On 1/22/16 12:25 PM, David Gallatin via TowerTalk wrote:
It would appear he is in Vermont and maybe intends the same thing,
just with a yagi. I'm going to say this is not a good idea. You're
going to have about 50-55 lbs at the top of a 50 foot long moment
arm, giving 1200-1300 lbs effective moment and only a few hundred at
the bottom. Not good enough. If you truly feel the ground is too soft
to use guys then you might try looking up the term "drive on antenna
base" and hamgeneering together something like that. 73, David, AA9G
ex W5DCG and KC9EEV
The "drive on base" really doesn't support much of a bending moment. It
will keep the base from moving laterally (skidding), but you'd need guys.
I ran into this when designing a "drive on" base that clamped to the
roof rack as well, and found that you'd probably rip the rack right off
the car.
http://home.earthlink.net/~w6rmk/antenna/mastcar.htm
Figure it this way, if you have a 50 lb force at the top of a 50 foot
tower, that's 2500 lbft. If the base is 2 feet wide, it will push up
with 1250 lb. That will lift the wheel right off the ground.
On the other hand, if you have rope guys out at 45 degrees, the 50 lb
horizontal force turns into a 70 lb tension and a 50 lb down force on
the mast/tower.
At the ground, the 70 lb tension in the guy is a lifting force of 50lb
and a skidding force of 50lb. If you have a 200lb block of something
sitting there, you're all set. The down force of the block on the ground
is 150 lb (200lb - the 50lb lift) and if the coefficient of friction is
0.3, there's 50lb to resist the sliding force.
it's worth thinking about the general magnitude of these forces, so that
if someone proposes "fill a couple 5 gallon jerry cans for a guy anchor"
you can say, Uh, I don't think so.
A 3" mast, 50 feet tall, in a 50 mi/hr wind, has a moment of about 2000
ft lb (the wind load is 80 lb, and arm is 25 ft).
That's for just the mast. Something lightweight like a 20m spiderbeam is
about 4 square feet, and a 50 mi/hr wind is just over 6lb/sq ft, so you
could add another 25*50 = 1250 lb ft of moment.
I used 50 mi/hr here.. that's a pretty low speed, but for a field
installation, using rope guys, etc.; you're probably willing to have it
fall over, because you can put it back up.
if you want to go to 70 mi/hr, double all the forces.
A car could make a good guy anchor, if there's a secure place to attach
the guy.
From: Larry Banks <larryb.w1dyj@verizon.net> To:
towertalk@contesting.com Sent: Friday, January 22, 2016 11:05 AM
Subject: Re: [TowerTalk] field-day mast anchoring question
We (W1HP) put up a "two-el" inverted-V. Only takes two fiberglass
poles for the centers and no guys -- the wire acts as the side guys
and the poles are strong enough to not need guys in the fore/aft
direction. If you are not going to rotate it, why go to the trouble
of a 2-el yagi? (From the Boston area we just point WSW.)
73 -- Larry -- W1DYJ
-----Original Message----- From: dw Sent: Friday, January 22, 2016
10:31 To: towertalk@contesting.com Subject: [TowerTalk] field-day
mast anchoring question
For those familiar with the AB-621 military launcher mast units. Lets
say there was an interest in using this on a field day event, where
there is a concern that the ground is too soft to safely establish
anchors. Is there a possibility of using concrete wall-barrier blocks
bolted together, as dead-weights to safely hold the anchors? For
example, I think I can come up with a number of 60lbs cement
wall-barrier slabs that can be bolted together for each anchor
point. If this would be a viable option, how much weight on the
anchor blocks would guarantee a safe installation? The intent is to
put up a 40 meter 2 element yagi, without a rotor, at a height of 50
feet.
Thanks in advance. N1BBR
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