[TowerTalk] TowerTalk antenna TOWER - Homebrew

Fri Feb 28 09:51:18 EST 2014

When mounting a tower on top of your roof you need to consider the down 
thrust on the tower base  and what part of your structure holds up 
against that thrust.  Guy tension increases these down forces. Wind can 
greatly increase these down forces, even to the point of structural 
failure of the roof structure because sideways wind forces translate 
into down forces via the guys.

Of course the tower base should be placed right above good strong roof 
structure but note that the roof was not designed to hold up against the 
force of the mast.  There are simple modifications/additions that will 
greatly strengthen the roof in the area of the mast's down force. A 
thrust member, a good solid timber or metal piece (tube or rectangular 
metal) can be mounted vertically from the point under the roof where the 
mast is positioned on downward to something capable of taking the load.

OK, what can "take the load?" a timber or stiff metal reinforcement 
positioned on and at right angles to the ceiling joists can distribute 
the down force of the mast over several joists.  Ideally you will mount 
the mast over or very near to a load bearing wall in the building and a 
timber, tube, or other brace from the top of the wall to the point below 
the mast will pass the down force to the wall.

Personally, for vertical timbers I prefer at least a good straight 
grained 4x4 or a metal tube that is stronger than the mast.  For a 
horizontal item to place at right angles across ceiling jopists I prefer 
what are called "TJI" (trus joist,  maker's spelling, not mine) These 
are very strong and stiff for their weight.  Essentially a TJI is a 
wooden "I" beam. Think of two 2x3 boards with grooves down their lengths 
attached to say a 12 inch wide length of OSB (Oriented Strand Board, 
cheap substitute for plywood, NOT PARTICAL BOARD!) These TJI are used as 
"I" beams and when placed on top of ceiling joists at right angles to 
the joists the load placed on the TJI is distributed to the underlying 
joists.  The TJI being quite stiff can span several joists.  They can be 
shimmed to account for their flex under load placing thicker and thicker 
shims as you progress out from the center where the mast down load is.

I am not a structural engineer nor an architect by degree or 
certification so consider the above as food for thought not detailed 
instructions for any specific undertaking.  Offer void where prohibited 
by law, your mileage may very, no refund without a valid receipt, no 
tickee no laundry, I didn't stay in a motel, I didn't play anyone on 
TV.  In fact I was never there and I have witnesses to that...

Patrick NJ5G

.
On 2/28/2014 12:31 AM, Roger (K8RI) on TT wrote:
> On 2/27/2014 12:50 PM, Wilson wrote:
>
> I hope this is easily translate and hope it is useful.
>
> Wilson has asked the important, basic questions.
>
> These are some additional thoughts and a question:
>
> I have to ask if the 20 meter height above ground will be mounted on 
> the roof, or ground.  On a 5 to 7 meter high flat roof we'd be looking 
> at a 13 to 15 meter tower (roughly 40 feet). "Depending" on the roof 
> structure and antenna size, it might be a lot for the roof to support 
> when the wind is strong.  Even a 10 to 12sq ft (About 3.6 sq meters) 
> is a lot of leverage when applied to a 15 meter lever. Concrete  and 
> heavy timber roofs should be able to handle it.
>
> A 20 meter ground mounted tower might be simpler from a structural 
> approach, but getting it up could be difficult. At present there are 
> still too many unknowns to be able to recommend one approach over another
>
> The tower in the following link is well beyond the capabilities most 
> of us have.
> http://www.rogerhalstead.com/ham_files/skyhook.htm It is 60 meters, 
> self supporting and is entirely home brew, but it was engineered by 
> professional and licensed engineers. It was completely assembled by 
> two hams. Note the one ham working at a 100 or 120 feet (30 to 37 
> meters) and the gin pole they used. This story was in QST
>
> 20 meters (65 feet for those in the US) is not out of the question, 
> but I need to emphasize it can be very dangerous work. I don't know 
> your background or ability for climbing and working at heights, access 
> to safety equipment, or help.
>
> With the steel available on the market and a safety factor for home 
> built it will take the equivalent of 4 sections and would need to be 
> built in the shape of a windmill tower. Triangular, not square.
>
> It would need to be built in place unless you have access to heavy 
> equipment to lift it up and stand it in place.
>
> Using steel angle, which could possibly be purchased with a galvanized 
> coating could be done with little or no welding. Bolting the sections 
> together would require care to make sure the joints are strong 
> enough.  Because of the likely wind load it would require much 
> stronger bracing than a typical wind mill tower and a reasonable 
> overlap of vertical sections.
>
> Wood is normally easier to work with and easier to get, but does 
> deteriorate much faster than steel with age. The wood available here 
> is no longer of high quality and is not well cured/dried. 2 X 4s tend 
> to twist, or warp. I lay them out on supports so air can get to all 
> sides. I rotate them (top to bottom)twice a day for the first week and 
> once a day after that in a heated (10 to 15C) shop with a 
> dehumidifier. Higher temperatures may cause them to dry too fast. Out 
> doors in summer They need to be protected from rain. The sections and 
> pieces should be bolted together, rather than nailed although I'm sure 
> opinions will vary on that.  Use large washers on the bolts.
>
> I tried clamping new 2 X 4s to a steel frame (without rotating) while 
> they dried, but in less than 4 days they twisted enough to break with 
> enough force to throw pieces a meter long as much as 3 to 5 meters. 
> When it broke, it was loud enough to scare me
>
> 73
>
> Roger (K8RI
>
>
>> Hi Vikas,
>> Others will chime in, but I think 20m freestanding is more than you 
>> could likely get built/installed safely.
>> So please tell us the following:
>>
>> Do you have a flat roof, or peaked?
>>
>> Roof material?
>>
>> What services are available locally, welding, woodwork?
>>
>> What materials are available locally, steel, aluminum, wood?
>> Many towers have been built of wood, which is available nearly 
>> everywhere.
>>
>> Do you plan to do the building, or hire it done?
>>
>> A modest installation will be many times easier than an ambitious one.
>> With 3-5m, on a rooftop, a three element Yagi or two element quad 
>> will help you greatly, with reasonable effort and danger.
>> It's reasonable to use a short tower on the roof and let the mast go 
>> through the roof and be rotated by hand, if you have trouble finding 
>> a rotor.
>>
>> Wilson
>> W4BOH
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>>
>>
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