On Dec 1, 8:28*am, Dave wrote:

Can you show us one of these C-POL receive antennas?

Here's a link to one example for VHF and UHF TV...

http://www.kathrein-scala.com/catalog/HDCA-5CP.pdf

Of course the Lindenblad, or even 1/2 of one could be used, also.
Using half a Lindenblad would make it true c-pol only in two
directions, but if you can aim it in the right direction that may be
OK.

RF

On Dec 1, 9:07*am, Richard Fry wrote:
Here's a link to one example for VHF and UHF TV...

Oops. That one is VHF only.

"Richard Fry" wrote in message
...
On Nov 29, 2:18 pm, Roy Lewallen wrote:
Jerry wrote:
Just for conversation, I submit that an antenna with good hemispheric
CP coverage could be made with 4 dipoles.

Yes, there's the quadrifilar helix which I believe fits that
description. Another, which I built decades ago at 450 MHz, is the "skew
planar" antenna which resembles a cloverleaf but with the "leaves"
rotated 45 degrees...

Getting back to Jerry's idea - yes, four linear dipoles can generate
nearly perfect omnidirectional c-pol. This is a design of Nils
Lindenblad many decades ago, and I've done some NEC-2 modeling of it.
The link below leads to a rendered view of that model.

http://i62.photobucket.com/albums/h8...adRendered.gif


RF

Hi Richard

The Lindenblad is Omniazimuth CP. The QHA is hemispherical CP. Some
explanation of the DCA is shown in the Feb 2008 QST.

Jerry

Roy:

[snip]
"Roy Lewallen" wrote in message
treetonline...
Dave wrote:

Circular polarization means you have equal H-Pol and V-Pol radiation,
with one polarization 90 degrees ahead (or behind) of the other.
..
..
..
That's not a very good description, although it's correct. But here's a
little more complete description: The electric (E) field of a horizontally
polarized wave is horizontal, and the E field of a vertically polarized
wave is vertical. But the E field of a circularly polarized wave rotates
at the transmission frequency, one revolution per cycle. The instantaneous
amplitude of a vertically or horizontally polarized field is sinusoidal,
varying at the transmission frequency. The amplitude of a circularly
polarized wave is constant.
[snip]

Hmmmm....

Roy, I don't believe that is a "complete" description of circular
polarization either.

My understanding is that circular polarization is what we call the
polarization of electromagnetic radiation when the electric field vector E
rotates with an angular velocity rather than oscillating back and forth in a
single (linear) direction.

Roy, your description of circular polarization above seems to imply that the
angular velocity of the E vector of a circularly polarized wave is always
"synchronized" with the signal frequency since you stated that it rotates at
one revolution per cycle, or one radian per radian per second.

This of course is the (normal?) situation if the antenna is say a helix
firing along its axis or say crossed dipoles fed with a 90 degree phase
shift, but... to be more complete, we should note that...

Circular polarization does not have to be "synchronous"!

Consider...

What would you call the polarization type of the radiation emitted by the
following radiator?

An ordinary linear dipole fed with RF from a feedline through two slip rings
arranged such that a mechanical drive is able to rotate the dipole at some
arbitrary mechanical angular velocity completely unsynchronized with the RF
carrier.

For example, say it's a 5m dipole driven with 30MHz carrier signal and
mechanically rotated at 1000 revolutions per minute.

Such an arrangement would result in circularly polarized radiation with the
E vector having angular velocity of 1000 revolutions per minute and a
carrier frequency of 30MHz which definitely is not one revolution per cycle.

Thoughts, comments?

--

Pete
Indialantic By-the-Sea, FL

"Peter O. Brackett" wrote in message
m...
Roy:

[snip]
"Roy Lewallen" wrote in message
treetonline...
Dave wrote:

Circular polarization means you have equal H-Pol and V-Pol radiation,
with one polarization 90 degrees ahead (or behind) of the other.
.
.
.
That's not a very good description, although it's correct. But here's a
little more complete description: The electric (E) field of a
horizontally polarized wave is horizontal, and the E field of a
vertically polarized wave is vertical. But the E field of a circularly
polarized wave rotates at the transmission frequency, one revolution per
cycle. The instantaneous amplitude of a vertically or horizontally
polarized field is sinusoidal, varying at the transmission frequency. The
amplitude of a circularly polarized wave is constant.
[snip]

Hmmmm....

Roy, I don't believe that is a "complete" description of circular
polarization either.

My understanding is that circular polarization is what we call the
polarization of electromagnetic radiation when the electric field vector E
rotates with an angular velocity rather than oscillating back and forth in
a single (linear) direction.

Roy, your description of circular polarization above seems to imply that
the angular velocity of the E vector of a circularly polarized wave is
always "synchronized" with the signal frequency since you stated that it
rotates at one revolution per cycle, or one radian per radian per second.

This of course is the (normal?) situation if the antenna is say a helix
firing along its axis or say crossed dipoles fed with a 90 degree phase
shift, but... to be more complete, we should note that...

Circular polarization does not have to be "synchronous"!

Consider...

What would you call the polarization type of the radiation emitted by the
following radiator?

An ordinary linear dipole fed with RF from a feedline through two slip
rings arranged such that a mechanical drive is able to rotate the dipole
at some arbitrary mechanical angular velocity completely unsynchronized
with the RF carrier.

For example, say it's a 5m dipole driven with 30MHz carrier signal and
mechanically rotated at 1000 revolutions per minute.

Such an arrangement would result in circularly polarized radiation with
the E vector having angular velocity of 1000 revolutions per minute and a
carrier frequency of 30MHz which definitely is not one revolution per
cycle.

Thoughts, comments?

--

Pete
Indialantic By-the-Sea, FL

Hi Pete

Your idea of rotating a linearly polarized 30 MHz dipole at 1000 RPM does
not relate to the term "Elyptical Polarization" which is often called
Circular Polarization. The Wikipedia information is fairly complete as it
defines CP..

Jerry KD6JDJ

Peter O. Brackett wrote:

Hmmmm....

Roy, I don't believe that is a "complete" description of circular
polarization either.

My understanding is that circular polarization is what we call the
polarization of electromagnetic radiation when the electric field vector
E rotates with an angular velocity rather than oscillating back and
forth in a single (linear) direction.

Roy, your description of circular polarization above seems to imply that
the angular velocity of the E vector of a circularly polarized wave is
always "synchronized" with the signal frequency since you stated that it
rotates at one revolution per cycle, or one radian per radian per second.

This of course is the (normal?) situation if the antenna is say a helix
firing along its axis or say crossed dipoles fed with a 90 degree phase
shift, but... to be more complete, we should note that...

Circular polarization does not have to be "synchronous"!

Consider...

What would you call the polarization type of the radiation emitted by
the following radiator?

An ordinary linear dipole fed with RF from a feedline through two slip
rings arranged such that a mechanical drive is able to rotate the dipole
at some arbitrary mechanical angular velocity completely unsynchronized
with the RF carrier.

For example, say it's a 5m dipole driven with 30MHz carrier signal and
mechanically rotated at 1000 revolutions per minute.

Such an arrangement would result in circularly polarized radiation with
the E vector having angular velocity of 1000 revolutions per minute and
a carrier frequency of 30MHz which definitely is not one revolution per
cycle.

Thoughts, comments?

That certainly doesn't fit the classical definition of circular
polarization. Circular and linear polarizations are special cases of
elliptical polarization, which all fields actually have, and which is
universally defined in terms of polarization change over each period.
Whatever you want to call the field your mechanical arrangement is
producing, it's not circular polarization in the sense universally used
in the literature.

Roy Lewallen, W7EL

Richard Fry wrote:
On Nov 29, 2:18 pm, Roy Lewallen wrote:
Jerry wrote:
Just for conversation, I submit that an antenna with good hemispheric
CP coverage could be made with 4 dipoles.

Yes, there's the quadrifilar helix which I believe fits that
description. Another, which I built decades ago at 450 MHz, is the "skew
planar" antenna which resembles a cloverleaf but with the "leaves"
rotated 45 degrees...

Getting back to Jerry's idea - yes, four linear dipoles can generate
nearly perfect omnidirectional c-pol. This is a design of Nils
Lindenblad many decades ago, and I've done some NEC-2 modeling of it.
The link below leads to a rendered view of that model.

http://i62.photobucket.com/albums/h8...adRendered.gif

There's still the problem of ground reflection, though. I didn't state
it very well in my last posting -- what I meant was that the sum of the
direct and ground-reflected rays tend to produce a linearly or nearly
linearly polarized wave even when you start out circular.

This isn't true at least at VHF and UHF, where the ground reflection
mostly just reverses the polarization sense of the incident wave.
This have been demonstrated by the much-improved images seen on analog
TV receivers in city centers when using c-pol transmit and receive
antennas, because multipath reflections ("ghosts") tend to be
suppressed by the receiving antenna.

The polarization reversal on reflection occurs only when the wave is
normal to a large (in terms of wavelength) flat surface. If it reflects
at a glancing angle, the sum of the direct and reflected rays end up
being nearly linear, or at least elliptical, depending on the reflection
angle and reflection coefficient. Glancing reflections from ground are
just about impossible to avoid at HF, but the also occur at VHF and above.

Roy Lewallen, W7EL