"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