Originally Posted by dattier
Until someone else gives a more detailed response, I'll chime in: lower channel numbers are at lower carrier frequencies and need less power.
Sort of. Many stations went to UHF because the noise floor is lower, the signals penetrated buildings downtown better, and in this part of the country it is relatively flat making UHF ideal. VHF doesn't necessarily allow lower power. The noise floor is significantly higher at 50 MHz (VHF LO) than 600 MHz (UHF) and the VHF Hi (200 MHz) is somewhere in between. You need a good signal to noise ratio to decode ATSC. The only good thing about VHF is that it refracts (bends around hills, trees, etc) better than UHF. VHF happens to be vonerable to static crashes from thunderstorms and man made noise like power lines, florescent bulbs, furnaces, etc.
So... there are less noise problems on UHF and the urban canyon dwellers can receive the signals easier so that's why UHF is better for Chicago. Montpelier Vermont or Marquette MI might want to consider more VHF due to terrain, no urban canyon, and the lower noise floor.
But the point made about the FCC's maximum allowable VHF ERP is very valid. Their models probably only considered the S part of the S/N (signal to noise) equation. So, a 3 dB increase (double the Watts) from their original limits is pretty fair. they definitely underestimated the amount of noise in their models. Some stations have already started that, i.e. WWTV in Cadillac, MI