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Nice write up.  I recently hooked some friends up.  They desperately wanted to dump Comcast, so they picked up a TiVo for $20 off Craigslist, and I have a connection at Radio Shack for the antenna.  Using the coax runs from their old Comcast lines, we set them up in just under a couple hours.  They love the HD picture, and can still record their shows in decent SD.  Good times!

I receive OTA in oposing directions.   I want to mount an antenna in my attic; can I mount 2 side by side facing opposite diections and join them ?

I have heard about signal loss doing this but some say you can?

Kryspy

HD radio is on the exact same frequency as the stations analog broadcast.  For radio broadcasts the FCC assigns a center frequency and a channel bandwidth.  For FM the center frequency is and odd decimal (ie; 96.9 102.5 etc) and a channel bandwidth of 200kHz (+/- 100kHz from the center frequency).  So, take for example 102.5-FM has a channel that stretches from 102.4MHz to 102.6MHz.  To prevent interference with adjacent channels the FCC specified that an FM station could only use 150kHz of their 200kHz channel, leaving 25kHz at the extream upper & lower end of their channel unused. 

HD radio uses that spare 25kHz at the extreme ends of the FM channels to broadcast the digital HD broadcast.  The technique that the FCC approved was developed by iBiquity and is heavily pattented so I haven't been able to find exact technical details about the modulation and digital encoding techniques used, but suffice it to say, it works. 
Since HD radio is using the extreme ends of the channel bandwidth the possibility for interference is increased.  However since that bandwith is being occupied by digital data the interference primarily will cause data errors which can be corrected for.  If, however, the BER (Bit Error Rate) is too high to be corrected for, an HD radio receiver reverts back to the analog broadcast.  Yes, as I said before, the analog and HD radio broadcasts coexist on the same frequency so analog radio is not going to go away like analog TV. 

Once the prices for receivers comes down I think you will see HD radio included in most cars and home audio receivers.   

For more reading, here is Wikipedia's article;
http://en.wikipedia.org/wiki/HD_Radio

Yes, you need a combiner to do this.  Or else use an antenna that is omni-directional.  The addition of the combiner will cause some signal loss.  Any time you lengthen the signal run or add more connectors to the path the signal will get weaker.  You may be just fine without any amplification and I would try it that way first.  You can always throw an ampllifier at the antenna feed or at the tv/tuner.

Perhaps I'll do a follow up later on the different types of antennas and their strengths and weaknesses.