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[TowerTalk] TOP HAT FOR BB3 Screwdriver Antenna

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Subject: [TowerTalk] TOP HAT FOR BB3 Screwdriver Antenna
From: (Phil Clements)
Date: Thu, 22 Jul 1999 19:15:22 -0500
>Now a few questions:
>o Did I just improve the efficiency of my mobile antenna?

No, you did not. You need to get a copy of Don Johnson's book,
"40+5 Years of HF Mobileering." You will see that the radiation
above a top hat is nil. That is why it has to be mounted at the very
top of the whip to improvr efficiency.

>o Can I start visualizing more current climbing up that 12 inches of
>whip between the coil and
>   the top-hat?

Yes, you can, but that is where it stops. You have effectively shorted out
the whip above the hat.

>o Is the claim of the screwdriver builders true -- that this little
>top-hat is worth at least 1db
>   improvement?

Fewer turns  required on the coil get you a little  bit. 
The trouble with most screw driver antennas is that the coil diameter
and wire size are too small for good efficiency  below 20 meters..

>o Is it true that the higher up the whip the top-hat is placed the
>greater the effect?


>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?

Again, the power rating is determined by the huskiness of the coil.
If you push the ratings, you end up with a bathroom heater on your

Get Don's book if you can; it is full of great data.

Phil, K5PC

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