The following information might be of interest to you.
http://www.weaknees.com/tivo-directv-parts.php#h20
AM21 Off-Air Tuner for HR21 and HR21Pro
High Definition Off-Air Tuner for DIRECTV. Requires separate DIRECTV HD DVR (HR21 or HR21-Pro), not included in this purchase. (Same footprint and color as HR21.)
$79 - FREE GROUND SHIPPING!