I missed the comment early on about Low Power and referencing KPDX. KPDX was running just above the LPTV limit I think, and in Dallas and Salem it was coming in okay (not great) even at that power level. The problem is most Full power antenna design is set for 1/2 deg or 3/4 deg of beam tilt. The assumption is that the close population will get the signal just because they are close in. This is the reason I think KPDX was good in the mid valley (not great mind you) but was viewable unless there was a lot of rain or wind. When you design an antenna system for a LPTV it needs 3 or more deg of beam tilt if it is in town because you are NOT looking for the horizon you are looking for close in. Look at channel 5 (was channel 4 when I designed it) it had little gain (thereby a wide beam) and had a good reach into Portland even downtown. The other item that UHF can do because wavelength is so small, so they do it, is to make these antennas high gain that signal comes form somewhere and normally it is by reducing the beam width vertically I have seen antennas with only a 3 db point 2 deg wide.
So it is not the power you have it is how you use.