After-action report:
We got the antenna mounted to the chimney and grounded with #8 copper wire running to the house's ground rod (we got a grounding block connected to the ground rod as well). Somewhere around 100' of cable brings the signal to the television, using an assortment of coax cable, cable runs, grounding blocks, and connectors left behind by a cavalcade of predecessors. (Over the years the house appears to have been connected to the cable company and both satellite providers, and there are signs of a previous rooftop antenna from even farther back.)
I've got all the channels I wanted, and they look beautiful on my 4-year-old Panasonic plasma. Sunday night football on NBC was eye-poppingly gorgeous; and even an afternoon game on Fox, which seems to have lower standards, looked really good. Heck, even some of the 480i multiplex channels looked impressive for what they were--the best of them looked more like upconverted DVDs than like the lousy standard-def channels on satellite.
I watched the TV's signal strength meter fluctuate between 92 and 100 for ABC and PBS; The CW ran from 86-92; CBS and Uni from 73-84; NBC and Fox from 66-73. I hope that means I've got the antenna decently aimed, and that they'll all continue to come in, under pretty much any conditions, for some time to come.
To compare to the signal analysis, here's the plot I ran for 15' again (I don't know the precise height the antenna wound up at, but it's surely much closer to 15' than to 25'):
http://www.tvfool.com/?option=com_wr...ec2f0beadcd046
All the channels down to 11.0 NM(dB) come in, except for those four that are located close to due north, or about 90 degrees off from the direction the antenna is pointed. (The weakest one I receive--WGGD-LD, right at 11.0 dB--has a signal strength that runs from about 44-51. Luckily there's nothing I want to watch that far down on the meter, since the TV appears to lose channels altogether at around 40.)
Thanks so much to those here who provided helpful information and advice. You may see my friend who helped with the install (or rather, did most of it...he's the homebuilder after all) show up here in the near future, as I think he's got the OTA bug now.