[TowerTalk] Wind Antenna Damage
jim.thom at telus.net
Mon Apr 21 06:41:21 EDT 2014
Date: Sun, 20 Apr 2014 11:53:09 -0700
From: Dan Hearn <n5ardxcc at gmail.com>
To: towertalk reflector <TowerTalk at contesting.com>
Subject: [TowerTalk] Wind Antenna Damage
## what do u mean by lost ? Did the yagi fall off the tower ? Did you trash the rotor ?
Min windload on HF yagis occurs with the boom broadside to the wind. On VHF yagis..it usually
the other way around, with eles having less windload than the boom . Spacing between
elements doesn’t even enter into the equation. IF you have an equal amount of boom on either side
of the mast...it is now tq balanced. Even with just the REF installed...and no other elements.
## I used kurt andress K7NV s yagi stress software to design a plate for a buddys 20m yagi.
Plate is installed just in front of the 20m REF..which is the short end of the boom. OK, now the
boom is tq compensated. End of problems. B4 that, it would trash the brake in several T2X rotors !
## we tested the plate concept by installing the 5 el 20m yagi on a short 30 foot tall tower. No rotor
used at all..and just 2 thrust bearings. I could climb up in a 35 mph wind..and turn the yagi by hand....
and point it in any direction I wanted....and it stayed put ! Remove the TQ comp plate..and all hell breaks
loose...and it will weather vane every time.
## Plan B is mount the boom at the exact center of the boom... and then use a counterweight on the light end
of the boom. Yagi stress would also design the counterweight for you. Both methods work.
## method A adds more total windload to the boom. Method B adds more total weight to the yagi.
## Imagine a boom that is 40 feet long....with mast mounted at the 19 ft point. You now have a boom that is just
one foot less than 20 ft at on end.....and one more foot at the other end...21 ft of boom. However you now have a
TWO foot difference between the boom halves.
## Mount the same boom at say the 17 ft point... then its 17 ft on one side....and 23 ft on the other side.
One end is now SIX foot longer than the other. Short end needs to be compensated with a plate to make it
## Dunno why yagi makers don’t grasp the simple concept of tq balancing. IF more than one yagi on the mast,
u alternate sides of the mast. Once the ants are ALL TQ balanced..... its easier to turn the ants..and also hold em in a
wind. f12 used to publish TQ specs for their yagis...in a 70 mph wind. Watch out..... in a 80-90-100 mph wind, the TQ
goes through the roof. In a 100 mph wind... the TQ is double that of a 70 mph wind. Use either method A or B to
do the TQ compensation..and you can reduce the TQ to virtually nothing. Or use a combo of methods A + B.
## You also have to factor stuff in like the coax going out the booom..and whether it is on the side of the boom..or
on top or below the boom. Use LMR-600 on top of a boom....or below a 2 inch boom...and now the wind thinks that
portion of the boom is 2.6 inchs in diameter. Mount it along the side of the boom... and the wind thinks its just 2 inch OD.
### U also have to factor in the balun.
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
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