roof mount antenna tripod install questions
im about to install a antenna tripod to my roof. How do i align the feet of the tripod with the rafters? If i cannot what is the best way to add support structure underneath? ? ?
Forget the tripod not worth the money for any type of beam except a small 6M beam or a vertical. Read the current posts on QRZ this week on the Glen Martin roof mount tower. You can not practically align the feet of a tripod with the rafters and you don't want to. You need to re-enforce the rafters underneath the roof for a mini- or full size beam.
Glenn - K4ZOT
well i just have a 2/440 omni , 450-470 7el yagi
Yes, your good with 440-470 antennas. I was referring to mainly HF type or large 6M beams.
A couple of ways to find the rafters, although unless you have a very smart tripod, probably all three legs won't land on them. You might get two to do so.
One is to go into the attic and site the rafters you think you'd like to use. Drill a small diameter hole using a long bit to go all the way through the center of one until it goes through the roofing, the stick a wire coat hanger or stiff piece of wire through that small hole. Go up to the roof and you'll see it sticking out and know exactly where the center of that rafter is.
Another way is to use a stud finder from the roof side: This may not work because roofing can be too thick to get the desired results, but it sometimes works.
Another way is to use a small hammer to bang on the roof and move around until you hear a difference in the sound. The roof will sound more hollow when you're not over a rafter, and more solid with a different pitch when you're over a rafter. Then use a small diameter drill bit to explore with a few test holes until you find the center of the rafter, and patch the holes that aren't used.
I actually use that last approach most of the time. It's fast, it's easy, it works.
As has been said, it's not going to be easy to find a roof joist. Since you don't have a lot of wind load with those VHF-UHF antennas, you shouldn't need the extra strength. You should be able to just pass machine bolts thru the roof & sheathing, and use large OD "fender" washers & nuts on the underside of the roof. Use either nylon insert (Ny-Lock) nuts or doubled ordinary nuts.
If you feel you need a stronger mounting, you can drill holes through the roof, locate them on the underside of the roof, and install 2X blocks between the roof joists under the drilled holes. You can then secure the tripod to the blocks using lag screws, or machine bolts through w/ washers & nuts. In any case, all hardware should be either hot-dip galvanized, stainless steel, or electrogalvanized steel. Seal the tripod legs to the roof w/ roof sealer.