TV antenna far from box - how to ground?
I would mount the antenna on the same side of the house as the current box, and run straight-shot to the cold water bond point
Given that you can't rely on the integrity of metal plumbing to bond things together, your best bet is to place the antenna so that you can run the antenna ground wire as a straight shot to the existing water pipe bond point. This will give you a decent place to tie into the grounding electrode system at, provided you clean up the paint and such so that a good connection can be made with a split-bolt or other listed connector; with a bit of work, you could even retrofit an Intersystem Bonding Termination device there.