Andy:

Hey, thanks for your input.

I know about VOR systems and other similar systems such as TACAN. Indeed
they do use rotating antennas.

However VOR and TACAN use rotating antennas in the same way as rotating PPI
radar antennas, that is they emit linearly polarized
waves whilst rotating the direction of highest directivity/gain.

They do not emit circular polariztion as such.

Rather they emit linear polarization whilst aiming or directing the 'beam'
of linear polarized waves as they rotate.

Sort of like rotating a flashlight, or the beam of a searchlight or coastal
lighthouse.

I'm not sure that anyone yet (that includes Roy Lewalen) has fully
understood exactly what I was trying to convey.

I'm afraid that the true nature of circular polarization is not well
understood by many.

Perhaps opitical scientists understand circular polarization best, if only
because most of the important 'applications' of circular polarization are in
the field of optics rather than radio.

-- Pete K1PO
-- Indialantic, By-the-Sea, FL


"AndyS" wrote in message
...


Peter O. Brackett wrote:

Now transmit on that dipole antenna whilst mechanically spinning it
clockwise [RHCP?] (with a mechanical motor of some kind).

The dipole antenna is linear and thuse emits linear polariztion, except
it
is mechanically spinning, and so the E vector emanating from the antenna
will be rotating with respect to its direction of travel.

In this case the angular velocity of the motor that spins the linear
antenna
need not be synchronous with the frequency being radiated.

For example we could mechanically spin the antenna at 330 rpm while
transmitting a carrier of 1 GHz.

This would most certainly produce circular polarization. For is not the
E
vector spinning at 330 revs!


Andy writes:

It sounds to me like you are describing the technique for generating
an aircraft VOR signal, which has been in use for well over 50 years.

The VOR band is 108 - 117 Mhz, and the antenna is a cardoid
pattern
that is rotated mechanically at a 30 hz rate. At a distant point
this
results in a 30 hz amplitude modulation of the received signal, which
is one of the components used in the signal processing for the
receiver to determine the direction to or from the ground VOR station.

Simply rotating the antenna does not result in circular
polarization, but
rather it changes the field strength of the radiated signal at a point
in
space.... The received signal is therefore modulated in amplitude as
the pattern passes a singular distant point in space.....

I just wanted to throw this in the mix, since rotating the antenna
has
been around for a long time.

Of course it can be done electronically now, but the initial
systems
were simply turned by a motor.

Andy W4OAH , ex- aircraft nav system
designer....long retired.