[TowerTalk] field-day mast anchoring question

[jimlux]

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 at verizon.net> To:
> towertalk at 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 at 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
>