I did a Google search on what the NEC (National Electric Code) says about grounding of satellite dishes, and everything I read said that satellite dishes must conform to the same grounding requirements as TV aerials.
Here's my search
https://www.google.com/search?q=NEC+...f+dish+antenna and this article seemed quite topical
http://ecmweb.com/qampa/code-quandaries-7
If it were mine, I would ground the mast to the building ground using #6 copper wire, as indicated by the NEC. You can buy whatever length of solid #6 copper wire on eBay - I found that the eBay seller was much cheaper than the local home center or any other online source, even with shipping added.