I want to use an OTA HD antenna...

I looked in my manual, and on the back of the TV. I have DISH connected through S-Video and composite plugs, and I do have an available Antenna-in co-ax connection. The manual has the external antenna connected to the cable box or the DVD, not directly to the TV. Do I need to do that, or can I run it directly into the Antenna-in on the TV? Do the roof mount antennas work a lot better than the table top ones I see for sale? For 20 miles, do I need an amplifier? Do I get all this at Radio Shack?

The table top models work well for some . It will not do as well as a roof or attic mounted antenna. Radio shack will have most of what you need. Walmart sells the table top versions fairly cheap. Try it without the amplifier first. You may not need it. You can get coax from Home depot reasonably.

Gmood1 - You stated that OTA HD is uncompressed. It is always compressed to fit into standard 6MHz bandwidth but cable companies compress it even more to save on cable bandwidth.

Same is for directionality - it is as directional as antenna is at given channel and being analog or digital has nothing to do with it.

Also sombody mentioned HD Antenna. There is no HD Antennas - they just call them HD or DIGITAL to promote sale. Frequencies are the same (VHF/UHF)and bandwidth is the same (6MHz). I got HDTV in 2001 and started using it with existing cheap antenna (15 year old Radio Shack) with good results.

I use now Winegard HD-7084P 68 element 131" long antenna (VHF/UHF). Directionality is pretty poor as stated by Winegard and tested by me on the roof. I live about 30 miles from transmitter. For example - beam width at half power is 69deg at channel 2 and 41deg at channel 32.

Actually I stated I wasn't sure if compression was the cause for the difference I see compared to cable . It probably has more to do with the miles of copper and equipment used by cable companies.
Yes this is true..UHF..is used for digital transmission. As far as directional goes, the analog channels are much easier to receive here without the extra fine tuning needed for the HD channels ..of course they don't look nearly as good either.

Thanks for the in depth explanation.