In the case of an attic installation your tv antenna will be sheltered from the environment so there s no need to ground it for that reason.
Do you need to ground an antenna in the attic.
To answer the question yes you can install your outside antenna in the attic if needed.
However the connection of your antenna coax to various ac operated equipment in this case your converter box or tv introduces the potential of electric shock.
If you have no access it s hard for me to tell you to go to extremes to find a way.
This isn t the ideal situation for installation of an outside antenna but sometimes it is necessary based on a number of factors.
Agreeing with adtech i don t see the nec requiring you to ground an indoor antenna system.