Sounds like you don't have SWM, since the dish works without a power inserter. In that case you DO need B-Band converters for HD channels. The older HR21's didn't need them, but the 24's do. The good news is that they are cheap, and DTV might send them to you for free if you ask them nicely.
Correction - The HR21 DOES need B-Band converters for non-SWM HD use. I guess I forgot until I had to get behind it last weekend to run another device's power cord.
Linear Mode
