As mentioned, you need an HD source hooked up to the TV. Most people don't know this because electronics shops and cable satellite providers like to hide the fact, but depending how close you are to broadcasting towers, you will be able to get most, if not all network channels through antenna in HD.
I use a Channel Master 4228 8-bay antenna. I'm lucky enough to be close to all the towers in the Buffalo area to get all the network channels, and also have a TV with a built in ATSC tuner (many do these days). The only station I have to rotate for is CBC in Toronto as it's about 90' from Grand Island. I had a smaller 4-bay antenna without a rotator on the deck prior to this one about three years ago, but upgraded about two years ago to get CBC. I even added an amp in the hopes of getting a few more of the weaker channels from TO, but I am too close to buffalo and the signals from their overpower the amp and kill any chance, so I took the amp down. Regardless, you can't beat free HD!
All you need is a UHF antenna, which most rooftop units from decades ago would have done both VHF and UHF... so if you never took it down, you're good to go. What's old is new again
If you're close enough to the towers, you can put the antenna in your attic, but it does kill about half the signal strength. As mentioned, the nice part about digital OTA is that it is MUCH easier to lock into a channel, and there is no bad signal... it's either there or it's not - the only in between you will get is sattelite-like breakup of the image.
Antenna HD is the best quality picture you can actually get - cable and satellite actually compress the signal which deteriorates the picture quality slightly.
