1) First of all, there is NO such thing as a 'hdtv' antenna. There are UHF and VHF antennas and that is all there is. Its the same frequencies that TV has been using since before I was born.
That being said, most all Digital OTA tv signals in most communities have moved to UHF, so what you really need is a very good UHF antenna. Most of the big TV stations used to be on VHF so most indoor antennas typically really focused on VHF and often just gave you a tiny little loop for UHF and that is it. This will not work very well for all the UHF signals you want now.
I built a VERY simple and quick to build UHF antenna using a 2x4 board a few pieces of 14inch long wire, and a couple of cheap metal sheets.http://www.tvantennaplans.com/
I built this design in less than an hour, and had it up in my attic with incredible reception.
Being a ham, I had plenty of extra wire around. I used some of the same 3/16" braizing rods I use to build antennas for 2m, 70cm etc.
I had some extra metal sheeting to use as the reflector. You can get some for a few dollars at the hardware store if you want to make yours a little more sturdy like I did.
Also, UHF is very directional, so you can't really just 'eye ball' it and get good results a few minutes with a compass, and you will get the signal you want.
Go here to get the compass bearing to the stations you want to view from your location.http://www.antennaweb.org/aw/Welcome.aspx