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Thanks for the clarification.
I'm not entirely convinced that the ammeter is the best idea. There are enough internal wires and coils to introduce a real possibility of error when in close proximity to an inductor. It shouldn't be as much of a problem with a toroid, but I'm still a little leery. I agree it would be difficult to do the measurements well with a scope anywhere but at the base of the antenna. Current probes and a detecting meter might be ok, but you'd have to take a lot of care to avoid making an unintentional loop which would couple to the inductor, and you'd have to calibrate the potentially nonlinear detector. Phase information would be lacking, too. I'm waiting for Cecil's response, since by his theory, as I understand it, we should be able to get a decent phase shift through an inductor at the base of an antenna providing the antenna is significantly longer than a quarter wavelength. And if I understand your theory, we should be able to see a full 30% change in magnitude and 45 degree change in phase in the current through a base mounted inductor, if it's loading a 45 degree radiatior to resonance. Am I correct? I could measure that with the same setup but with an antenna removed from the mount. And 30% and 45 degrees should be much easier to resolve with any accuracy than the 2.5 or 5 percent you predict for the setup I did measure. Incidentally, I take it that your prediction for the setup I did measure includes an 18 degree phase shift of current from input to output of the inductor? Roy Lewallen, W7EL Yuri Blanarovich wrote: I apologize. I read and was referring to the same quote, and interpreted it to mean that the first measurement was made with the coil at the base of the antenna. So where was it -- 78" from the bottom? Roy Lewallen, W7EL Yes, mast 78" - coil - 38" top whip we keep saying, looking at typical mobile antenna with loading coil about 2/3 up the quarter wave radiator. The lower the frequency, more loading, more pronounced effect. Caution, using toroid current transformers with scope leads would detune the antenna setup and introduce errors. You can get away with this at the base, but any stray capacitance up the radiator will detune it and skew the results. Need to use thermal RF current ammeters or current probe with detector and small meter together, no wires. Yuri, K3BU |
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