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).