The HR20 relies on DIRECTV's guide data to know what OTA channels are available and at what RF channel to find them, it doesn't scan for channels like a TV will. It has been stated in these forums that DIRECTV isn't adding any more OTA sub channels to their guide data due to space issues.
The new DIRECTV TiVo and the HR34 can scan for channels but if it finds channels that aren't in the guide data, you won't have any program info but at least you can tune to them.
The new DIRECTV TiVo and the HR34 can scan for channels but if it finds channels that aren't in the guide data, you won't have any program info but at least you can tune to them.