Hi Dave,
> Don't know what that equates to in inductance... I presume less inductance
> means better efficiency.
Maybe, maybe not. Depends on how you get there.
> Now a few questions:
>
> o Did I just improve the efficiency of my mobile antenna?
> o Can I start visualizing more current climbing up that 12 inches of
> whip between the coil and
> the top-hat?
Yep, more current flows through the coil to the hat. Problem is you
need to also visualize LESS current climbing up the whip above the
hat. So you made current in the coil increase, while current in the
main part of the radiator decreased.
If you got lucky and didn't reduce current in the whip, and if it
stayed the same (which is a long shot, and the best you can hope
for) you increased current in the coil for the same antenna current.
Losses in the coil increase by the square of the current. Power
loss is I squared R. Adding a capacitor across a coil increases
loss by the square of the inductance reduction, if all other things
remain equal.
Now someone will say, "but it has less turns and less resistance!"
It has less turns, but maybe not less resistance. Depends on the
coil.
If the ESR (series loss resistance) drops at a linear rate and
mutual coupling is high between turns, you will reduce resistance
at 1/2 the rate you reduce inductance. That's the best you can
hope for.
In this case you only double the loss if you halve inductance with a
hat at the end of the coil and little or no radiator below the coil.
Most systems fall right in between the two.
> o Is the claim of the screwdriver builders true -- that this little
> top-hat is worth at least 1db
> improvement?
Only if the whip is almost totally insignificant as a radiator, and
there is a lot of radiator below the hat that is out in the clear.
Otherwise, you can be sure you made things worse.
> o Is it true that the higher up the whip the top-hat is placed the
> greater the effect?
No, that isn't exactly it. The higher the hat is placed, the greater
the improvement. When the bulk of the radiator is above the coil
and hat, the closer the hat is placed to the top of the coil the
greater the loss.
Three things happen when you install a hat at the correct spot:
1.) Radiation resistance goes up. It goes up as much as FOUR
times for a given antenna length with a big enough hat at the top.
If you don't do anything else, you just cut loss something up to four
times.
2.) The inductance required and the voltage down low in the
antenna decreases, both of which increase efficiency.
3.) Bandwidth increases, and feedpoint resistance increases.
(There goes the wive's tale that wider bandwidth and higher
feedpoint resistance means less efficiency...out the window.)
Of course if you park the hat right above the coil, feedpoint
resistance and bandwidth often decrease...along with
efficiency..but that makes some people think they made things
better.
> o If the reduction of inductance does significantly improve effeciency
> then is it reasonable to
> presume that one could raise the power rating of the antenna?
It probably lowers power rating. With the hat close to the coil power
loss increases, and there is less coil area to dissipate the power.
73, Tom W8JI
w8ji@contesting.com
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