|To:||"Mark - AA6DX" <firstname.lastname@example.org>, <TowerTalk@contesting.com>|
|Subject:||Re: [TowerTalk] review of calculations requested|
|From:||Jim Lux <email@example.com>|
|Date:||Fri, 20 Aug 2004 16:52:40 -0700|
At 04:04 PM 8/20/2004 -0700, Mark - AA6DX wrote:
Jim .. I like your idear for temp portable .. butcha gotta remember a 3000 lb vehicle does not have 3000 lbs on the rear wheels .. so, I propose this .. think about it ......
Put a .. say 1Ø foot .. "pull up" pole, mounted 90 deg from the vertical mast, near the base. This would enable the mast to be raised at full height, or at least the upper sections erect... so easier to raise... you THEN hoist it up using this device. Conveniently, you have a very simple apparatus which the front wheel(s) have driven upon, with a clamp of some sort that will hold this lever DOWN .. .. and become part of the overall stabilization! You also gain the effect of having the front wheel weight as part of the system! You could hold this hoister in a semi-erect position with a lightweight chain, or rope, to facilitate installing the antenna ... vert, yagi, quad, whatever .. HEY! Are we partners, or what! I had a patent attorney years ago, bet he would help us! (W6OAT) .. 73, Mark .. AA6DX
I've contemplated a number of schemes where you have plates under each wheel that anchor to various rigid struts. You need some adjustability to allow for uneven ground. When it comes down to analysis, it looks like, any scheme where the ballast is a 3000 pound car is going to be limited to lower heights and/or smaller antennas and lower wind speeds.
It's that 15,000-20,000 foot pound overturning moment that you need to resist. If you're limited to 3000 pounds, and you want to keep the load on the car to less than a third of that, say, 1000 pounds (keeping things reasonable), then you need to make the lever arm from mass to pivot point at least 15 feet long. A 6 foot wide, 12 foot long car just isn't going to give you that kind of base (sideways overturning is 9000 lb ft, and I wouldn't want to get even close to that). There's also a very significant problem (in the catastrophic case) in that the CG of the car is a fair ways above the ground, so as it starts to tip, the restoring force gets less (as folks driving Ford Explorers with soft tires and making fast swerves have found). A 35 foot long, 8 foot wide RV weighing in at 8000 pounds is another story. One also has to consider that the wind is also trying to tip the car over (which is something I entirely neglected in my earlier analysis, but should be considered... the cross sectional area of a car is many square feet, and the Cd from directions other than the front is fairly high, fortunately, the lever arm is quite short (a few feet))
One could throw out very wide outriggers, with the mass of the car in the center, but it starts to get complex.
Time to revisit the original requirements... is 50 feet really needed? Is 4" in diameter really what you need? Is 10 square feet really needed?
Off hand, I think that 40-50 feet would be nice (clears the trees and 2 story houses)
4" is probably unecessarily large in diameter. Flexing under a gust wind load and moving several feet at the top is inconvenient, but if it doesn't fail, that's ok. Cutting the mast diameter to 2" cuts the loads by 30%-40% or so.
A SteppIR 3 element Yagi is 6 sq ft, a 2 el SteppIR is 4 sq ft.
So... now you're looking at half or a third of the original speculative load. Limit yourself to 50-60 mi/hr instead of 80 and it gets even better. I'd be pretty nervous about designing for 30 mi/hr. That's not an unusual gust in Santa Ana season (don't forget, you're 50 feet up, and the wind is stronger up there), and if you were operating on a hilltop where there would be some natural speed up.... (The fastest mile wind speed charts I've seen, though, specify the wind at a 10m (33ft) height).
However, some thought about a design that deliberately flexes in a way that reduces the wind load (like a sailboat heeling) might be useful. You don't need to operate 100% of the time, you just don't want that sudden downburst or gust to tip the car over. There's a big difference between continuous use, repetitive momentary deformation, and catastrophic failure.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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