"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