The Facts. What the FCC main studio rule actually says.
The Main Studio Rule:
http://edocket.access.gpo.gov/cfr_2005/octqtr/47cfr73.1125.htm
For FM this is the 70 dBu (50,50) contour.
How the Commission interprets the rule:
(Copied from:
http://www.fcc.gov/eb/Orders/2006/DA-06-664A1.html):
Section 73.1125(a) of the Rules requires the licensee of a broadcast
station to maintain a main studio at one of the following locations:
(1) within the station's community of license; (2) at any location
within the principal community contour of any AM, FM or TV broadcast
station licensed to the station's community of license; or (3) within
twenty-five miles from the reference coordinates of the center of its
community of license as described in Section 73.208(a)(1). In
addition, the station's main studio must serve the needs and interests
of the residents of the station's community of license. To fulfill
this function, a station must, among other things, maintain a
meaningful managerial and staff presence at its main studio. The
Commission has defined a minimally acceptable "meaningful presence" as
full-time managerial and full-time staff personnel. In addition, there
must be "management and staff presence" on a full-time basis during
normal business hours to be considered "meaningful." Although
management personnel need not be "chained to their desks" during
normal business hours, they must "report to work at the main studio on
a daily basis, spend a substantial amount of time there and ... use
the studio as a home base."
Doesn't matter if the transmitter is or isn't located in the city of license. A microphone and mixer at a transmitter site in a rack next to the transmitter wouldn't qualify for a main studio (no meaningful staff presence, public file, etc.).