I'm not sure what you mean by "Without any signal", because I'm seeing an SD simulcast on 13-2.
Everything I've read says that a 1080i HD signal can easily take up the full bandwidth of an OTA frequency. That means that any bandwidth dedicated to additional subchannels will cause the need for more aggressive compression on the subchannel. The more motion that is present in the particular program at the time, the more noticeable this increase in compression will be. Football in particular has a lot of shots where there is a lot of motion (Many players moving during the play, Zoomed in shots of a running player where the background is panned quickly, shots of thousands of people moving in the crowd, etc.)
Here's a link explaining the #s, (although his example is for a 720p signal which wouldn't be as adversely affected by an SD subchannel.):http://broadcastengineering.com/digi...ultiplexing_5/
As far as how much bandwidth that is in this case, I'm not sure because I don't have any measurements on the bitrate for 13-2. It looks slightly better than the SD version I receive through DirecTV, however, which means it's probably around 2-3 MB/s, which cuts the bandwidth for 13-1 considerably.
My real point is that one of the following has to be the case:
1. 13-1 PQ is being compromised for satisfactory PQ on 13-2
2. Both 13-1 and 13-2 PQ are being compromised for the sake of having both
Either option seems silly when there, IMO, isn't any real target audience for 13-2. I can't imagine anyone with an ATSC wouldn't prefer the HD version, even if they have to downscale it for display on their particular TV.