"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