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![]() "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 |
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