Yo Beerman-
The signal is digital and the television decodes the coaxial signal into HD. Set up right, the picture is a magnitude of order better than the best cable. So the coaxial is a conduit, only as good as the signal it carries, which is digital.
New, digital antenna + new Quallty coaxial + new digital 1080p tele equals the best signal right now.
As far as your iMac, you will get at least 720p resolution going through a $100 Motorola modem, into a new Airport Extreme Time Capsule ($250--3tb hardrive) for any stream or app signal into an AppleTV 3. Movies that you 'find' on the Internet will be Blu-Ray qualty or Black Ray. Be sure to get the .mp4 designated codec that you can just drag into iTunes and boom goes the dynamite.
Alternatively, look at free Mac thrd party stuff like HandBrake to encode movies to the smallest file size and into .mp4 from .avi (standard DVD file codec). Far easier than it all seems. Hit me up on pm if you need specifics.
Hope this helps.