Wouldn't WRIU Kingston, RI also prevent them from gaining much ground to the south/southwest (if they were interested in doing so)?
Don't know why I didn't reply to this sooner...
It's been a while since I've looked at this, and the landscape (WICN and WSMA) has changed somewhat. But back in 2004 or so, WRIU
did limit WZBC somewhat but not as much as you'd think. The key here is that there's two ways WZBC could improve their signal: either they move north or north east to take advantage of a big "hole" around 90.3 north of Boston, or they move downtown to maximize downtown coverage.
In both cases, the real limiting factor is WSMA. In fact, I'm not sure it's physically possible for WZBC to move downtown anymore. Assuming they could find a place to put a tower that won't cost an arm, leg and firstborn child (unlikely) the interfering contours from WSMA reach pretty far north from their transmitter site in Manomet. Moving north to Medford...or maybe Zion Hill or the old WTTT site by Rt.2 in Lexington...would allow for a killer omni signal and big power boost. It'd boost WZBC's already-decent range from just being "inside Rt.128" to being all of the north shore. Catch is, it doesn't really put more actual signal over the core listening audiences in Boston and Cambridge proper...and with WSMA's signal there now, it probably would get a little worse. Especially for in-home listening (as opposed to in-car) which is, AFAIK, most of WZBC's audience.
I remember that at the time WZBC couldn't move west to the Newton/Needham towers because of WICN but I'm not sure if that's still true now that WICN moved. Still, you'd have issues with WSMA's interfering signal over the downtown area; anything but a move to downtown will exacerbate that. And a move downtown would be both difficult and fiendishly expensive.
BTW, in all these cases, WZBC would not gain anything southward because of WRIU and WSMA, but WRIU is far enough away that it's pretty easy to deal with...either through transmitter location or a directional antenna.
Of course, a big problem with any of these scenarios is that it'd result in substantially
less signal
on-campus and that's important, politically. You can't have a student radio station that the students can't hear on their radios...and BC has a lot of thick-walled steel-and-concrete dorms. They need to be inside that 100dBu contour just to hear the station. Unless WZBC were to radically change its mission and format, there's little reason for them to move from their current location. It has free rent, the xmitter facility itself is pretty good (especially since they added A/C a few years back), it has fairly decent signal coverage over the market (it's not as good as it could be, but it's certainly not
bad or anything), and it covers campus excellently.