Here is what Carlson says about their towers based off the GM designs.
This scares the hell out of me.
The model IO-810 roof tower was originally based on the Glen Martin
RT-832 roof tower. I have placed a rating of 10 square feet at 80 mph
on it, slightly less than what the RT-832 has. This was done because of
the many unknowns. The major unknown is what the weakest point of the
structure is. I estimate that the roof itself may be the weakest point,
as there are many variables, including bolt size, deck thickness,
shingle composition and thickness, and what was used for back-up under
the deck.
"I have placed a rating".......?????? I would hope that he actually
calculated this. I agree that there are far too many unknowns on each
installation.
A TH3MK2 if that is what you have is a bit over 3 sq ft. The 6m beam is
just over 2 sq. Probably ok on the Wind load.
Can the top of the tower take more than 3 ft of mast out of the top?
You are doubling that spec. I would assume the tribander is going at
the bottom. The 6m on top and the 2m in between. The taller mast will
act like a much bigger lever. toppling force applied to the tower then
to the roof.
I agree with Tom that it probably all depends on how your tower was
mounted to the roof. Was it simply lagged in, was it through bolted in
with what I would call spreaders that span multiple joists to carry the
load or some other method.
I would not even bother with a trapped tribander today. They are not
very good. A spider beam or Hexbeam would work and give you more
bands. Wind load about 5 sq ft. You are probably going to have a bit
of interaction with the 6m antenna if you use something with 17m on it.
How much is unknown and maybe acceptable. You could put the Hex on top
with 3 ft of mast and the 6m on the bottom. It still comes down to how
the tower was installed on your roof and how confident you are in that
installation.
Maybe there are alternatives like putting up another tower that you can
do reasonably that would not possibly jeopardize your 20k investment.
I am not an engineer. People have gotten away with all sorts of things
and many have payed the price for over burdening there antennas towers.
Mike W0MU
On 4/6/2025 6:22 AM, Tom Hellem wrote:
Dave-
One piece of info that figures heavily into this equation-
What are you using for fasteners to attach the legs of
the tower to the roof rafters, how many, and if they are
some type of lag screws, are you sure they are all into
solid wood, and not, for example, screwed into the joint(s)
between the double and triple members?
Tom
K0SN
On Sun, Apr 6, 2025 at 1:31 AM Dave WB2PJH<wb2pjh@gmail.com> wrote:
Hi Jon,
There is a lot of expertise in this group but if you wouldn't mind looking
over my plans and letting me know if I'm FUBAR or OK, I'd sure appreciate
it.
My plan is to the following: I have an existing GM RT-832 roof tower with a
G800DXA rotator and Yaesu GS-065 thrust bearing. It's currently holding a 5
el 6 m and 70 cm yagi on a 10 ft length of 2" aluminum conduit. The roof
joists are doubled or tripled where the mounting feet are located. I just
replaced my roof and don't want to trash it by doing anything stupid like
overloading this light duty roof tower and taking my $20K roof out.
I bought a 2 inch 12 ft 15 lb 6061 mast and want to install a Hy-Gain 2 el
trap TB, 20 lbs, 3 sf, 14' radius, an M2 6m 5 el yagi, 2.2sf 13 lbs, 11 ft
radius, and, if possible my 2 meter M2 9 el, 1.2 sf, 6 lbs 8 ft radius on
top (very optional). The 70 cm is coming down and will be sold.
The K factor in the Yaesu manual is for this rotator is 180. If I only put
the 2 el HF yagi and 6 meter on the mast the calculated K factor is 231, 51
higher than recommended by Yaesu. If I add the 2m then I'm over by 77. The
Yaesu manual equation doesn't count wind resistance, just antenna and mast
weight and turning radius. The thrust bearing takes most of the weight of
the mast and antennas but there's nothing in the manual to account for
that.
I'm OK on weight for the GM roof tower but the manual calls for no longer
than a 9 ft mast, and the rotator plate is 6 ft from the top, not much room
for antennas! I think I'll be OK with 12 ft even if I add the little 2
meter yagi at the top, but the tribander and 6 meter both have to go on the
mast and the 6 meter won't be anywhere near the top since it's a big
antenna.
I got a 12 ft mast because that's what my friend had in his garage - if he
had 9 ft or 11 ft it wouldn't make much difference to me since I won't have
enough room for optimal spacing in any case..
In your collective experience, am I cutting things too close for comfort?
I have a TA-33 but I think it's too much antenna for this roof tower and
rotator with a K of 262 which is why I'm going with the 2 el Hy-Gain,
lightweight with low wind resistance.
Most of this project except for the roof tower and meter M2 yagi are used
items from from fellow hams who are trustworthy.
Thanks for your assistance.
73,
Dave WB2PJH
FRC, CWOPS
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