If I remember correctly, the rough rule of thumb is ATSC requires about 1/10th the power of NTSC to cover the same area (yeah, I know that's not an absolute).

So, for CHCH originally at 325kW, that would mean they should have been looking at around 32kW. Even 20-25kW would have probably been OK.

But 6.1kW? That's just 1.877% of their original power. There's no way they could ever have reasonably expected to cover the same geographic area with that, especially in the troublesome VHF range.

Frankly, even before the change I was unable to get anything usable on analog 11 in Markham... but 18 came in just fine (and I do have a VHF antenna and VHF preamp).