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eHam Forums => Elmers => Topic started by: KG4WXP on September 24, 2012, 05:47:00 AM



Title: Roof mount: 3 ft or 5 ft tripod???
Post by: KG4WXP on September 24, 2012, 05:47:00 AM
Out of curiosity, if I was going to use it for... say... a end mounted 3 element 2 meter yagi (Fixed to the north - vertical for fm, no rotor needed), and a 10 meter vertical antenna (maco 5/8 wave tuned to 10 meters), would a 3 foot tall tripod work okay? Or would I need a 5 foot tripod?

Thanks!


Title: RE: Roof mount: 3 ft or 5 ft tripod???
Post by: KB3HG on September 24, 2012, 06:43:50 AM
Maco antenna tuned for ten meters about 228" +/- or about 19 feet. Mast at least  tower 3' or 5' + 3' for 2 meter beam if mounted to the mast plus separation space between 2m and 10 meter antenna above tripod and below 10m antenna.19" each for another 3 feet. now you are about 10 to 11 feet plus the 19 feet of antenna. go with the 5 foot tripod securely mounted to the roof.  that's about 29 ' x wind load area.  I have not done something like that since the late 70's but it will work. follow tower manufactures mounting advice. Be safe.
Tom Kb3hg


Title: RE: Roof mount: 3 ft or 5 ft tripod???
Post by: K8POS on September 24, 2012, 09:14:55 AM
Make sure to get up in the attic and place some 2x6 under the sheeting to lag screw the tripod into.
The sheeting alone won't hold up over time.
Usually the 5' tripods are a bit heavier duty.

My 2 cents

Bob


Title: RE: Roof mount: 3 ft or 5 ft tripod???
Post by: K3VAT on September 24, 2012, 02:35:29 PM
... that's about 29 ' x wind load area.  ... Tom Kb3hg 

Tom's calculations appear sound.  29 feet is a lot of height which, depending on your QTH's wind parameters may require guying.  You can shorten this configuration significantly by going to a quarter wave 10M vertical without much sacrifice in performance.

Make sure to get up in the attic and place some 2x6 under the sheeting to lag screw the tripod into.
The sheeting alone won't hold up over time.  Usually the 5' tripods are a bit heavier duty. My 2 cents
Bob

These are good recommendations.  If you're serious about roof-mounted tower systems, then search the Antennas Forum for some detailed postings I made this summer.

GL, 73, Rich, K3VAT




Title: RE: Roof mount: 3 ft or 5 ft tripod???
Post by: KL3HY on September 25, 2012, 11:29:10 AM
The 5 ft. should have leg separation allowing you to lag the feet into rafters instead of plywood.  With the 3 ft. you'll only be able to get two of them into rafters (assuming 16" centers") and you'll have to do what Bob suggests and install some extra bracing for the third leg. 

I installed a 3 ft. tripod my roof for a 10' 2m vertical, and I wish I'd done the 5 ft. tripod instead.

Mike


Title: RE: Roof mount: 3 ft or 5 ft tripod???
Post by: AA4HA on October 03, 2012, 02:06:58 PM
Maco antenna tuned for ten meters about 228" +/- or about 19 feet. Mast at least  tower 3' or 5' + 3' for 2 meter beam if mounted to the mast plus separation space between 2m and 10 meter antenna above tripod and below 10m antenna.19" each for another 3 feet. now you are about 10 to 11 feet plus the 19 feet of antenna. go with the 5 foot tripod securely mounted to the roof.  that's about 29 ' x wind load area.  I have not done something like that since the late 70's but it will work. follow tower manufactures mounting advice. Be safe.
Tom Kb3hg

Yes, that is quite a bit of mass up on the roof, it only takes one leg to tear out on a tripod tower and it is game over.

Definitely re-enforce in the attic first and even consider mounting galvanized steel 90 degree angle to the outside of the roof (to get more places to spread out that load. Look at the joists in the attic at the crown, sometimes they are using nothing more than particle board for roof sheeting and not something like 3/4" marine grade plywood. I have been surprised as I designed, spec'ed out and oversaw the construction of my house. The day I came home from work and inspected I saw they were trying to install this really cheap particle board type stuff and had a big argument with the builder who had to take that stuff back and put up what was on the specs they signed the contract for.

There are four legged tower roof kits (quadpods? <lol> Glen Martin roof mounts). I would be nervous about having that much of a three legged "crowbar" mounted on my roof. Sooner or later it would tear free and I would be looking at skylight kits to fill up this gigantic hole. (or a submarine conning tower on top of the house).