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This question will probably be best answered by you ham folks. How much concrete would make a good base for a 40 foot tall, free standing tower? I would use that common 9 foot long section stuff that bolts together. I don't want to put up guy wires. It will have a broad spectrum VHF/FM/UHF high gain (read large) antenna with a rotor at the top, and negligible wind loading further down (like a DBS dish and/or weather station sensors).
A Ham checking in here!
If you know the tower manufacturer, the information is probably available on their website. That aside, one of my friends has a Rohn HDX at 40' and his base is about 4'x4'x6' deep. A free standing tower requires abigger base, anchor, than a guyed tower. When I was planning on putting up some Rohn 25G, the wind loading un-guyed at 35' was about 5 Sq. Ft. if I recall.
All the numbers as well as the diagram for the base, including rebar, was on their site.
Do you know the manufacturer and model of the tower?
If you can anchor the tower to the house at the roof line, the wind rating will be higher than if it's not anchored.
It would also be nice to know the antenna's wind load in Sq. Ft. That's a factor.
Thank you Gary. From the dimensions I can measure, I think this may be a Rohn tower I've inherited. Unfortunately, my e-mails to Rohn (addresses provided on their website) have all bounced back. I need to find out how to tell if it's their stuff, ie, do they stamp their tower sections somewhere. Otherwise their site has a lot of useful info, and the accessories available are also worth consideration.
If you go to http://www.rohn.net/ and click on "On Line Catalog" then pick "GT Series Towers" Then pick 25G, you'll be able to access all the details and drawing you require.
They've changed the website since I was there last, so it took me a while to find what I wanted.
For example a 25G tower at 40 Ft. is rated for 1.5 Sq. Ft.
But the same tower, bracketed to the house at the 15 foot level is rated for 15 Sq. Ft. of wind load.