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#9
April 3rd 06, 06:16 PM
posted to rec.radio.amateur.antenna
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Current across the antenna loading coil - from scratch
Cecil Moore wrote:
wrote: In a loading coil with very small distributed capaciatnce to the outside world compared to termination impedance, current has to be equal. Phase shift in current at each end has to be nearly zero. That is a false statement and is at the root of the misconceptions. Standing wave current does not have to be equal. I assume you are meaning that the RMS current at one physical point must not equal the RMS current at some other point. I have shown how current at the bottom of the coil can be zero while the current at the top of the coil is one amp. Do you think the coil is sucking that one amp sideways from somewhere else through its distributed capacitance? (snip) Of course that is what is happening. It is what happens in any transmission line like device. There is a standing voltage wave, also, and that produces displacement current through any capacitance, just as the antenna does. Aren't you claiming that the coil has transmission line like properties, in that it takes time for a wave to pass through it? Any such device needs two mechanisms for storing energy, one magnetic (inductive) and one electrical (capacitive). Even free space has both. If you eliminate either mechanism (or make one of them insignificant, as would happen to the capacitance if the inductor approaches zero size), you lose the transmission line like properties as the dominant mechanism. |
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