On Dec 6, 10:11*am, "Peter O. Brackett"
wrote:
Roy:

Thanks for your well thought out responses.

See my comments below interspersed with snippings of your response.

[snip]"Roy Lewallen" wrote in message

treetonline... Peter O.. Brackett wrote:
. . .
It is commonly understood that polarization of electromagnetic waves may
be either linear or circular.

Then some education is in order. Electromagnetic waves are elliptically
polarized. The two extreme special cases of this are linear and circular
(with axial ratio of zero -- or infinite depending on your choice of
definition -- and one respectively). There are an infinite number of other
possible elliptical polarizations with different axial ratios.

[snip]

I agree. *My statement was not quite precise.

I should have stated something like, "it is commonly understood that
polarization of waves may be categorized as being either linear or
elliptical, and
in the elliptical category the special case of circular polarization occurs
whenever
the major and minor axes of the elliptical polarization are equal."

[snip] Of course linear polarization can have any orientation, not just vertical
or horizontal. And even those terms lose meaning when away from the Earth.
However, it's often convenient to mathematically separate waves into two
superposed components of horizontal and vertical polarization.

[snip]

Agreed!

[snip] The polarization of the dipole signal will be purely horizontal only
directly broadside. The signal off the ends are purely vertically
polarized, and in other directions neither horizontal nor vertical.

[snip]

Agreed! *It is relatively difficult, and perhaps even impossible to arrange
the physical configuration of an antenna such that it emits (or receives)
wave of purely one category of polarization.

In practice though many antennas concentrate a major part of their emissions
in one polariztion form.

[snip] 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.

[snip] It's also difficult to get the polarizations of the antennas exactly
right.

[snip]

Agreed!

[snip] There's no advantage at HF of having the antenna orientations the same if
the path is via the ionosphere.

[snip]

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.

*[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.

[snip] Only if it strikes the surface directly head-on. Otherwise you get an
elliptically polarized wave. The axial ratio depends on the angle of
incidence and, if the reflector isn't perfectly conducting, on the
impedance of the surface.

[snip]

Agreed!

A very intersting optical phenomena to observe is to look at a mirror
through
an optical circular polarizer (polarizer in tandem with a 1/4 wave retarder)
which
renders the "image" of the circular polarizer to be black. *i.e. the optical
circular polarizer eliminates the reflection. *This technique is widely used
to eliminate reflections from information displays that must operate in high
sunlight with good sunlight readability. *High quality high transmissivity
optical circular polarizers are relatively expensive, and so one does not
find such technology applied to consumer displays like computer
monitors, TV sets or IPhones, however optical circular polarizers are
widely used by the military for eliminating sunlight reflections from their
(expensive) information displays.

[snip] CP propagation is often used in Satellite communications where a
satellite may use both RHCP and LHCP transmitting antennas on the same
frequency for communicating independently with two different ground
stations using R and L CP *antennas on the same frequency. *CP frequency
diversity doubles channel capacity!

I think you mean that polarization (not frequency) diversity doubles
channel capacity.

[snip]

Yep that's exactly what I meant, but my fingers did not type it that way.
Thanks!

[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]

[snip] I believe that the well known and understood situation of purely
synchronous CP is NOT necessesarily the only form of CP.

It's the only one which fits the definition. If you choose to rotate the
polarization at some other rate, you should call it something else.

[snip]

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]

[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.

[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 ...

read more »



It was stated above that the purely horizontal polarisation will occur
when the dipole is broadside
This is not correct
Using an optimiser and inserting a one liner where all dimensions are
different allows for the design to conform to Maxwell
laws in their entirety, which means the inclusion of the "weak" force
required for equilibrium
Regards
Art