Ted, that is only correct if and while the element is perfectly
horizontal above the boom and perfectly centered on it. As soon as the
element tips away from horizontal, the center of gravity of the element
shifts to the side of the boom and tries to tilt the element even more.
The situation is reversed for an element mounted below the boom ... the
shifted center of gravity tries to correct the tilt. It's the same
reason that airplane fuselages usually hang below the wing. The thinner
the antenna element the less the effect, but I can't think of a single
reason why a designer wouldn't make use of it and hang the element below
the boom. If the element mass isn't perfectly balanced on the boom, the
benefit is even more compelling.
Dave AB7E
Ted Bryant wrote:
> If I remember my Statics & Dynamics correctly, the gravitational forces on a
> horizontal element(and
> the resulting torque on the boom) are the same whether it's mounted above or
> below the boom. For a
> symetrical element mounted at its center, rotational torque due to gravity
> should very close to
> zero.
>
> I suspect the decision whether to mount the element above or below the boom
> is affected more by
> other factors.
>
> 73, Ted W4NZ
>
>
> On 22 Nov 2006 at 19:42, Bill Turner wrote:
>
>
>> Nearly all of the all-metal yagis I've seen have the elements mounted
>> on top of the boom. I wonder why?
>>
>> If they were mounted under the boom, gravity would be working to keep
>> the boom in place instead of constantly trying to turn the boom over.
>> This is such a common design I'm wondering what am I missing?
>>
>
>
>
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