Just a note to begin that there is no such thing as an "HD" antenna; every TV antenna is capable of receiving HD. Don't be fooled by manufacturers latching on the HD concept and adding "HD" to all their model names... when the designs probably haven't changed in 30 years! :-)

With that said, you need an antenna system appropriate to your location and what you're trying to receive.

So: what city are you in, what type of home are you in (condo, house, etc.), and what would you like to pick up?