I could be wrong here but ... I agree there's no such thing as an HD TV antenna. However, back in the analog days most of the TV stations were broadcasting on VHF (a lo and hi band) with few in the UHF broadcast bands. As a result antennas sold back then were better tuned for VHF as opposed to UHF. With the move to digital broadcast the FCC has been moving a lot of the old VHF stations up into the UHF broadcast bands, so the older antennas may not work as well. Many antenna companies that are advertising "HD TV antennas" are really marketing antennas that have been re-tuned to better receive UHF transmissions. Some have totally dropped VHF as the lo-VHF band no longer exists for TV.
There are still some stations that are broadcasting in the hi-VHF band though, so when looking for a replacement antenna one might want one tuned for UHF as well as hi-VHF.
The real problem with digital is that they've moved to higher frequencies which don't penetrate obstacles like building and trees as well as the lower frequencies. You see the same sort of thing with cell phone coverage.