Well....
I'll give you the semi-short version. Most HDTV I've run into or heard about is running in the low-mid UHF freq's (400-675MHZ). Here in Seattle/Portland areas, ch 18 to 48 UHF. VERY line-of-site, doesn't do well over hills like VHF (and that ain't saying much!), VERY sensitive to noise, specifically signal to noise ratio. I found that early Mitsubishi decoder boxes are lousy, Samsung very forgiving, Pioneer good, Zenith Good, Panasonic spotty, Sony ok-not consistant. Some will work with very weak signal levels, (-40db!) if it's clean. And AGC effect is lousy, compared to regular TV's. It's dependent on how many buffers and how big they are in the box. That's why mis aiming the antenna works, it has less multipath (ghosting) to deal with, and all you need is one fair signal and it looks great. Antennas make a big difference depending on what your problem is. I've found the long Radio Shack UHF yagi works well where multipath exists, the older style "bedspring" ones where gain is most important. Booster amps can help in the outer areas, but weed out the VHF before hand. Experimentation is the key. Done dozens of HDTV installs since November, been putting up VHF and satellite since late 60's. Very frustrating at times, too, as some TV stations are still experimenting with power and antenna location/designs, etc. What works one day fails the next.
Does that make you feel warm and fuzzy ?!
Can give more details if you tell me some, like what your freq's are, distance, brand of box, terrain, etc.
Hey, it ain't all bad. Once it's running, pictures are usually awesome, when they actually transmit actual HDTV and not up-converted NTSC stuff.