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