This is a list of broadcast station classes applicable in much of North America under international agreements between the United States, Canada, and Mexico. Effective radiated power (ERP) and height above average terrain (HAAT) are listed unless otherwise noted.
All radio and television stations within of the US-Canada or US-Mexico border must get approval by both the domestic and foreign agency. These agencies are Industry Canada/Canadian Radio-television and Telecommunications Commission (CRTC) in Canada, the Federal Communications Commission (FCC) in the US, and the Telecommunications Regulatory Commission (CRT) in Mexico.
AM
Station class descriptions All domestic (United States) AM stations are classified as A, B, C, or D.
A (formerly I)—clear-channel stations—10 kW to 50 kW, 24 hours. Class A stations are only protected within a radius of the transmitter site. The old Class I was divided into three: Class I-A, I-B, and I-N. NARBA distinguished between Class I-A, which were true clear-channel stations that did not share their channel with another Class I station, and Class I-B, in which a station operated with 50 kW at night but shared its channel with at least one other I-B station, requiring directional operation. This distinction was superseded by the Regional Agreement for the Medium Frequency Broadcasting Service in Region 2 (Rio Agreement), which instituted the ABCD class system in 1983. The former Class I-As are omnidirectional, with the exception of 870 WWL New Orleans and 1030 WBZ Boston, which use directional antennas to put a better signal over their largest population areas. Most former Class I-Bs are directional at night, although a few are also directional during days. (A handful of I-Bs did not have to use directional antennas: 680 KNBR San Francisco, 810 WGY Schenectady, 850 KOA Denver, 940 XEQ Mexico City, 1070 KNX Los Angeles, and 1070 CBA Moncton. KNX and CBA were far enough apart that both could operate without using a directional antenna. XEQ is far enough from Montreal that it did not need a directional antenna. KNBR and KOA are the only Class Is on their frequency but share those frequencies with several Class II-Bs.) Former Class I-N stations exist only in Alaska, where they are too remote to interfere with other clear-channel stations in the contiguous 48 states. They are only held to Class B efficiency standards (although higher efficiency is acceptable). No new Class A stations are licensed in the conterminous United States, although the FCC states it may be possible to license additional Class A stations in Alaska. B (formerly II and III)—regional stations—250 W to 50 kW, 24 hours. Stations on the AM expanded band, 1610 kHz to 1700 kHz, are limited to 10 kW days and 1 kW nights, non-directionally. Several expanded band stations operate DA-N or even DA-2 with up to 10 kW during all hours, after providing proof that such operations will not cause co- or adjacent-channel interference. If under 250 W at night, the antenna must be efficient enough to radiate more than 140.82 mV/m at 1 km. C (formerly IV)—local unlimited-time stations—250 W to 1 kW, 24 hours. Class C stations that were licensed at 100 W are grandfathered. Rare Class Cs operate with directional arrays, such as KYPA and KHCB. D (formerly II-D, II-S, III-S)—daytimers—daytime 250 W to 50 kW, nighttime under 250 W, or off-air. Field strength is limited to 140 mV/m (millivolts per meter) at 1 km. No new class D stations are licensed, with the exception of Class B stations that are downgrading their nighttime operations to Class D (i.e., less than 250 W). The station's daytime operation is then also reclassified as Class D. If a Class D station is on the air at night, it is not protected from any co-channel interference. TIS/HAR—travelers' information stations / highway advisory radio stations—Up to 10 W transmitter output power. Stations within US national parks are licensed by NTIA and not the FCC. Unlicensed broadcasting—(see low-power broadcasting)—100 mW DC input to final amplifier with a maximum length radiator, no license needed, may be measured at edge of campus for school stations and neighborhood broadcasters.
Notes: In the Western Hemisphere's (ITU region 2), medium wave AM broadcasts are on channels spaced 10 kHz apart from 530 kHz to 1700 kHz, with certain classes restricted to subsets of the available frequencies. With few exceptions, Class A stations can be found only on the frequencies of 540 kHz, 640 to 780 kHz, 800 to 900 kHz, 940 kHz, 990 to 1140 kHz, 1160 to 1220 kHz, and 1500 to 1580 kHz. The exceptions are cited in relevant international treaties. While US and Canadian Class A stations are authorized to operate at a maximum of 50,000 watts both day and night (and a minimum of 10,000 watts at night, if grandfathered), certain existing Mexican Class A stations, and certain new Cuban Class A stations are authorized to operate at a higher power. Certain Mexican Class A stations are authorized to operate at less than 50,000 watts at night, if grandfathered, but may operate at up to 100,000 watts during the day. Class B and D stations can be found on any frequencies from 540 kHz to 1700 kHz except where frequencies have been reserved for Class C stations. Class C stations can be found in the lower 48 US states on the frequencies of 1230 kHz, 1240 kHz, 1340 kHz, 1400 kHz, 1450 kHz, and 1490 kHz (commonly known as "graveyard" frequencies). Other countries may use other frequencies for their Class C stations. American territories in ITU region 3 with AM broadcasting stations (Guam and the Northern Mariana Islands) use the 9 kHz spacing customary to the rest of the world. All stations are class B or lower. Canada also defines Class CC (Carrier Current, restricted to the premises) and LP. (less than 100 watts) TIS stations can be found on any frequency from 530 kHz to 1700 kHz in the US, but may only carry non-commercial messages without music. There is a network of TISs on 1710 in New Jersey. Low-power AM stations located on a school campus are allowed to be more powerful, so long as their signal strength does not exceed roughly 14 to 45 μV/m (microvolts per meter) (depending on frequency) at a distance of 30 meters (98.4 ft) from campus.
Former system AM station classes were previously assigned Roman numerals from I to IV in the US, with subclasses indicated by a letter suffix. Replacement class A is equivalent to the old class I; class B is the old classes II and III, with class D being the II-D, II-S, and III-S subclasses; and class C is the old class IV.
The following conversion table compares the old AM station classes with the new AM station classes:
{| class="wikitable" |- ! Old Domestic Station Class ! New Domestic Station Class |- | I | A |- | II | B |- | III | B |- | IV | C |- | II-S | D |- | III-S | D |- | II-D(Daytime Only) | D
AM station classes and clear channels listed by frequency
The following chart lists frequencies on the broadcast company band, and which classes broadcast on these frequencies; Class A and Class B, 10,000 watt and higher (full-time) stations in North America which broadcast on clear-channel station frequencies are also shown.
By international agreement, Class A stations must be 10,000 watts and above, with a 50,000 watt maximum for the US and Canada, but no maximum for other governments in the region. Mexico, for example, typically runs 150,000 to 500,000 watts, but some stations are grandfathered at 10,000 to 20,000 watts at night; by treaty, these sub-50,000 watt Mexican stations may operate with a maximum of 100,000 watts during the daytime.
Because the AM broadcast band developed before technology suitable for directional antennas, there are numerous exceptions, such as the US use of 800 (kHz) and 900 non-directionally in Alaska, limited to 5 kW at night; and 1050 and 1220, directionally, in the continental US, and without time limits; each of these being assigned to specific cities (and each of these being Mexican Class I-A clear channels). In return for these limits on US stations, Mexico accepted limits on 830 and 1030 in Mexico City, non-directionally, restricted to 5 kW at night (both of these being US Class I-A clear channels).