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Re: Re: Up and dwnlink polarization of HAMSAT



Dave,

Well, you make a common mistake (lots of folks do this, so do not feel
bad).  You try to analyze polarization from only your antennas viewpoint.
It is correct when you transmit on it:  If the signal rotates clockwise it
is RHCP.

But what you must ask for a signal you receive is:  "what direction is the
signal rotating from the other station's viewpoint.  In other words: "what
polarization does the other station have use to produce a signal your
antenna can receive?  The answer for your RHCP helix is another RHCP
antenna.  The exercise is to point two helical antennas at each other and
view how the signal "corkscrews" thru space.  A signal leaving your antenna
must rotate clockwise as viewed from your feedpoint; a signal received by
your antenna must rotate counter-clockwise (since it is moving in the
opposite direction down the helix wire) as viewed from your feedpoint.
They still are both RHCP signals.

And if your helix is LHCP everything I said, above, reverses.

A cross yagi produces a electrical vector aligned with the first driven
element (DE) (and say for argument sake that is pointing vertical and
straight up).  For the x-yagi to produce a RHCP signal the electrical
vector must point horizontal and to the right, exactly 1/4 wavelength (WL)
down the boom from the first DE.  Both DE's are fed by coax cable with the
signal split and the second DE having an extra length of cable (1/4 WL
long) to properly phase the signal for CP.

Now when driven this way, the x-yagi will produce a electric vector that
rotates 90-degrees to the right (clockwise) as it travels a 1/4 WL down the
boom.  (i.e. at 1/2 WL from the first DE the electric vector will point
straight down; and at 3/4 WL it will point to the left; and 1 WL from the
first DE it will again point straight up...continuing to rotate clockwise
as it travels down the boom).

Now I admit that a yagi is more complicated than this simple analysis but
the principle holds how a CP signal is generated.

I hope this helps understand this better...it helps if you have good
imagination to visualize things :-)

73's Ed - KL7UW

At 07:30 AM 7/3/05 -0700, Dave G. wrote:
>> The satellite will see the waves going towards earth in
>> clock-wize way... Now imagine the waves are comming towards
>> you (on earth), then they will be like going like
>> anti-clockwize. This seems opposite of your helical winding,
>> BUT the waves are going the other way.
>
>If my own "Rule of Thumb" is correct, the way I visualize it 
>is;
>
>Starting from the helical antenna feed point, if the electron 
>flow is clockwise then it is RHCP. And, viewed from the same 
>feed point on the same antenna, any arriving electrons will 
>travel to down the helix as LHCP.
>
>But I don't have a "Rule of Thumb" for crossed yagi's or 
>diversity antennae.... they totally confuse me...
>
>Dave KK7SS
>----
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