OK...here are the numbers. (percent on the signal meter, showing bit-error-rate derived measurement) Channel AM21/H21/Indoor AM21/HR21/Outdoor HR20/Outdoor 5-1 71 92 93-B 8-1 92 93 90-B 11-1 100 100 100 13-1 85 100 100 17-1 52 84 84 23-1 82 94 84-B Notes: A "-B" suffix indicates considerable "bounce" symptomatic of multi-path issues. All transmitting antennas are at the same location, elevation 2200 feet. Distance is approximately 28.7 miles at a bearing of 138 degrees. Indoor Antenna = Philips MANT510 (UHF Plate + Rabbit Ears), 4 dB Noise Figure Outdoor Antenna = CM medium sized VHF/UHF combo at 32' (roof top) All my stations are currently UHF, so the Telescoping Rabbit Ears are fully collapsed, vertical, and parallel to the UHF plate. The preamp off/on produces a 30% signal improvement on the H21/AM21 combo, with a gain setting of 4 out of 5 LEDs. Summary Conclusions: 1. Notice only the HR20 OTA tuner shows significant bounce (even on the same outdoor antenna, signal level balanced between the HR21 and HR20). This demonstrates better multi-path resistance of the AM21 tuner. (confirmed from prior measurements) 2. Notice even on an antenna that cannot discriminate effectively against multi-path (Phillips MANT510), the AM21/H21 combo shows no significant bounce. This is outstanding performance. BTW, this indoor antenna is available at Wal-Mart for around 29 bucks. It is labeled Philips, 50 dB gain and is all silver. (the gain is not important, look on the side panel for its Noise Figure..it should say 4 dB) All in all, the tuner performance of the AM21 is terrific, both on the HR21 and the H21, even with a very modest, but well designed, antenna (in this area, at this distance).