In a message dated 7/3/05 3:22:21 AM Greenwich Standard Time, ne0p@lcisp.com
writes:
I am in the planning stages for getting a tower up in the next couple of
years (hopefully) and am hoping to get some feedback from the group
concerning
this. Here is my situation: I don't have much room for guy wires in my
yard,
nor a desire to have guys running all over my yard. I wonder if I could go
with
Rohn 25 or 45 unguyed, as I could use house brackets on 2 sides to help
support it. I would like to have 40 feet or so of tower, with a 2 element
tribander (16 pounds), a cushcraft 13B2 (5 pounds), and a Cushcraft 719B (3
pounds)
mounted on the mast-whatever that weights. Is this a reasonable setup, or
would
Rohn 25 or 45 have to be guyed, even with the house brackets?
-0-
K4XS correctly observed that house bracketed might be ok, but 'self
supporting'
would not.
I replied directly to ne0p, but will add comments here, for the general
good:
I had 50' of 45G bracketed to my house in VT @ 6' and 16'. It had 10' of
mast
coming out the top, and an lpda and an r7000 mounted on it. The rotor was
10'
below the top. The lpda was at 55', and the r7k 5' above that. It was up
for
several years, and withstood several ice storms and 75kt+ winds.
THAT said, I made my own house brackets from wood, rather than use the Rohn.
The reason was both standoff distance, flexibility (or wiggle), and the fact
that Rohn's brackets had too small an attachment to the house.
My brackets spanned 3 roof joist and 3 floor joist bays, (48") and were
calculated
to withstand 100mph windloads with 20% dynamic reserve. Details provided
privately, if anyone's interested, but suffice it to say that you absolutely
must pay attention to windload calculations, and the strength of the
fasteners
to the house.
Thrubolts to backing plates are critical. If lag bolts must be used,
properly
sized bolts and pilot holes are critical to strength, spread over enough of
the
structure.
If I remember correctly, Rohn specifies 25g as withstanding 2sq ft @ 70mph,
30' above a house bracket. This is too light for even a small HF beam.
I don't recall the 45G spec, but it's viable for the application, with
proper care.
What you WON'T get with that system, however, is an engineer's approval.
So, if your building permit requires an engineer's seal, or if you don't
have clear fall-radius to nearby structures, you will have a problem.
I was a scofflaw...simply put it up, after careful calculation, knowing
that the worst thing I could do was some damage to my own home. 5 years
after having had it up, I applied for a permit for a 100' tower, located
150' from my home. Got it with no problem, although the code enforcement
officer DID say...geee, was that a tower I saw on the side of your house?
n2ea
jimjarvis@ieee.org
_______________________________________________
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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