The AM21 works with the HR21/22/23 and the H21 and H23 receivers. As for what channels you'll receive it's all based on what DirecTV says you can get OTA for your zip code, the AM21 doesn't scan for channels like some other ATSC tuners. Of course that assumes that you have a good OTA signal and antenna so you can receive those stations.
Dish - Slimline 5LNB
Switches - SWiM16
Receivers - HR34-700, HR44-700, C31-700, C41-100, C41-700, WVB, C41W-100, Samsung RVU client
GenieGo + 1TB USB drive
Networking - 100/1000Mbps switched ethernet and DECA for DirecTV network
TimeWarner RoadRunner internet service