[TowerTalk] TOP HAT FOR BB3 Screwdriver Antenna

[Tom Rauch]

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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