UNC TV channels no signal, why?

I use antenna only (don't pay for cable service), because I am not much of a TV-holic. Om my downstairs TV, I get all local stations fine. I decided to buy a small, flat screen digital TV for my upstairs office. The initial setup scan found all of the same channels as the downstairs set, except the four University of North Carolina stations (all "public broadcasting"), which are 4-1, 4-2, 4-3, and 4-4. The menu option to show signal strength shows 60-77 for the other channels, but zero for each of the 4 UNC channels. I repositioned the antenna five times and re-scanned for each position, but the UNC channels were never found. Are these stations somehow broadcasting in a special way that makes the signal zero unless I do something special, and if so, what must I do?