[TowerTalk] New Yagis on Existing GM Roof Tower

Sun Apr 6 12:23:50 EDT 2025

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 at 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
>>
>> <
>> https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail
>> Virus-free.www.avast.com
>> <
>> https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail
>> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk