Date: Wed, 13 Apr 2016 08:55:43 -0500
From: Stan Stockton <wa5rtg@gmail.com>
To: "Joe Subich, W4TV" <lists@subich.com>
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] W6NL 40m Moxon
If calculating wind load for the purpose of deciding which rotator to use, I
would think the boom length would factor in. Surely a square plate 4'x4'
mounted to the mast would put less stress on a rotator than a 50 foot boom
antenna with the same 16 SF calculation?
At any rate if the difference in the various ways of calculating the wind load
mentioned makes the difference in what you believe will survive, I would think
you are cutting it too close.
Stan, K5GO
## Boom length has nothing to do with it....provided you have an equal amount
of boom on either side of the mast. If one end of the boom is heavier than the
other, then the fix is a counterweight is used on the lighter ( LAST DIR)
end.
IF instead you mount at the center of gravity, then you will have a gross tq
imbalance. Fix for that is a tq compensator plate down at the REF end of the
boom.
## I designed a TQ comp plate for a fellow here in town. We tested it by
installing the yagi on a short 30 ft tall test tower, with just 2 x
bearings..and no coax and no rotor. It was free to spin 360++ degs.
Buddy climbs up there in a 35 mph windstorm..and could easily turn the boom in
any direction he wanted..and it stayed put. It wont weather vane. Remove
the tq comp plate..and all hell breaks loose.
It will weather vane. It also destroyed 2 of his ham-4 rotors in the
process....( b4 the tq comp plate was added), the previous 2 winters. In
actual usage you also have to factor in the coax + balun heading towards the
REF end of the
boom..terminating on the DE. Running the coax down the side of the boom will
minimize the windload on the boom. We factored all that in in the latest
version of the tq comp plate.
## Done right, the end result is zero tq..with boom broadside to the wind.
With boom pointed into the wind, you will still have a slight tq
imbalance..due to the fact that the boom is mounted to the SIDE of the mast.
A 2 inch OD
boom mounted to a 2 inch OD mast is not an issue. A 3 inch od boom, mounted
to a 3 inch OD mast creates slightly more imbalance. If more than one yagi
on the same mast, then alternate sides of the mast = almost near zero tq.
## Of course you require enough rotor tq to overcome the inertia from the mass
/weight of the heavy yagi. No different from my buddy turning his yagi on the
30 ft temp tower..by hand.
## If the yagi is not tq balanced, then you require a huge amount of tq to
get it to turn..esp in high winds. Not tq compensating the array is like
using more HP on a car....since you have deployed a chute.
## if you think your tq imbalance is bad at 70 mph, its worse at 100 mph.
The issue is easily fixed. Dunno why ant makers don’t tq balance their
products, its very simple.
Jim VE7RF
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