A friend educated me to the fact that our local cable providers carry broadcast HD signals on their cable lines, and one can simply connect the cable line to the ANT IN connector on a tuner to get the broadcast HD signals (but obviously not the cable HD signals). Apparently, the cable companies are required by law to carry the broadcast signals, but they don't advertise this for obvious marketing reasons.
So, if you have cable, then you don't require an antenna, but you still need the digital HD tuner.
So, if you have cable, then you don't require an antenna, but you still need the digital HD tuner.