Green Egghead wrote:
. . .
Karo brand corn syrup has an interesting property.
It will rotate the linear polarization of light passing through it
by different amounts depending on the frequency.
This can easily be seen by placing a small jar of
Karo syrup between to linear polarizers and rotating
them. Different angles between the linear polarizers
will result in a different color being seen in the Karo jar. . .

Dextrose (for right) and levulose (for left) -- aka glucose and
fructose, the components of sucrose, ordinary table sugar -- are named
for the direction in which they rotate the polarization of light.

Roy Lewallen, W7EL

Dear Roy:

Stupendous explanation, I also think, as Denny, that should be included
in the antenna book to highlight this aspect, because a lot of friends
tends to believe that the horizontal dipole only emits with horizontal
polarization...

In fact, my question arose when comparing the relative difference
between the vertical field and the horizontal one in the free space and
on the proximities of the real ground (or with the perfect ground also)
and to notice a bigger intensity of the vertical field near to the
ground (relative to the horizontal one), taking reference with the same
relationship in the free space.

My hypothesis was that this would be due to an asymmetric distribution
of the displacement currents taken place by the presence of the graund
that would cancel part of the vertical field in low angles.

I was looking for the correct explanation of such an asymmetry. Now I
think that your explanation, based on difference in the reflection
coefficient, seems to be more appropriate.

It put my accent in "the low height", because I was in a friend's shack
that uses EZNEC 3 (I am more familiarized with MMANA which I use
because it is free and not because I prefers it to yours, that is very
good) and I am got confused with automatic way of locating the outer
ring, suggesting me a great intensity of vertical field (reason why the
results they were too optimistic). It was my fault because the text on
the results gives the outer ring value (but one so many times doesn't
read the tiny letters...).

When, at your suggestion, download EZNEC 4 Demo, I realized (when
opening Desc Options) that the 0 dB corresponded to an external ring of
approximately -10 dB or less, (maybe it would be good a "warning" to
avoid troubles at peoples as fool as me ;) )

Those first results took me to think that the vertical field could have
more relevance on comunnications at short distance by terrestrial waves
(very low angles) on the lowest bands...

Anyway, maybe the most interesting thing that has arisen of this
question is to remember us that the horizontal dipole ¡doesn't sends
only polarized horizontally waves..!

Thank very you for your quick answer and my congratulations for your
great program.

Miguel Ghezzi (LU 6ETJ)
---------------------------------------

Roy Lewallen wrote:
At HF considerable fading, including selective frequency fading, is
caused by polarization shift. But it's not easy to create a receiving
antenna that's circularly polarized when a ground reflection is involved
(because ground reflection characteristics are functions of both
reflection angle and polarization), and even more difficult to do it in
more than one direction. If you can build the antenna, it should reduce
polarization shift fading. You still have the problem of fading due to
multipath interference.

To get a circularly polarized field (again, relative to the
transmitter's coordinates irrespective of any receiver)
feeding the two linearly polarized antennas in quadrature
would be equivalent to:

B_h = A(t)*cos(0) = A(t)
B_v = A(t)*sin(0) = 0
and
C_h = A(t+90)*cos(90) = 0
C_v = A(t+90)*sin(90) = A(t+90)

Where A(t+90) represents the signal A(t) shifted
90 degrees relative to the carrier frequency.

Signal A(t) is not equal to A(t+90) at the every point in
free space and so they will interfere. This would create
a spatially and temporally changing carrier amplitude?

Yes, that's correct.

So I don't understand how two same frequency carriers
where one is 90 out of phase with the other creates a
circularly polarized wave since their resultant is not
in the polarization plane but along the direction of
the field's propagation.

Here's your error. In free space in the far field, there is no tilt in
the E field in the direction of propagation; the field is what we call a
plane wave. At any instant, the E field is oriented normal to the
direction of travel. If you look at a circularly polarized wave at a
fixed location, you'll see it rotate in the plane normal to the
direction of propagation. If you freeze the wave in time, you'll see
that the field orientation is a rotating vector, again rotating in a
plane normal to the direction of propagation. Think of the path of an
airplane propeller as the plane flies.

I don't yet see how the B_h and C_v signals, A(t) and A(t+90),
(which appear serially on the feed line as a superposition)
get physically split into their respective h and v dipoles
(I can see that if they are, circular polarization results).

Besides the 90 carrier phase shift and the 90 angular shift
of the crossed dipoles, I figure there has to be one more part
that splits the orthogonal signal components in the feed line
into their respective dipoles (it would be a waste of energy to
send the B_h component through the vertical dipole).

Is this why circularly polarized antennas like this one
seem to have a vertical and horizontal radiator combined?
http://www.ccbroadcasters.com/images/antenn3.jpg
That's what had me thinking that circular polarization
had something to do with the E and H field phase difference
since it looks like a horiz loop integrally combined with a vert dipole.


What amount of radio signal attenuation is typically
attributed to polarization mismatches?

I commonly see fades of 20 - 30 dB on 40 meters which I can reverse by
switching between horizontal and vertical antennas -- that is, at the
bottom of the fade I can switch to the other antenna and restore the
signal. So it's mainly due to polarization shift. On line of sight
paths, I believe the attenuation can be quite severe. I don't know what
proportion of the frequency selective fading you hear on distant AM
signals is due to polarization shift and how much to multipath interference.

. . .

There should be some good explanations (and undoubtedly also some bad
ones) on the web, and the topic is covered to some extent in most
electromagnetics texts.

Thanks for your experienced help getting through these rough parts for me.
I'll keep studying.

Roy Lewallen wrote:

Green Egghead wrote:
. . .
Karo brand corn syrup has an interesting property.
It will rotate the linear polarization of light passing through it
by different amounts depending on the frequency.
This can easily be seen by placing a small jar of
Karo syrup between to linear polarizers and rotating
them. Different angles between the linear polarizers
will result in a different color being seen in the Karo jar. . .

Dextrose (for right) and levulose (for left) -- aka glucose and
fructose, the components of sucrose, ordinary table sugar -- are named
for the direction in which they rotate the polarization of light.

Roy Lewallen, W7EL

To make things worse, there's a d-fructose and l-fructose.

D-tagatose is the all-natural un-sugar:
http://www.jhu.edu/~jhumag/1102web/sweet.html
I think I've seen it on store shelves. I'll check it out
if it's not too expensive.

A few years ago someone discovered an efficient
way to grow crystals of a specific handedness
from solution. So I'm surprised L-tagatose hasn't
totally replaced saccharin and phenylketoneurics.
Perhaps because some diseases ()like phenylketoneuria)
are associated with certain chiral forms?

Astrobioligists have suggested using chiral tests to
determine the presense of life on other planets since,
as far as they know, all life on earth has a preference
for (or immunity against?) one handedness over the other.
This suggested to them that perhaps once life developed
on a planet it would quickly bias all life on that planet
towards one of the two forms.

More on topic, should we be testing for circular polarization
radiation exposure levels on field strength meters?

It is interesting that Faraday rotation of linear polarization
can be described in terms of circular birefringence.
I can't tell if that's an analytical more than a physical description.

I see though the Faraday Effect is used in astronomy with
oscillating pulsars but otherwise making a radio tuner in
this manner would seem to present a problem of scales.

I'm confused about this since I've seen it said the higher
frequencies of light are rotated more by Karo syrup, while
another website says the higher radio frequencies are
rotated less by the ionosphere due to the Faraday Effect
than are the lower frequencies.

If I had to guess, I would think there would be only a certain
band of radio frequencies where polarization rotation would
make a practical radio tuner. Spintronics?

On Wed, 23 Aug 2006 16:44:28 -0000, Green Egghead
wrote:

If I had to guess, I would think there would be only a certain
band of radio frequencies where polarization rotation would
make a practical radio tuner. Spintronics?

Perhaps if your dial is marked in THz.