I have an HD antenna mounted to a storage shed, not even as high as the house roof. It's close to where the electrical service enters the house. There's no ground rod in sight. (House built in 1971.) However, electrical is underground, and there's a metal conduit from the meter into the ground.
I'm guessing (!) that the conduit is grounded. My thinking is that, since it goes underground and constitutes a ground itself, it must (by code) be connected to the main ground.
I could pay an electrician a few hundred dollars (or, in my experience, much more) to provide a ground to the antenna, or I could just ground it to the conduit, which has a very handy clamp where it's attached to the brick wall.
Probably 90% of home antenna installations aren't grounded at all, but I thought I'd make an attempt to do it right.
Above grounding would also apply to the ground block which will be attached to the outside wall, near where the coax enters the house.