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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