Re: Circular polarization... does it have to be synchronous??

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
.

Relevant Pages

  • Circular polarization... does it have to be synchronous??
    ... Conventionally electromagnetic wave 'polarization' refers to the relative physical spatial orientation of the electric field vector of an electromagnetic wave. ... For example most common linear amateur antennas produce and/or respond to waves of linear polarization, and these antennas produce either either horizonally or vertically polarized waves depending upon the orientation of the antenna with respect to the earth's surface. ...
    (rec.radio.amateur.antenna)
  • Re: Redshift without expansion
    ... that's what the word "analogy" means. ... later as a *literal* explanation to explain polarization. ... It is like a three-dimensional mexican wave. ... at different speeds. ...
    (sci.astro)
  • Re: A thought experiment
    ... "In electrodynamics, circular polarization (also circular ... The>> magnitude of the electric field vector is constant ... How can a wave propagate at a fixed point in space at one ...
    (sci.physics.relativity)
  • Re: Redshift without expansion
    ... that's what the word "analogy" means. ... later as a *literal* explanation to explain polarization. ... wave as a dipole that travels at c. ... the dipole is always oriented perpendicular to ...
    (sci.astro)
  • Re: A thought experiment
    ... "In electrodynamics, circular polarization (also circular ... The>> magnitude of the electric field vector is constant ... How can a wave propagate at a fixed point in space at one ... calmed down at the thought of the sum of two linear waves. ...
    (sci.physics.relativity)