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.
Do you need to ground an antenna in the attic.
If you have no access it s hard for me to tell you to go to extremes to find a way.
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Antennas mine are receive only for fm tv and public safety bands will be generally restricted to the attic.
To answer the question yes you can install your outside antenna in the attic if needed.
Agreeing with adtech i don t see the nec requiring you to ground an indoor antenna system.