Hi Tom,
I have followed this discussion for a little while and there are several things
that make me worried. I went through some mistakes myself erecting my first
"antenna support".
Aircraft wires: No, no! I used them in my first setup. Flexible wire and cheap
but they corroded very fast for me. Use galvanized utility wires instead. I did
and have found no corrosion after several years up.
No Z support between the legs: OK, it might work but the diagonal support
transfer the bending force from one leg to the other, giving a lot more rigid
structure. I have seen small towers with only perpendicular cross brassing but
then the brassing consists of wide plates, not round tubes or rods.
Don't guy self supporting towers: This is a misnomer. You can add extra
strength to a self supporting tower but the problem is the crank-up tower as
the guy wires will add force to the winch cable(-s), something they are not
designed for. If you have a fixed, self supporting tower you can add guy wires
to add to the strength.
Tower support: I, myself, would not use my house for support. I would be very
affraid the tower would rip my house apart. I have seen towers attached to
houses at the edge of the roof. I also seen guy anchors screwed into the roof
and I have asked myself; "What would happen when the tower is exposed to a near
destructive wind. What would give in first? The support point or the house?" I
would, possibly, anchor a short tower (~10 feet) to a roof but not anything
large.
I have a 85 feet, rigid, self supporting tower in my back yard. It can
withstand 90 mph wind. When I added guy wires it can now support 135 mph wind.
The guys are attached about 60 feet up. They were placed where they reduced the
bending forces the best. That point is not at the top nor half way up.
That's my penny thoughts. My tower has been up for 7 years now. Nothing much
has happend to it. The eye of hurricane Irene went through here and a lot ot
trees came down. My tower is still standing.
73 de,
Hans - N2JFS
-----Original Message-----
From: Tom Hellem <tom.hellem@gmail.com>
To: towertalk <towertalk@contesting.com>
Sent: Mon, Oct 14, 2019 9:13 pm
Subject: [TowerTalk] Tower Guy Calculations
Thanks to all who responded and for the link to K7NV's excellent work.
What prompted my original question was a visit to the home of a member of
our local ham population and upon learning of his proposed installation. A
description follows:
The tower appears to be one which was originally designed and sold as a
self supporting approx. 55' tower, consisting of two telescoping sections
that nest down into a fixed base section, all of steel.There is no "z"
bracing like what is found on a Rohn type tower, only horizontal slats
every 14" or so. Raising and lowering is accomplished with a "boat winch"
type device, a few pulleys and what looks to be 1/8" aircraft cable. I do
not know who the manufacturer was. The op is using this tower in a guyed
configuration, with guys (1/8" aircraft cables) attached near the tops of
the 2 movable sections and the base section fixed near its top end to the
roof overhang of the house with a flimsy piece of 1-1/2" aluminum angle. 4
of the guys are anchored to 1/2" screw eyes that are threaded into various
framing members (of unknown configuration) of the house's roof structure.
The screw eyes are not the forged variety where the eye portion is
continuous, but the Home Depot variety where the eye portion is simply bent
into a loop. He is proposing to mount a rather large multi-monoband beam,
similar to C31-XR, onto a 2" mast about 4' above the top of the tower.
There are 13 elements on a 30' long, 3" diameter boom.
After looking at K7NV's info it seems safe to predict that the guy loads
might well approach 3000 lbs or even more on the top section. I am very
concerned with both the ability of the screw eyes to remain closed and with
their resistance to being withdrawn from the wood that they are screwed
into. I am also concerned with the strength of the cable that is used to
crank the tower sections up and down, given that is what holds the entire
assembly in the fully extended position and the fact that there is
considerable downward pressure on it when the wind is 60 degrees off the
line of a guy.
Does anyone else share my apprehension about this setup or am I
overthinking it? I'm trying not to sound like an old nag with this fellow
and I think he's getting tired of me preaching at him, but I sure don't
want to see his new beam laying in a crumpled heap on top of his roof, or
worse.
Input and suggestions will be appreciated.
Tom
K0SN
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