|Believe I may have found the answer, inadvertently.|
Brought the box inside the house and hooked it up--RIGHT next to my router (not on purpose, that just happens to be where the KVM is). Lo and behold, there's a signal. Not a strong signal mind you, but enough that I'm able to connect and do some test surfing.
I could swear when I was researching this wireless card that it said it didn't need an antenna, but the evidence doesn't really bear that out! I think my next move is to buy an antenna kit and hook it up, see if that doesn't solve my problem.
Thanks, everyone, for your time and diagnostic skills. If an antenna doesn't fix it, I'll come back and let you know.