fixture it
There are two different thrust loads for an antenna mast... vertical and lateral.
The vertical thrust bearing is built into the rotator, as a set of balls running in grooves just inside the perimeter. If you're using a gear or chain drive, then the bearing would just mount in a locating frame bolted to the tower plate.
Lateral thrust can be a much more serious problem. If you're running a single yagi for 440 on a 5' mast it's not much of an issue. But if you have an array of HF mono-banders, some serious engineering is required.
You should have two lateral bearings, one at the bottom of the mast just above the thrust bearing, and one as high on the mast as possible, usually fixtured in the top of the last tower section.
If you're supporting a large array, it's advantageous to mount the tower plate lower than usual, to increase the distance between the two lateral bearings thereby reducing the stress.
If I were doing it, I'd get a big block of stainless and bore out a hole for the lateral thrust bearing, with a smaller hole below it to accommodate the vertical thrust bearing.
Leave about 1/4" of material between the bottom of the vertical bearing and the bottom of the fixture, with some drainage channels or holes to let the water out. You don't want the bearings submerged in water.
Here's a little drawing of what I would do. Exact dimensions and mounting depend on your details.
So what are you putting up?
Good luck with it.
- 73 de Leigh W3NLB