Been using galvanized fence posts for years on the GEOSATpro dishes without any issues with the clamp. As pointed out, there are many post options (don't try PVC as a few newbies have attempted).

As the op is likely to mount the dish on a motor, I would suggest using a larger 2"- 2.5" post anyways.
If you ever want to add extra grab to a post mount, place a strip of the non-slip stair tread tape in the clamp or on the post prior to mounting. Once clamped, the grit on the tape secures the mount from easily rotating.