We have the RF Mogul automatic dish which is excellent and $$$. It is HD and SWM technology. This dish requires a receiver with SWM capablity and we used it on the HR24 and HR44 receivers.
Sometimes when we're on the fringe of local channels we can still get SD channels.
It's easy and quick to set up a 18" round dish to receive SD only. HD dishes are more involved to setup.
Be advised that there are some areas that DTV does not transmit SD local channels. The first one we discovered was the Eugene, OR area and nearby coast.