C-Band project

[pjcl]

After much thought and planning, I finally moved ahead with the project of "planting" a 8-footer. It has to be on the roof due to LOS reasons on the property.

My immediate question now is: what is the best way to ground the dish? The same way I grounded my 90cm KU-Band dish? Any special requirements?

I'm attaching some pictures of the pipe installation.

 

[FaT Air]

Just some thoughts- I don't think that plumbers tape is going to hold up against the wind load exerted by the dish. I'd box it(the pole) in with doubled up material at least equal to the rafters. Run it on both sides of the pole to the next rafter. Then put a bolt through the pole and the box.. Then spread the load to more of the roof by running ties across the bottoms of the rafters, and maybe at an angle down to the sil.
The base was held by a pipe, that the pole slid into, that had a flat base welded to it, (sleeve) that was bolted to the floor. About a foot tall with 3 or 4 bolts through it and the pole. (There's going to be some terrific torque on that pole in high winds)
Ground it (10 gauge copper?) with no sharp bends.
That's what I remember of how one up here was done.
Disclaimer: I am not a structural engineer, so all advice in this should be run by one. Huh Moment, are there any building codes, in your area, that may apply?

 

[cracklincrotch]

You should definitely look at this before putting a dish on your roof with your setup regarding the wind load you should expect. I think you might be under-built here.

Satellite Footprints by Dish Size - Ground Pole Installation, Pad Mount Installation, Wind Loading

 

[turbotoy]

I'm a structural engineer, and by inspection can tell you that your mount is far from satisfactory. Furthermore, the chart in the link above, which seems to provide moment as a function of dish size and wind speed, makes no sense without supplementary information.

I quickly searched and did find this thread that discusses wind loading on a 1.8m Andrews dish with values that actually make sense. I would try to find something similar for your or an equivalent dish.

Satellite Antenna Wind Load Test Data - Satcoms UK - Global VSAT & Satellite Community

Absent better data for your dish, one could scale those forces/moments by the ratio of dish area as a reasonable approximation. Make a free body diagram of the pole and apply the corrected dish load at the top of the pole to determine reaction forces at the rafter and at the base of the pole. This would allow you to size your pole mounts correctly.

If we use the ~1800 lbf horizontal force for the 1.8m dish, the load at the rafter is going to be substantially greater than 1800 lbf (depending on pole support locations, likely 2-3 times greater). The rafter, by itself doing its intended job of supporting only the roof, is not designed or capable of taking ANY horizontal load.

I can help you figure out loads, but I can't/won't help you determine a satisfactory mounting system due to liability concerns. If I were to design a support system that did not extend the pole to the ground, it would need to be much more elaborate and many more structural aspects of the home would need to be understood and properly analyzed. Also understand that a structure that simply "works" under static loading would be much less robust that one designed for worst case loading, which an engineer must consider.

 

[fastscirocco]

If you mount a dish on that you are going to have problems.
I strongly recommend you either purchase or build (have built) a non penetrating peak mount like the ones available here: Baird Mounts

 

[turbotoy]

Note that the Baird UR10, at $1,458, would be just undersized for an 8' dish and they would likely recommend the UR10D at $2,030. The UR10D weighs ~840 lbs, and I would want to run some calculations to verify that the roof could support that increase in deadload. This probably gives you a better idea of the magnitude of the forces at work here, and what is really required to support them using only the house structure.

I'm not trying to be discouraging, just trying to prevent potentially serious home damage before it happens.

Setting a long pole in the ground adjacent to the house is likely a much more economical and practical approach to this problem.