Mac, this can be analyzed with one of the mast calculators. Assuming a
robust bracket 12ft above the ground level and the antenna about 5 sq ft,
mounted at 35ft above ground, that leaves 23 ft unsupported.
With 6061T6 aluminum, 2" OD. .25" wall, the mast should survive 55 mph
winds. Lowering the antenna to 30ft, it should survive 64 mph winds.
This is a very conservative result based on the old TIA222 rev F method of
calculating wind loads. Using rev G method would show much higher wind
survival, on the order of a 15 mph increase. This is for exposure B, meaning
there are trees and houses around to help mitigate the winds.
You are in a 70mph rev F, 90 mph rev G county. What you are proposing would
be considered border line so you will have to consider the risks involved.
If you could use a larger diameter for the lower part of the mast the wind
survival could be increased substantially. For example, using 2.5" diameter
and .25" wall, the wind survival increases from 55 to 70 mph, 3" OD
increases to 84 mph. These numbers are for a height of 35ft.
Sounds like an interesting project.
73,
Gerald K5GW
GM Texas Towers
.
In a message dated 9/14/2016 6:59:19 P.M. Central Daylight Time,
wt4bt@blomand.net writes:
My aim: Vertical support for a Hexbeam at about 30-35 feet.
Space for guys is severely limited by property lines, trees and other
structures
I cannot use a lattice type tower for several reasons.
Ground here is clay over ledge/bedrock and only 18 - 24 inches deep so
anchors may be marginal without rock drilling.
I am in a 70 mph wind zone (90 mph @ 30 sec. gust zone) here on the
Cumberland Plateau in So. Central TN.
What I am thinking of:
Mast system like the Penninger Radio “Tipper” 35 foot using 2” x 0.25”
(rather than the stock 0.125") wall tubing (8’,10’ or 12’ sections) which
would be bracketed to the shack building (separate from the main house) at
about the 10-12 foot level.
Yaesu G-450, or similar, rotator at top of mast turning a Hexbeam with
maybe a 6 meter short-boom yagi (3-5 elements).
There would be 1 or 2 pulleys below the rotator for inverted V or L
antennas and “experimental wires” as the mood strikes.
Base of mast would be in concrete (TC-24 concrete base mount) but the
amount poured might be sub-optimal (can go somewhat wide but not deep).
As noted above, guying will be a compromise situation.
Due to roof overhang on main house bracketing to it, or trying to mount a
shorter mast off the face of a dormer is not a strong option. Roof mount a
la Glenn Martin is possible but the roof pitch is greater than 12 in 12 so
getting up there to do any work is not an option for me.
Questions:
Can I get away with the top 25 or so feet of mast being unsupported
(unguyed)?
Are there any other suggestions or thoughts for doing what I want?
Different mast or tubing suppliers?
Mac / WT4BT
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