Share Time ("ST"): Two stations share time on the same frequency. The
FCC may specify the division of time in the stations' licenses, or the stations
may reach a private agreement which is then filed with the FCC and considered
part of their licenses.
It is possible for more than two stations to share a
channel. No such situation currently exists, though there was a three-way
sharing arrangement in Chicago well into the 1990s. It is also possible for
stations to share time at night and operate simultaneously during the day.
(when they are far enough apart that simultaneous daytime operation won't
cause interference) Again no such situation currently exists. At one time,
KFAB and WBBM operated simultaneously on 780 - with separate programming
during the day, and synchronized simulcasting at night.
Share-time stations are required to operate at least
2/3 of their authorized hours between 6am and 6pm, and at least 2/3 of their
authorized hours between 6pm and midnight.
Daytime ("D"): Operation at full power is permitted between average
monthly sunrise and average monthly sunset. The FCC attaches a document
(obtained from the U.S. Naval Observatory's American Nautical Almanac) to
the license specifying these sunrise and sunset times. The station must
operate at least 2/3 of its authorized hours between 6am and 6pm, and at
least 2/3 of its authorized hours between 6pm and midnight. (Much of the
year, daytimers have no authorized hours after 6pm, so the latter period
doesn't apply.)
In most cases, daytime stations may begin operation
under "Presunrise Service Authorization" (PSRA) at 6am. Stations within
the 0.5mV/m contour of a Class A station to the east must wait until sunrise
at the Class A station. (this means that if sunrise at the Class A station
is after 6am, the daytimer has no PSRA) This authorization is at no greater
than 500 watts, and usually quite a bit less.
Daytime stations also usually have "Postsunset
Service Authorization" (PSSA). This too is at no greater than 500 watts and
usually less. PSSA operation begins at sunset and in most cases continues
for two hours. Stations within the 0.5mV/m contour of a Class A station to
the west must stop PSSA operation at sunset at the Class A station. (this
means that if the Class A station is *east* of the daytimer, no PSSA
operation is possible)
Limited Time ("L"): Limited time stations are allowed to operate during daytime, and until sunset at the Class A station on the frequency if the Class A station is to the west. A limited time station is also permitted to broadcast during any nighttime hours that the Class A station doesn't use. (probably irrelevant today with all Class A stations operating 24/7, but when most stations had overnight silent periods it was quite applicable) Limited time stations must operate at least 2/3 of their authorized hours between 6am and 6pm, and at least 2/3 of their authorized hours between 6pm and midnight.
Specified Hours ("SH"): The operating hours are specified in the station's
license. Additional hours are permitted for special events. While the FCC
regulations don't say so, this category seems to allow for stations in
sparsely-populated places where it's economically impossible to meet the 12
hours a day minimum schedule. There are very few specified hours stations
in existence today, in fact I'm unable to find *any* in the NRC Log.
The regulations are somewhat unclear as to whether
the hours specified in the license are a minimum, or if the station is only
required to operate 2/3 of the hours specified. I suspect the figures in
the license are a minimum.
Non-commercial *FM* stations must operate at least 36 hours a week, and at least 5 hours a day at least 6 days a week. Stations licensed to educational institutions, however, need not operate on weekends or when school is not in session.
Non-commercial FM stations that don't operate at least 12 hours every day
are subject to time sharing, upon application by someone else for the same
frequency.
In the first 3 years of operation, TV stations must operate at least 2 hours a day and 5 days a week. They must also operate at least 12 hours a week during their first 1-1/2 years on the air; this figure increases by 4 hours every six months after that.
After 3 years of operation, TV stations must broadcast at least 2 hours
every day (no Sunday exception) and at least 28 hours a week. Test patterns
don't count.
An Act of Congress (not a FCC regulation) requires the FCC to revoke the license of any station which remains off the air for over a year, even if the Commission was notified of the outage. However, there seems to be a loophole in this regulation: it doesn't seem to specify any minimum operating period between silent stretches. Some stations have been known to operate for just a day or two every few months simply to keep this law from kicking in.