A couple of questions:
1. Are you sure that the TV has an HD tuner? Many 16x9 sets only have a traditional NTSC tuner.. If the set can truly receive HD, you will see the HD signal on a channel such as 7.1 (or 7.2 or 7.3) if your local non HD (same station) is channel 7. As far as antennas are concerned, the HD (digital) picture is either there and very pristine, or not there. No fading in and out and no noise. The only thing a better (rooftop) antenna will give you is either additional stations or a more solid picture. If you can receive the HD now on the stations that you want, a new antenna won't be any technical benefit.
Hope that this info helps...
I just bought a mitsu 52525 ($2750 at Best Buy after tax - great deal) Anyway, I'm using the built in HD Tuner and have a Cheesy $9 antenna from Walmart. The image of an HD signal comes in but has some video noise, not digital blocks from a bad signal just a little noise and maybe not as sharp as I expected or saw at the store. Could this improve with a better antenna and is there also a way to know what the HDTV mode of the broadcast is, ie. 720p/i or 1080i on this Mitsu TV? The TV just says HD16:9.
Any help would be appreciated, Thanks!!!