No matter what city I travel to, I know I’ll always find my NPR at the low end of the dial, between 88 and 91, and never at the high end of the dial.
Why? Are the low numbers cheaper? Is the low band somehow better for transmitting the measured voices of NPR commentators? Am I just not travelling far enough from home?
Whatever the reason is, it’s specific either to NPR or to the United States, rather than public radio in general. The CBC has stations in different parts of the country on every point of the dial pretty much, FM and AM (though mostly FM now).
The Federal Communications Commision has set aside frequencies within a certain range (ARRRGH! I can’t find the exact range anywhere, sorry!) for public, religious, non-profit type broadcasting.
91.9 and lower on your FM dial are reserved for public radio, talkradio, religion, etc.
In the U.S., the FM frequencies of 87.9 to 91.9 (channels 200 through 220, to be precise) are reserved for non-commercial operations. 92.1 to 107.9 (channels 221 through 300) are available for commerical operations, although there are some non-commercial stations on these frequencies too, most commonly low-powered “translator” transmitters, which rebroadcast the parent station’s programs at distant locations.
There’s a PDF doc giving a list of all the frequencies for NPR stations here:
http://www.npr.org/members/ (link at top of page)
There are some on the AM dial, and a few in the upper part of the FM dial, like WFIU, Bloomington, IN, 103.7 or WMEM, Presque Isle, ME, 106.5. However, most, like you suggest, are on the low end of the dial. As Podkayne suggests, an FCC policy reserving up to 91.9 for non-commercial stations is at work here.