[GreenKeys] polarization. hmm.

[Chris Elmquist]

On Wednesday (11/06/2013 at 03:43PM -0700), gil at baudot.net wrote:
>    Hi Ralph, Roy, Ron, and Chris:
> 
>    Hmm.  I think I have the polarization issue confused.  I was thinking
>    that as a horizontal dipole rotates under the balloon, it changes the
>    polarization, but that is no different than a horizontal dipole on land
>    being rotated about its feed point.  On land, the nulls would be
>    problematic since reception is to the side, but looking up to a
>    horizontal dipole in the air, the nulls are not a problem.  This
>    scenario would be the same as a receiver in the air, that was looking
>    down at a rotating land horizontal tx dipole. That is not a changing
>    polarization, is it?  Or is it?  Sheesh, electromagnetics class in
>    college was too many decades ago.

So, I think polarization gets confused a lot because the reference plane
is not ever really clearly stated.

And I'm not really that good at explaining it either but I'll try a few
things.

If you put a 1/4-wave whip on the roof of car and the car is sitting
flat on the road, that 1/4-wave whip will radiate a vertically polarized
signal.

If you tip the car on its side, that 1/4-wave whip will radiate a
horizontally polarized signal.

Similarly, if you put a dipole suspended between two trees and feed it
in the center, that dipole will radiate a horizontally polarized signal.

If you suspend the dipole between the top of one tree and the ground
directly below that tree and feed the dipole half way up the tree,
that dipole will radiate a vertically polarized signal.

If the dipole is on a slant, 45 degrees say, then you do actually radiate
"slant" polarization--  some component vertically and some component
horizontally.  Then the ground, the tree, other structure nearby all
affect how much of each of those components you get.

>    To actually change polarization, I think, the dipole axis would need to
>    move from horizontal to vertical, which we aren't doing on the balloon
>    -- a vertical dipole in the sky would be that same as an inverted
>    quarterwave vertical, would it not (with a big null pointing straight
>    down)?

Yes. Exactly.

So, ideally a dipole on the balloon needs to remain parallel to the earth,
not perpendicular.

>    Which leads me to another question:  what exactly is the "circular"
>    polarization on the turnstile crossed dipole antenna, and what is its
>    radiation pattern?

Circular polarization has both horizontal and vertical components, in roughly
equal strength, with one polarity 90 degrees out of phase with the other.
Depending on which polarity leads, you get either right-hand circular or
left-hand circular polarization.

FM broadcast evolved to transmitting with circular polarization so that
they get the best coupling to both vertical whips on the fenders of
cars and horizontal yagis that were on the roofs of people's houses.
There is also a benefit in reducing multi-path because the reflected
signal will be the opposite circularity and "rejected" by the antenna.

Here's one writeup that doesn't seem too bad,

http://www.astronwireless.com/topic-archives-antennas-polarization.asp

and another from the RC crowd that I thought was pretty clear too,

http://rcexplorer.se/educational/2011/08/circular-polarization-explained/

>    It sounds to me like 2m land radio is all using vertical antennas and
>    vertical polarization, so does this mean that a horizontal tx dipole
>    (land or sky-based) would get virtually no signal to a vertical rx
>    antenna?

Not "virtually no signal", but much less signal.  30dB less.  The cross
polarization penalty is roughly 30dB.

>    One last thing:  What is the polarization of a yagi, let's say if it
>    was aimed at some arbitrary angle to the balloon?

The polarization of a yagi depends on its orientation to the earth.
If you hold it parallel to the earth, with the elements coming off right
and left from the boom, then it is horizontally polarized.  If you hold
it perpendicular to the earth, with its elements going up and down from
the boom, then it is vertically polarized.  If you hold it at an angle,
then it is slant polarized.

If the yagi has two sets of elements, one set horizontal and one set
vertical and one set is mounted a 1/4-wave length ahead of the other on
the boom, and you feed those two sets of elements in parallel, then you
generate a circularly polarized signal.  Whichever horz or vert leads
the other decides the handedness (RH or LH) circularity of that yagi.
We use these all the time for 2m and 70cm amateur satellite work.

Chris N0JCF

-- 
Chris Elmquist