Can an older outdoor antenna receive HDTV, or can I modify it somehow? I have a Directv HDTV receiver. The installer said I could receive local high def programs if I plugged a HDTV antenna into the back of the Directv HDTV receiver. Before I go buying one I'd thought I'd ask you guys if that was necessary, or could use the one already on my roof?
HDTV over the air signals are in the UHF frequencies. ANY UHF television antenna will receive them properly. It was pure sales hype that you somehow needed a "special" antenna for HD signals. I currently receive 6 separate HD channels and all their sub-channels (i.e. 46.1, 46.2, etc.) on a cheap Radio Shack $24 yagi UHF antenna. Aligning your antenna to the source can be very critical. Digital signals require much more precise antenna alignment to the transmitting tower direction than older analog signals. Where one antenna aimed in a general direction would work for analog, it might take a rotator motor to swing your antenna and point it more precisely, or multiple UHF antennas each pointed directly at the source.
Your older antenna is probably a VHF/UHF model, but it is possible it is VHF only. If it does not receive UHF (channels 13-69) than it will not work on the UHF HDTV signals.
You may want to consider new RG-6 cable, new 300 ohm to 75 ohm balan at the antenna, and bring the antenna cable all the way into the set top box. Don't scrimp and lose signal with combiners/splitters - your box should have a separate externall antenna connector and a cable connector for the connection to dish/cable co.