I have probably linked to a similar comment already, but I think it's important to mention it in this thread...
If you choose to drive a ground rod it needs to be bonded (*) to the existing grounding system of your electrical service. Isolated ground rods leave you with the illusion of safety but due to the resistance of the soil, very high voltage differences can occur during fault conditions such as short circuits or lightning events. This is a recipe for property damage and personnel injury.
I have to advise against 'grounding' the antenna, mast or coax to an isolated ground rod. If you add a ground rod, you must bond it to the existing ground system with #6 AWG (or heavier) copper.
I would be inclined to contact the electrician that performed the sub-panel install, unless you are already sure they are not a 'stand-up' kinda person.
Here's a moderately technical discussion of the concepts of grounding and bonding as it applies to electrical systems in general:
http://ecmweb.com/bonding-amp-ground...ding-part-1-12 (Parts 2 through 12 should be easy to locate if you're interested.)
(*) NEC Art. 100 Defines:
Bonding (bonded). The permanent joining of metallic parts together to form an electrically conductive path. This path must have the capacity to safely conduct any fault current likely to be imposed on it