a receeding wave or one that is going away from you.
[You have one minute to flap your lips with your finger and read the
above paragraphs again.] ;-)
Ok, now we know a reference that we can base "right-hand circular
polarization" on. Now I have an experiment for you to try. Obtain
a hard rubber ball from the kids or a neighbor's kid and try this: drop
the ball with a "right-hand polarization" spin and watch which way
the ball spins as it bounces towards you. You'll find it will spin
in the opposite direction [this works great with a super ball]. This
is exactly what happens in EME work. The "right-hand circular polarized"
signal returns in the opposite polarity. The same holds true for passing
satellites using circular polarization. An approaching satellite's
circular polarized wave will reverse as it passes.
Another quote from the ARRL Antenna Handbook states that the
circular polarized wave and a linear wave are mathmatically special
cases of an elipical wave. When the magnitudes are the same and phase
angles are 90 degrees in time, we have circular polarization.
-WS