Chris & Bill Shell wrote:
> I just finished putting the 40m element, X940, on my Cushcraft X-9. The
> instructions, step 7, have you move the boom-to-mast adapter 30.5 inches to
> the rear between the 2nd 10m and 15m driven elements.
>
> My question is, why? The manual just states, "This is necessary to balance
> the antenna on the tower mast."
>
Hi Bill,
Well, the answer depends on what one is trying to balance.
If the object is to obtain a balance for the antenna weight distribution, then
you want to mount the mast as close as possible to the antenna's Center of
Gravity. This makes you happy when putting it on, or taking it off of the
tower.
If the CG is very far from the center of the boom, you will have an antenna
that produces rather high torque when the boom is exposed to the wind. I think
others call this type of antenna "A Wind Vane!"
If your rotor and tower like this, then mount the mast at the antenna CG and
let the tower and rotor absorb the torque loads.
If you wnat to have an antenna that is balanced under wind loading, mount the
mast at the center of the boom, and put a counterweight inside the light end of
the boom to move the antenna's CG back to the boom center. This is my
preference!
Be aware that all antenna manufacturers do not understand this. So, some may
tell you to do otherwise. Ignore that guidance! It may be well intentioned, but
is uninformed.
73, Kurt
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