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Author Topic: Screwdriver antenna with capacity hat question  (Read 392 times)
KG5KS
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« on: May 23, 2007, 06:10:23 PM »

I have a BMD screwdriver antenna and have a 66inch whip antenna for it.  I wanted to hear if capacity hat will make it a bigger signal, or what can be done to help this installation?  Any and all replies are greatly appreciated.  

Thank you
Kenny
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K0BG
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« Reply #1 on: May 24, 2007, 05:35:47 AM »

In a word or two, no.

Cap hats increase the capacitance of the antenna which is sometimes called end effect, which isn't very descriptive. This raises the radiation resistance to so extent, and lowers the amount of inductance required to resonant the antenna. While this does decrease the over all losses, the effect isn't as pronounced on the upper HF bands. On 160, and 80, it is.

The other side of the coin is, it takes a really BIG cap hat to notice any improvement, although you can measure it. It adds a lot of wind drag, and if the mounting integrity is minimal, this can become a problem.

Lastly, most cap hats are mounted too close to the coil, and the effect they produce is opposite of what it should be. The rule of thumb is 1.5 to 2 times the coil length ABOVE the coil as a minimum. The correct place is at the very top, but this adds even more problems.

I have more on the subject on my web page under Antennas, commercial.

Alan, KØBG
www.k0bg.com
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