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THE AM BULLETIN BOARD => Technical Forum => Topic started by: WA2SQQ on April 02, 2018, 02:40:49 PM



Title: Top Hat for 40M vertical
Post by: WA2SQQ on April 02, 2018, 02:40:49 PM
I started building a 40M vertical and Id like to add a capacitive top hat. My understanding is that it can broaden the bandwidth.  Just wondering how large it needs to be, is bigger better?


Title: Re: Top Hat for 40M vertical
Post by: PA0NVD on April 02, 2018, 04:56:53 PM
A top head doesn't broaden the bandwidth,. If you have to shorten the antenna and make it to resonate, a top head gives you a wider bandwidth and a higher radiation resistance COMPARED to a loading coil, so it is preferred. But the bandwidth of a shortened antenna will always be narrower than a full size.
I advice downloading MMANA, an antenna simulation program, works very fine, fun and is simple to use. I posted a 40 and 20 meter vertical in antennas


Title: Re: Top Hat for 40M vertical
Post by: Steve - K4HX on April 02, 2018, 09:14:14 PM
You would only need a top hat if your vertical is not a quarter-wave length (about 33 or 34 feet). If you want more bandwidth, use a larger (fatter) vertical element. This can be a larger size tubing or the use of multiple wires in a cage configuration.


Title: Re: Top Hat for 40M vertical
Post by: WA4WAX on April 03, 2018, 11:46:35 AM
Whatever you do, put the hat on the very top of the antenna.  Do NOT put it on the top of the loading coil if you are using a loading coil as well.

Try an 8 to 10 inch hoop with varying numbers of spokes.

Matt


Title: Re: Top Hat for 40M vertical
Post by: WA2SQQ on April 04, 2018, 10:36:38 AM
Thanks for the clarification. The radiator will be made from the aluminum tower legs from my tower than came down 6 years ago. They are about 1 1/4" in diameter so that alone may give me some extra bandwidth.
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