Recently I lost a 6l 20 yagi in a windstorm when my power went off and I
could not rotate it into the wind.
Some time ago I wrote a short note to post on our www.sdxa.org web page
under articals which explains how to minimize this problem. Here it is.
Parking Your Yagi – by Dan, N5AR <http://www.sdxa.org/?p=16>
Does it make a difference which way your yagi is pointed during a
windstorm? Of course it does. There are different opinions on this subject.
Here is mine.
You must first decide what you are trying to protect, your tower, your
yagi, or your rotator. Minimum tower wind loading occurs when the yagi
presents the least area to the wind. This will usually be at some angle
other than boom end or element end into the wind. Several articles have
been written about how to calculate the optimum angle using trigonometry.
Yagis seldom have equal spacing between the elements and the directors are
always shorter than the reflector so a yagi oriented as above will see
quite a bit of element and boom flexing as well as intermittent torque
loading on the rotator.
Yagis are typically clamped to the mast at their center of gravity (balance
point). Since the directors weigh less than the reflector, this is not the
center point of the boom. If the boom is positioned broadside to the wind,
the yagi will swing left and right as the wind varies, causing variable
wind loading forces on the rotator gears and brake if it has one. This can
be taken care of by adding additional wind loading on the short side of the
boom. This can be a plate or extra boom material. This technique has been
the subject of a paper by Dick Weber, K5IU.
I prefer to point the end of the boom into the wind. The elements are the
same length left and right so there is no variable torque on the rotator
and the boom during wind surges. The majority of the wind loading is then
on the elements. The wind here is usually from the southwest so I leave my
beams pointed that direction unless unusual conditions prevail. If someone
has a better solution, I would like to hear about it.
73, Dan, N5AR
--
Dan Hearn
N5AR
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