The about is where I started wondering about number of antennas. It's been a very long time since I've even looked at them, but from everything I've done and from what's in the about it's a 1 to 1. One antenna to one device. I wasn't aware of any multi tuner single antenna device. They could exist, and prob do, I just have never heard of them. The youtube stuff should be linked to the about or main page. If it had that kind of info in it, you'd think they'd want everyone to see it. It's the corner stone of their entire case... I think anyways. I seem to be wrong a lot on this..
I have an HDHomeRun Prime with 3 tuners and 1 RF input that can be used with an antenna for over-the-air (but I use it with cable). I also have a dual tuner HDHR.
I have 2x Hauppauge HVR-2250 cards with 4x tuners each (2x analog + 2x digital on each card). Each card has only one RF input.
A dual-tuner or quad-tuner TiVo works with a single antenna.
I can't say I've ever heard of a multi-tuner device that expected you to use multiple antennas.
However: The earlier version of the HDHR Dual had 2 inputs. Most users were expected to use a splitter to feed the same signal to both tuners, but had the option of connecting cable to one input and an OTA antenna to the other. A lesser-known option for people in areas with very poor reception was to set up two different
directional antennas and tuning specific channels on specific tuners. The revised HDHR Dual has only one input.
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My old Sony CRT HDTV had a "DualView" mode where it used 2x tuners simultaneously with a single RF input.
VCRs could always tune one channel to record, and you could press the TV/VCR button to switch to an RF pass-thru mode that allowed you to watch another channel on your TV's tuner while the VCR was recording from another channel...so that's also 2x tuners working simultaneously from a single RF source.