Something just occurred to me. I did get to thinking.

My previous answers were wrong. Peter's spinning antenna wouldn't
produce a circularly polarized wave (as universally defined) even if it
was synchronous with the wave frequency. As I've said, a circularly
polarized wave has constant E field amplitude; Peter's wave would have a
time-varying amplitude. If it were synchronous, the nulls and peaks
would always occur at the same places in the rotation cycle, so they
would occur at fixed angles relative to a rotational reference point. If
non-synchronous, the nulls and peaks would rotate at the beat frequency.

It seems to me that the way to mechanically generate a circularly
polarized wave would be to rotate a source of *static* E field, for
example, a short dipole with constant applied DC voltage at the
feedpoint. That should produce a circularly polarized wave with the
frequency being the rotational frequency of the dipole. At any point in
space, the E field would change with time, and would propagate, and it
would look exactly like a circularly polarized wave broadside to the
rotation plane.

If the scheme works and radiation is occurring, then power must be going
into the antenna, which in turn means it's drawing current that's in
phase with the applied voltage. When stopped, no current will flow, but
when rotating, it does. So how does the antenna know it's rotating? How
about this -- if you instantaneously move the antenna into some
position, a static E field appears there, and propagates outward at the
speed of light. Closer in than the leading edge of the propagating wave,
the field is static. When we rotate the dipole to a new position, it
moves through the field from its previous position, which induces a
current in it. Hence the current. It's fundamentally a generator, with
the field being in the air.

I'd be willing to bet a moderate sum that if you did apply a DC voltage
to a dipole and rotated it, you'd see an alternating current with a
frequency equal to the frequency of rotation, and a circularly polarized
wave broadside to the antenna. I suspect that the current and the
radiated field increase in amplitude with rotational speed, so you might
have to get it going really fast before you can detect the effects.

Now there's some food for thought.

Roy Lewallen, W7EL

In article tonline, Roy
Lewallen wrote:

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.

Hello, and that's quite correct, Roy. Having read the OP's statements and
others in this thread I would like to recommend that one step back from
antennas for a moment in order to examine the generation of an ellipse
(representing the locus of points of a rotating E (or H) field. The
parametric equations take the form x(t) = A*cos(2*pi*f*t) and y(t) =
B*cos(2*pi*f*t + phi). (These equations are of the same form that
generate the familiar Lissajous patterns except that for Lissajous the x
and y values differ in frequeny.)

While polarization is a convenient concept in electromagnetic wave
propagaion there's no reason that we couldnt just treat it as the
superposition of two separate Ex (or Hx) and Ey (or Hy) waves. Of course
we have to pay attention to amplitude and phase relationships.

I think investing some time with this math (it's not all that difficult)
will provide one with insight into the concept of polarization and perhaps
head off some misconception. If anyone is interested and has Mathcad,
I've got a worksheet that allows one to vary these parameters, plots the
resulting ellipse (or circle or line) and also calculates ellipticity
(axial ratio) and eccentricity. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

J. B. Wood wrote:

I think investing some time with this math (it's not all that difficult)
will provide one with insight into the concept of polarization and perhaps
head off some misconception. If anyone is interested and has Mathcad,
I've got a worksheet that allows one to vary these parameters, plots the
resulting ellipse (or circle or line) and also calculates ellipticity
(axial ratio) and eccentricity. Sincerely, and 73s from N4GGO,

All I have to know is that Circular Polarization always helps when one
end of the path is prone to random polarizations, even with the 3 dB
power loss.

"Roy Lewallen" wrote in message
treetonline...
Something just occurred to me. I did get to thinking.

My previous answers were wrong. Peter's spinning antenna wouldn't produce
a circularly polarized wave (as universally defined) even if it was
synchronous with the wave frequency. As I've said, a circularly polarized
wave has constant E field amplitude; Peter's wave would have a
time-varying amplitude. If it were synchronous, the nulls and peaks would
always occur at the same places in the rotation cycle, so they would occur
at fixed angles relative to a rotational reference point. If
non-synchronous, the nulls and peaks would rotate at the beat frequency.

It seems to me that the way to mechanically generate a circularly
polarized wave would be to rotate a source of *static* E field, for
example, a short dipole with constant applied DC voltage at the feedpoint.
That should produce a circularly polarized wave with the frequency being
the rotational frequency of the dipole. At any point in space, the E field
would change with time, and would propagate, and it would look exactly
like a circularly polarized wave broadside to the rotation plane.

If the scheme works and radiation is occurring, then power must be going
into the antenna, which in turn means it's drawing current that's in phase
with the applied voltage. When stopped, no current will flow, but when
rotating, it does. So how does the antenna know it's rotating? How about
this -- if you instantaneously move the antenna into some position, a
static E field appears there, and propagates outward at the speed of
light. Closer in than the leading edge of the propagating wave, the field
is static. When we rotate the dipole to a new position, it moves through
the field from its previous position, which induces a current in it. Hence
the current. It's fundamentally a generator, with the field being in the
air.

I'd be willing to bet a moderate sum that if you did apply a DC voltage to
a dipole and rotated it, you'd see an alternating current with a frequency
equal to the frequency of rotation, and a circularly polarized wave
broadside to the antenna. I suspect that the current and the radiated
field increase in amplitude with rotational speed, so you might have to
get it going really fast before you can detect the effects.

Now there's some food for thought.

Roy Lewallen, W7EL


A source of endless coffee-time debates where I used to work! No, the
current into the rotating dipole would be DC and the means of rotation at
the radio frequency would take the place of the 'transmitter'. If the
current were alternating then the radiated electric field would be
discontinuous but it isn't; it has constant magnitude. Between two such
systems separated by many wavelengths, if there were no anisotropic material
around, reciprocity would apply and a means of conveying DC by radio would
be created!

However, intriguing and amusing as this analogy might be I wonder if it
really has any practical value. For real mechanical rotating parts the
frequency would be limited to something rather low like the tens of kHz at
which Alexanderson alternators work, and then the wavelength would be so
long that it would probably be impossible to construct an efficient
radiator*. The quickest moving antenna I've encountered was a commutated
plasma antenna, using a construction similar to a 'dekatron' tube, but even
then the length of the radiator was so small that SHF would be needed to
achieve worthwhile radiation efficiency* and the maximum commutation speed
was limited to a few MHz by the time it takes to establish the plasma at
each step in the commutation cycle.

*(Of course, the conventional principles of radiation resistance vs. loss
resistance may need 'massaging' to bring them into line with the concept of
creating transverse waves by rotating a dipole connected to a battery!)

Chris

"christofire" wrote in message
...

"Roy Lewallen" wrote in message
treetonline...
Something just occurred to me. I did get to thinking.

My previous answers were wrong. Peter's spinning antenna wouldn't produce
a circularly polarized wave (as universally defined) even if it was
synchronous with the wave frequency. As I've said, a circularly polarized
wave has constant E field amplitude; Peter's wave would have a
time-varying amplitude. If it were synchronous, the nulls and peaks would
always occur at the same places in the rotation cycle, so they would
occur at fixed angles relative to a rotational reference point. If
non-synchronous, the nulls and peaks would rotate at the beat frequency.

It seems to me that the way to mechanically generate a circularly
polarized wave would be to rotate a source of *static* E field, for
example, a short dipole with constant applied DC voltage at the
feedpoint. That should produce a circularly polarized wave with the
frequency being the rotational frequency of the dipole. At any point in
space, the E field would change with time, and would propagate, and it
would look exactly like a circularly polarized wave broadside to the
rotation plane.

If the scheme works and radiation is occurring, then power must be going
into the antenna, which in turn means it's drawing current that's in
phase with the applied voltage. When stopped, no current will flow, but
when rotating, it does. So how does the antenna know it's rotating? How
about this -- if you instantaneously move the antenna into some position,
a static E field appears there, and propagates outward at the speed of
light. Closer in than the leading edge of the propagating wave, the field
is static. When we rotate the dipole to a new position, it moves through
the field from its previous position, which induces a current in it.
Hence the current. It's fundamentally a generator, with the field being
in the air.

I'd be willing to bet a moderate sum that if you did apply a DC voltage
to a dipole and rotated it, you'd see an alternating current with a
frequency equal to the frequency of rotation, and a circularly polarized
wave broadside to the antenna. I suspect that the current and the
radiated field increase in amplitude with rotational speed, so you might
have to get it going really fast before you can detect the effects.

Now there's some food for thought.

Roy Lewallen, W7EL


A source of endless coffee-time debates where I used to work! No, the
current into the rotating dipole would be DC and the means of rotation at
the radio frequency would take the place of the 'transmitter'. If the
current were alternating then the radiated electric field would be
discontinuous but it isn't; it has constant magnitude. Between two such
systems separated by many wavelengths, if there were no anisotropic
material around, reciprocity would apply and a means of conveying DC by
radio would be created!

However, intriguing and amusing as this analogy might be I wonder if it
really has any practical value. For real mechanical rotating parts the
frequency would be limited to something rather low like the tens of kHz at
which Alexanderson alternators work, and then the wavelength would be so
long that it would probably be impossible to construct an efficient
radiator*. The quickest moving antenna I've encountered was a commutated
plasma antenna, using a construction similar to a 'dekatron' tube, but
even then the length of the radiator was so small that SHF would be needed
to achieve worthwhile radiation efficiency* and the maximum commutation
speed was limited to a few MHz by the time it takes to establish the
plasma at each step in the commutation cycle.

*(Of course, the conventional principles of radiation resistance vs. loss
resistance may need 'massaging' to bring them into line with the concept
of creating transverse waves by rotating a dipole connected to a battery!)

Chris

Hi Chris

I am not smart enough to analyze the effects of rotating a dipole with DC
applied to it, but I have doubts that it would create a "far field". Did
you guys ever figure out how the "DC dipole" generates a Far Field?

Jerry KD6JDJ

christofire wrote:

A source of endless coffee-time debates where I used to work! No, the
current into the rotating dipole would be DC and the means of rotation at
the radio frequency would take the place of the 'transmitter'. If the
current were alternating then the radiated electric field would be
discontinuous but it isn't; it has constant magnitude. Between two such
systems separated by many wavelengths, if there were no anisotropic material
around, reciprocity would apply and a means of conveying DC by radio would
be created!

Now that I think about it, you're right -- the current would have to be
DC, so there would be only DC power into the dipole.

Interesting that you and your co-workers thought of and debated this.
I've given it less than an hour of thought since it popped into my head,
so you've had a lot more time to work out the details. Sounds like it
might work something like I described, then.

However, intriguing and amusing as this analogy might be I wonder if it
really has any practical value. For real mechanical rotating parts the
frequency would be limited to something rather low like the tens of kHz at
which Alexanderson alternators work, and then the wavelength would be so
long that it would probably be impossible to construct an efficient
radiator*. The quickest moving antenna I've encountered was a commutated
plasma antenna, using a construction similar to a 'dekatron' tube, but even
then the length of the radiator was so small that SHF would be needed to
achieve worthwhile radiation efficiency* and the maximum commutation speed
was limited to a few MHz by the time it takes to establish the plasma at
each step in the commutation cycle.

I can't see where this could possibly be of any practical use. For me it
was simply a mind exercise spurred by Peter's musings, resulting from
wondering just how a mechanical system could be made to generate a CP wave.

*(Of course, the conventional principles of radiation resistance vs. loss
resistance may need 'massaging' to bring them into line with the concept of
creating transverse waves by rotating a dipole connected to a battery!)

Indeed. And it seems there wouldn't be any skin effect, then, with only
DC going to the wire. And what about current distribution on the dipole?

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
treetonline...
christofire wrote:

A source of endless coffee-time debates where I used to work! No, the
current into the rotating dipole would be DC and the means of rotation at
the radio frequency would take the place of the 'transmitter'. If the
current were alternating then the radiated electric field would be
discontinuous but it isn't; it has constant magnitude. Between two such
systems separated by many wavelengths, if there were no anisotropic
material around, reciprocity would apply and a means of conveying DC by
radio would be created!

Now that I think about it, you're right -- the current would have to be
DC, so there would be only DC power into the dipole.

Interesting that you and your co-workers thought of and debated this. I've
given it less than an hour of thought since it popped into my head, so
you've had a lot more time to work out the details. Sounds like it might
work something like I described, then.

However, intriguing and amusing as this analogy might be I wonder if it
really has any practical value. For real mechanical rotating parts the
frequency would be limited to something rather low like the tens of kHz
at which Alexanderson alternators work, and then the wavelength would be
so long that it would probably be impossible to construct an efficient
radiator*. The quickest moving antenna I've encountered was a
commutated plasma antenna, using a construction similar to a 'dekatron'
tube, but even then the length of the radiator was so small that SHF
would be needed to achieve worthwhile radiation efficiency* and the
maximum commutation speed was limited to a few MHz by the time it takes
to establish the plasma at each step in the commutation cycle.

I can't see where this could possibly be of any practical use. For me it
was simply a mind exercise spurred by Peter's musings, resulting from
wondering just how a mechanical system could be made to generate a CP
wave.

*(Of course, the conventional principles of radiation resistance vs. loss
resistance may need 'massaging' to bring them into line with the concept
of creating transverse waves by rotating a dipole connected to a
battery!)

Indeed. And it seems there wouldn't be any skin effect, then, with only DC
going to the wire. And what about current distribution on the dipole?

Roy Lewallen, W7EL

Hi Roy

I have problems with believing there will be any current in either dipole.
What am I missing?

Jerry KD6JDJ

Jerry wrote:

I am not smart enough to analyze the effects of rotating a dipole with DC
applied to it, but I have doubts that it would create a "far field". Did
you guys ever figure out how the "DC dipole" generates a Far Field?

Jerry KD6JDJ

It requires energy to create a far field, since the far field is a form
of energy. I explained why I thought power might be consumed by the
antenna -- current would flow due to coupling with the field still
present from previous positions (although I mentioned alternating
current while Chris correctly pointed out that it would have to be DC).
I don't see any problem with conversion of the DC into AC. It's done all
the time with spinning magnets -- look at the alternator in your car for
example. And in times of yore, RF was generated directly with high speed
alternators. The principle is very similar to, if not exactly the same
as, the scheme I described.

The whole thing is just a mental exercise to help gain a better
understanding of the nature of a circularly polarized field.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Jerry wrote:

I am not smart enough to analyze the effects of rotating a dipole with
DC applied to it, but I have doubts that it would create a "far field".
Did you guys ever figure out how the "DC dipole" generates a Far Field?

Jerry KD6JDJ

It requires energy to create a far field, since the far field is a form of
energy. I explained why I thought power might be consumed by the
antenna -- current would flow due to coupling with the field still present
from previous positions (although I mentioned alternating current while
Chris correctly pointed out that it would have to be DC). I don't see any
problem with conversion of the DC into AC. It's done all the time with
spinning magnets -- look at the alternator in your car for example. And in
times of yore, RF was generated directly with high speed alternators. The
principle is very similar to, if not exactly the same as, the scheme I
described.

The whole thing is just a mental exercise to help gain a better
understanding of the nature of a circularly polarized field.

Roy Lewallen, W7EL

Indeed, and I would add that the spinning dipole fed with a constant voltage
appears the same as a stationary dipole fed with an alternating voltage with
respect to any chosen linear polarisation.

I was once told of a method of measuring the radiation patterns of large
installed antennas by 'flying' near to them a small metal rod rotating about
an axis that passes perpendicularly through the middle of the length of the
rod. By detecting, synchronously with rotation of the rod, changes in the
terminal VSWR (or reflection co-efficient for voltage) the near-field
radiation pattern could be assessed (i.e. an impression of the aperture
current distribution) from which the far-field patterns could be derived by
Fourier transform in the normal way (acknowledgement is due to the late Dick
Manton). There is a range of 3D angles over which the axis can vary without
upsetting the measurement. I don't know if this was ever implemented, e.g.
to measure the patterns of a television transmitting antenna - a helicopter
carrying a measuring receiver is used in the far field nowadays.

Chris

"Jerry" wrote in message
...

"Roy Lewallen" wrote in message
treetonline...
christofire wrote:

A source of endless coffee-time debates where I used to work! No, the
current into the rotating dipole would be DC and the means of rotation
at the radio frequency would take the place of the 'transmitter'. If
the current were alternating then the radiated electric field would be
discontinuous but it isn't; it has constant magnitude. Between two such
systems separated by many wavelengths, if there were no anisotropic
material around, reciprocity would apply and a means of conveying DC by
radio would be created!

Now that I think about it, you're right -- the current would have to be
DC, so there would be only DC power into the dipole.

Interesting that you and your co-workers thought of and debated this.
I've given it less than an hour of thought since it popped into my head,
so you've had a lot more time to work out the details. Sounds like it
might work something like I described, then.

However, intriguing and amusing as this analogy might be I wonder if it
really has any practical value. For real mechanical rotating parts the
frequency would be limited to something rather low like the tens of kHz
at which Alexanderson alternators work, and then the wavelength would be
so long that it would probably be impossible to construct an efficient
radiator*. The quickest moving antenna I've encountered was a
commutated plasma antenna, using a construction similar to a 'dekatron'
tube, but even then the length of the radiator was so small that SHF
would be needed to achieve worthwhile radiation efficiency* and the
maximum commutation speed was limited to a few MHz by the time it takes
to establish the plasma at each step in the commutation cycle.

I can't see where this could possibly be of any practical use. For me it
was simply a mind exercise spurred by Peter's musings, resulting from
wondering just how a mechanical system could be made to generate a CP
wave.

*(Of course, the conventional principles of radiation resistance vs.
loss resistance may need 'massaging' to bring them into line with the
concept of creating transverse waves by rotating a dipole connected to a
battery!)

Indeed. And it seems there wouldn't be any skin effect, then, with only
DC going to the wire. And what about current distribution on the dipole?

Roy Lewallen, W7EL

Hi Roy

I have problems with believing there will be any current in either
dipole. What am I missing?

Jerry KD6JDJ


That's understandable.

Chris