An omni in the attic will produce little if any DTV. You will need an outside installation, as high as practical. I recomend a Winegard 8200U, and a Winegard 8700 preamp. To receive channels from the multipul dirrections, you will also need a rotor. With all of that you should receive most of the stations in your report that are within 80 miles out, but I would not expect to receive any of the stations over 90 miles out. If you use a rotor follow these instructions:
Test prior to installation using a short length of cable to run from end to end.
Leave the rotor in the counter clockwise end of rotation.
If you choose a rotor with a 360 degree read out you can alighn the rotor and antenna to magnetic North. This will you to use your TVF radar magnetic locations to tune in the stations.
Be sure that your set is configured for ANTENNA reception, not cable. Be aware that some sets will only allow you to scan one location. If that is the case locate by scaning all of your channels one location at a time to confirm station availabilty. Then look for an add on tuner or DVR that will allow for rescans or direct channel entry.
www.solidsignals.com will have all of these products.