On 11/29/11 12:25 PM, Rick Karlquist wrote:
> With a vertical up to 1/4 wave long, the base current
> is the same as the current lobe. Thus you can control
> the phase and magnitude of the current lobe by controlling
> the base current. When you exceed 1/4 wavelength, then
> the current lobe moves up the vertical above the base.
> And the current lobe has a different value than the base
> current.
>
> The more you exceed 1/4 wl, the harder it is to control
> the current lobe by forcing the base current. When the
> vertical reaches 1/2 wl, you have to force the base
> voltage to force the current lobe. However, the relationship
> between the forcing voltage and the current lobe depends on
> stray capacitance around the base, so it is unpredictable
> in practice. (However, it works great on EZNEC).
>
Ah yes.. excellent point.
For elevated vertical dipoles, too, the current forcing would work
fairly well.
But for end fed schemes (1/2 and 5/8 wave) I can see where getting the
right excitation would be tough and VERY sensitive to small changes in
feedpoint Z.
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