Re: [amsat-bb] Whose Right and Whose Left--CP

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

Ben,

Don't feel bad!  Many a RF engineer has been challenged with circular
polarization (especially when generated by crossed dipole driven elements).

I like to use this analogy (which is not far from your wood-screw comparison):

Take two bolts and a nut.  Normally these have right-hand thread so this
will example will apply for RHCP.
Thread the nut all the way onto one bolt and face the thread end of the two
bolts together.  Now as you run the nut from one bolt onto the other, you
rotate the nut according to the right-hand rule, rotating the nut clockwise
as viewed from the first bolt head toward the other bolt.  The nut threads
from one bolt onto the other bolt by continually rotating it the same
manner, but if you were looking at it from the other bolt head back toward
where the nut originated, it appears to be moving counter-clockwise.

Think of the two bolts as circular polarized antennas and the spinning nut
as the RF field....only difference from the bolt analogy is the two
antennas don't (usually?) touch each other.

This works pretty nice if you are visualizing using a helix as it looks
like a screw or bolt.  Crossed yagis is a whole nuther deal...I won't get
into that, now!

73, Ed - AL7EB

At 12:35 PM 10/31/02 -0600, Ben wrote:
>Hello
>  The only definition of RHCP and LHCP that I have heard that makes sense
>appears to be wrong. Let me explain the way that makes sense to me. Long ago
>in college I learned the right hand rule for electric current vs magnetic
>flex lines. It is based on the Cartesian Coordinate System that all of us
>that have had high school geometry should understand. In other words if one
>were to take their right hand with thumb extended and fingers curled. Than
>define the direction of wave propagation as being from the base of their
>hand to the tip of their thumb. Their fingers are now pointing in the
>direction of positive field rotation. With this definition in mind, pretend
>you are behind an antenna. Now point your right hand thumb at your face. If
>you follow the curvature of your fingers than a wave front that is traveling
>to you from a right handed transmitter will be rotating CCW from your
>prospective. Turn your hand around so that your thumb is now pointing up the
>boom in the direction a wave front would be moving if you were to transmit
>from a RHCP antenna. From your prospective the wave front would be rotating
>CW as it travels out into space. Think of it in another way. To screw a
>right handed screw into a piece of wood you would need to turn it clockwise.
>Your the transmitter. To retrieve the screw you would have to turn it CCW,
>your the receiver. With a left hand screw CCW to embed the screw and CW to
>retrieve it. This is all clear to me, but it is in direct contrast to how
>AMSAT defines polarization. AMSAT's definition seems incomplete. All that is
>stated is that if one stands behind an antenna and RECEIVES a signal that is
>rotating clockwise than it is RHCP. But its not. Amsat's definition holds
>water only if you make the definition that your thumb points in the
>direction that the wave is coming from. I wish they would make this
>distinction clear in there literature. There would be a whole lot less
>confusion.
>Sorry for the long post
>Ben K9BF
>
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