> I had a homebrew bugcatcher style mobile antenna I used
on 40m CW. I
> placed a tophat about one coil length above the loading
coil by using a
> second Delrin coil center rod on top of the coil. I did
that in order to
> reduce the capacitive loading between the tophat and the
coil. In your
> opinion, do you think that distance would be enough to
mitigate the possible
> coil losses?
I'm not sure what you are describing. Of course if you
removed the electrical connection the hat wouldn't do much
of anything, so I'm assuming the hat is still connected
through a conductor to the top of the loading coil.
Adding a dielectric in the path of an electric flux does not
reduce distributed capacitance through that area, it
increases it.
If you supported the hat on a thin uninsulated metal
conductor there would be less distributed capacitance below
the hat and to the coil than when that same conductor is
surrounded by a dielectric.
This is why a "big sin" when making a very high reactance
high frequency inductor is adding insulation in and around
the winding. While the dielectric increases effective
inductance by adding parallel capacitance across the
winding, it also increases circulating currents in the
conductor. The increase in circulating currents greatly
impact ESR of the inductor, since dissipation is current
squared times resistance.
This is why edge-wound air insulated inductors found in BC
transmitters typically measure with Q's in the upper
hundreds (nearly 1000) and close-spaced insulated wires of
similar size can have Q's in the low hundreds. Distributed
capacitance shunting across a high impedance inductor
seriously degrades Q, even though system bandwidth in the
presence of that capacitance can actually become narrower!
> Secondly, in terms of effective height, I am of the
opinion that so little
> current exists in the whip above the coil of a heavily
loaded short vertical
> that the effective height is not much higher than the coil
itself. So
> therefore, unless you have a very big tophat at the top of
the whip, such
> that you could greatly reduce the inductive loading
(thereby increasing the
> current above the coil significantly) the position of the
tophat doesn't
> make much difference to effective height. The mast below
the coil, and the
> coil itself probably contribute to almost all the
radiation.
That picture isn't accurate. Assuming the inductor's
distributed capacitance to the outside world is small (not
the turn-to-turn capacitance), the inductor has essentially
equal currents at each terminal.
It is very easy to build a loading inductor of high Q that
has essentially equal currents at each end provided the
antenna area above the inductor has some reasonable
impedance.
Assume we have three feet of thin mast, a relatively compact
inductor, and six feet of whip above the inductor. What we
would have is close to triangular or linear current taper in
the whip above the coil. The coil would have essentially the
same current at each end, and the mast below the coil
essentially uniform current.
The six foot whip would provide the same approximate
radiation contribution as the three feet of conductor
carrying uniform current below the loading coil, assuming no
other conductors paralleled the mast down low. If something
like a metal car body paralleled the lower section, the whip
might provide the bulk of radiation.
>also, I think
> a few feet difference in effective height on 40m is not
very important.
> Your reasoning might be a better argument for increased
radiation
> efficiency, rather than increased effective height.
More effective height means higher radiation efficiency.
This is especially true in a mobile antenna where ground
related loss resistances dominate overall system loss. On
160 meters changing current distribution from triangular to
uniform (effectively doubling height) can result in nearly
four times more field strength (6dB gain) for a given
applied power without ANY change in loading inductor ESR.
It all comes down to the current flowing over every inch of
vertical distance.
73 Tom
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