Peter O. Brackett wrote:
. . .
By "mixed" polarization, I assume you mean a single polarization which
is neither horizontal nor vertical and can be described as a "mixture"
of a purely horizontal and a purely vertical wave.
[snip]

No. What I meant by "mixed" was that, just as with daylight for example,
the field contains many polarization orientations. In fact usually outside
in daylight most of the light we see with our eyes contains very nearly
an equal distribution of all polariztions. An exception in the sky's light
is perpedicular to the suns rays where because of upper atmospheric
conditions light becomes slightly polarized. It is claimed that some
people
can actually "see" this polarized light differently than normal light.
(Haider's
Brush) Of course many people know that reflected light, for example
from the surface of a lake, becomes highly polarized. This is the
reason that "Polaroid" sunglasses are used by sportsmen and others
to reduce perceived glare from reflective surfaces.

That said, mixed polarization, is also largely the case of HF waves
received over ionospheric paths. In other words HF waves received
over long distances will contain a wide distribution of linear
and perhaps circular polarizations. Thus rendering the use of single
polarized antennas relatively useless at HF by amateurs. Unless of
course one is prepared to pay the significant price in space and
equipment to implement a polarization diversity receiving system.

There is only one E field associated with a wave and, if linearly
polarized, it has only one orientation or polarization. It's not like
incoherent light, but akin to a laser. There is no "mixture" of
polarizations in an EM wave.

. . .

True for a single antenna and receiver, which is the usual case for a ham,
see my remarks above.

However if one is willing to pay the price for several antennas and
synchronous
receiving systems then receiving gains can often be obtained by the
exploitation
of polarization diversity.

Actually, you don't want synchronous receivers, or else you get a single
effective polarization just as though the antennas were combined into a
phased array. For spacial or polarization diversity, you need
intentionally non-coherent receivers.

[snip]
Interesting. Can you work an example for us? I'm curious as to what
you use for theta in the "law's" equation.
[snip]

Theta is just the relative orientation of the polarization of the
transmitting
and receiving antennas, or in the case of an optical polarimeter, the
relative orientations of the polarizing and analyzing polarizer.

Theta is commonly illustrated in undergraduate optical laboratories and
science
experiment kits, using a couple of pieces of "Polaroid" film with the
polarization
angle marked on the film by a notch or other marking. When the
two films are aligned with their polariztion direction perpendicular
there is no
light propagation, i.e. theta is 90 degrees, and when they are aligned
with theta
equal to zero then light is propagated.

In the case of dipole antennas, theta is zero when two antennas are
co-linear and theta is 90 degrees when the antennas are perpendicular.

So in your equation, what are theta for RHP and LHP, since you've said
that the equation applies to circular polarization?

. . .

[snip]
angular velocity of rotation is one revolution per cycle of the RF
carrier, or in other words one radian of circular rotation for each
radian of frequency transmitted. In other words most well known CP
antennas produce ONLY synchronous CP, where the angular velocity of
rotation of the E vector is synchronized exactly with the frequency
of the wave being transmitted.

That is, in fact, the definition of circular or elliptical polarization.
[snip]

Agreed, both you and I and thousands of others know that. [smile]


Then why are you calling your non-synchronous system "circular
polarization"?


Definition! Gosh where is Cecil when you need him? The only
problem with definitions is that there are so many of them!

---------------------------------------------------------------------------------------------


"When I use a word, Humpty Dumpty said in a rather scornful tone,

"It means just what I chose it to mean - neither more nor less."

"The question is," said Alice, "whether you can make words mean so many
different things."

"The question is," said Humpty Dumpty, "which is to be Master - that's
all."

-- Lewis Caroll, from Through the Looking Glass

--------------------------------------------------------------------------------------------


[grin]


That's a great attitude for a politician, philosopher, or biblical
scholar. But engineers and scientists depend on universally understood
technical terms in order to communicate. I'm free to say that my car
gets a gas mileage of 30 miles/hour and weighs 420 miles. But it
wouldn't be a smart thing to do if I intend to convey information.

[snip]
Sorry, it doesn't. An unavoidable side effect of the synchronicity
change is that the amplitude of the E field still changes at a 1 GHz
rate, going through a complete cycle from max to zero to max to zero
to max each nanosecond. A circularly polarized wave doesn't change
amplitude with time. A non-circular elliptical wave changes amplitude
but not fully to zero each cycle.
[snip]

Here there is a bit of fuzziness...

I agree that the E field of a wave is always changing at the RF carrier
frequency
since it is an AC waveform. Alternating current is always changing!
And so a
1 GHz carrier will always have an E field that oscillates back and forth
at the
carrier (center?) frequency when analyzed by a (linear) polarimeter.

I disagree with you that a circular polarized wave has a constant E field.

Even in the case of a purely circularly polarized the E field still
oscillates
at the carrier (center?) frequency when analyzed by a linear polarizer.

i.e. if a purely CP wave is received on a linear polarized antenna the
detected E field (Volts per meter) will be observed to be oscillating
at the carrier frequency. However if received on a purely CP responding
antenna this oscillating E fileld will appear to be constant.

The E field vector can be considered to be similar to the image of a
spoke on a rolling wheel. The radius of the spoke is constant, but
it's projection on the ground over which the wheel is rolling will
always be oscillating in length.

When you receive a circularly polarized wave on a linearly polarized
antenna, you're seeing only the component of the wave that's linearly
polarized in the orientation of the antenna. This is exactly the same
process as filtering a complex waveform. You've removed part of the
field and are observing what's left after the filtering process, then
drawing conclusions about the original waveform based on those
observations, much like listening to a concert orchestra through a long
pipe and deciding that orchestral sound is very ringy and limited in
tonal range. It would benefit you to gain a bit of education about
circularly polarized waves. You'll find that a circularly polarized wave
can be created from (or broken into) two linearly polarized waves
oriented at right angles and in phase quadrature. So each of the
components has a time-varying amplitude, but the sum, which is the
circularly polarized wave, has a constant amplitude but time-varying
orientation. Your linear antenna filters out one of the components,
leaving you to observe only the other.

[snip]
Circularly polarized waves have many characteristics and particular
relationships to linearly polarized waves. The waves you're producing
don't have some of these characteristics, like the constant amplitude.
Your method doesn't produce circularly polarized waves even though the
polarization does indeed change with time.
[snip]

I beg to disagree. The waves that I am describing are exactly the same.

Consider if the mechanical motor that spins my linear antenna spins at
exactly the carrier frequency. There would be then no way to tell the
difference between the two.

That's right, in that case you would be producing circularly polarized
waves. But only with a synchronous spin speed. As soon as you separate
the rotational speed from the wave's oscillation, you have something
else with different characteristics, e.g., a time varying amplitude.

[snip]
Because a circularly polarized antenna responds equally well to all
orientations of linear polarization, the normal helix wouldn't be
aware of the polarization rotation -- unless the polarization rotation
was fast enough to be nearly synchronous.
[snip]

Heh, heh... what would you consider to be "fast enough"?

Would the rate of spin have to be 99-44/100 percent of the synchronous
frequency? Or would it have to be closer than that?

At what magic spin frequency would the two be indistinguisable.

FWIW... I can propose a scheme that will electronically rotate the
linear antenna
at any desired frequency, at least up to the accuracy of modern atomic
clock standards.

What you'll end up with is amplitude modulation with the modulating
frequency being the beat note between your spinning speed and the wave
frequency. This creates sidebands. You'll see this when the sidebands
are within the bandwidth of the helix. Outside that, the helix will
filter off the sidebands and you'll just see the "carrier" -- the
original wave with no modulation.


[snip]
Sorry, I didn't find it "mind-blowing".
[snip]

Roy, I don't belive you have thought about it hard enough yet, for
clearly this idea
has already "blown" your mind!

If you say so.

For did you not state above that a circular carrier wave has a constant
amplitude?

Yes, I did. Circularly polarized, that is.

A radio wave with constant aplitude, indeed! Something must be blown!

At zero frequency, how would a constant wave propagate?

Here's a really neat little trick you might want to add to your bag --
superposition. As I mentioned, you can create a circularly polarized
wave from two linearly polarized waves. The linearly polarized waves are
of course normally time-varying. As long as the propagation medium is
linear (such as air), superposition says you can split the circularly
polarized wave apart into two linearly polarized waves, study and
analyze how they propagate, then add the two components back together
again after the propagation. This is, incidentally, a very simple way to
see what happens when a circularly polarized wave reflects from a
surface -- analyze the linear components separately and add the results.

This assumption/view that zero frequency wave can propagate is akin to
Cecil's
view that there are no reflections at DC.

No, it isn't.

I don't mean to be facitious and I am quite serious about all of this.

Just because no one has ever considered non-synchronous circular
polariztion before
does not mean that it doesn't exist, or that it may not be useful.

Me? I have already thought of several potential uses for
non-synchronous circular
polarization. How about polariztion frequency modulation? Or... how about
polariztion phase modulation? Or...

Got you thinking yet?

Sorry, I don't recall having stopped thinking. If I have, this isn't the
way to get me started.

Thanks again for your clearly interesting comments and feedback.

More thoughts, comments?

-- Pete K1PO
-- Indialantic By-the-Sea, FL

That's about all I can do at this end. I can't make you actually pick up
a text and learn about circularly polarized waves, and until you do,
you'll have some fundamental misconceptions about them.

Guess I'm one of those folks who someone described recently as "having
the common sense educated out of me". It's served me well, since it's
enabled me able to spend a career designing a wide variety of state of
the art electronic circuits and antennas, successfully mass produced,
which work as designed. But I know it's not for everyone.

Roy Lewallen, W7EL