Barry,
Thanks for your reply comments. With regard to your observations of
the current taper's dependence on the loading coil's position (base
vs. center), did you compare the current taper in the coil to the current
taper in a linear radiator of the same overall length as the coil (Larry
mentioned that the coils you tested were fairly long). Another way of
thinking about this would be to consider the case where you would
replace a 20" long inductor (used in a center loading configuration)
with a very short closewound inductor (say 5" long) having the same
inductance value and Q and an equivalent amount of additional tubing
to make up for the shorter inductor length (in this case 15" of tubing).
Would the end-to-end current taper be the same in either case?
Would the resonant frequency be the same in either case? Would
the ordering of the short inductor and the 15" 'make-up' tubing matter?
>
> I'm not sure how I might have misled you, but altho the current decreases
> rapidly in the loading coil, it does not reach zero at the top of the coil
> if
> there is a capacitance above the coil like a whip or hat. The amount of
> current reduction in the coil varies inversely to the amount of
capacitance
> above the coil.
Qualitatively, this should also be true for any linear radiating segment.
Consider a 12" segment of aluminum tubing along the length of a 1/4
wave vertical. If you consider the bottom 12" of the radiator, the taper
is very little. The closer you get to the tip of the vertical, the greater
the taper in the 12" segment. How is this different from the case of
the physically long coils you measured? Or is it?
> So, to answer your question, for a particular length of monipole, there
will
> be an increasing amount of current taper as it is moved from base to top.
> Further, in a typical mobile set-up using a Hustler mast and resonator,
> about
> 99% of the radiation is derived from the field created by the current in
the
> mast and 1% from the current in the upper whip.
> In the case of a base loaded mast, a significant portion of the overall
> radiated field is derived from the loading coil itself. An example from
our
> study makes this clear. On 75 meters, a 9 ft. inductively top loaded mast
> was
> 20.5db down from a full quarter -wave on the test stand. The same 9 ft.
mast
> when base loaded was 36.5db down from the quarterwave....16db worse. But,
> moving the loading coil into a shielded box at the base cost us 8 more db!
> It
> was 44.5db down from the quarterwave.
>
>
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