You'd be better served posting this question on DBSTalk ; there's a dedicated 921 forum there.

A quick summary: for all of the Dish PVRs, you must be receiving signal from one of their satellites (110, I think) for anything on the receiver to work, including OTA reception. I seem to remember reading that OTA timers on the 921 are pretty unreliable, so this may end up being an annoying purchase for you.