Classes of U.S. AM stations:
U.S. AM radio stations come in four categories:
Class A: These are the former Class I stations - 50kw,
usually non-directional at all times, "clear" channel. Minimum permitted
power is 10kw but all currently licensed Class A's are using 50. Class A
stations have some of their skywave coverage protected from interference.
Class B: These include many of both the former Class II
stations (secondary operations on clear channels) and former Class III.
(regional channels) However, those Class II and III stations which were
or remain daytimers are usually now Class D. Class B stations must use at
least 250w at all times. (except that lower powers are possible with
unusually efficient antennas) They are also authorized a maximum power of
50kw, except in the expanded band where 10kw is the limit. In practice most
Class B stations outside the expanded band are limited by interference to
the old limit of 5kw. Class B stations' groundwave coverage is protected
from interference, but their skywave coverage is not.
Class C: These are the former Class IV stations, the "local
channel" or "graveyard" stations. They're designed to provide service to
their groundwave coverage area but even that area may be subject to
interference. Power must be between 250w and 1kw. A Class C station will
not be authorized 250w or more of night power unless it is able to use at
least 250w non-directional daytime without interference. Since 250w is the
minimum limit and there is assumed to be no difference between daytime and
nighttime coverage for Class C stations, in practice this means no Class C
station will be permitted if it requires a directional antenna to exceed
250w daytime. There are in fact very few directional antennas on Class C
stations, and I'm not aware of any using DAs at night.
Class D: These are those Class II and Class III stations
which used to be (or still are) daytimers. No new Class D stations are
being licensed, however existing Class B stations are permitted to change to
Class D. (this seems to happen when stations lose their lease on a lot and
can't find another place to re-erect their nighttime directional antenna)
Class D stations must use between 250w and 50kw during the day, and less
than 250w at night. (if they have more than 250w at night, they become
Class B) The daytime service area of a Class D station is protected from
interference, but its nighttime service is not protected.