In standard broadcast (AM), stations in the U.S. are classified by the FCC according to their operating power, protection from interference, and hours of operation.

A Class A station operates with 10–50 kW of power servicing a large area with primary, secondary, and intermittent coverage and is protected from interference both day and night.

These stations are called clear channel stations because the channel is cleared of nighttime interference over a major portion of the country. Class B stations operate full time with transmitter powers of 0.25–50 kW and are designed to render primary service only over a principal center of population and contiguous rural area.

Whereas nearly all Class A stations operate with 50 kW, most Class B stations must restrict their power to 5 kW or less to avoid interfering with other stations. Class B stations operating in the 1605–1705 kHz band are restricted to a power level of 10 kW daytime and 1 kW nighttime.

Class C stations operate on six designated channels (1230, 1240, 1340, 1400, 1450, and 1490) with a maximum power of 1 kW or less full time and render primarily local service to smaller communities. Class D stations operate on Class A or B frequencies with Class B transmitter powers during daytime, but nighttime operation, if permitted at all, must be at low power (less than 0.25 kW) with no protection from interference.

Although Class A stations cover large areas at night (approximately a 1000 km radius), the nighttime coverage of Class B, C, and D stations is limited by interference fromother stations, electrical devices, and atmospheric conditions to a relatively small area.

Class C stations, for example, have an interference-free nighttime coverage radius of approximately 8–16 km. As a result there may be large differences in the area  that the station covers daytime vs. nighttime. With over 4900 AM stations licensed for operation by the FCC, interference, both day and night, is a factor that significantly limits the service stations may provide.

In the absence of interference, a daytime signal strength of 2 mV/m is required for reception in populated towns and cities, whereas a signal of 0.5 mV/m is generally acceptable in rural areas without large amounts of man made interference.

Secondary nighttime service is provided in areas receiving a 0.5-mV/m signal 50% or more of the time without objectionable interference. Table 16.1 indicates the interference contour overlap limits. However, it should be noted that these limits apply to new stations and modifications to existing stations. Nearly every station on the air was allocated prior to the implementation of these rules with interference criteria that were less restrictive.

TABLE 16.1 Protected Service Signal Intensities for Standard Broadcasting (AM)
Signal Strength Contour of Area
Protected from Objectionable Permissible
Interferencea (μV/m) Interfering Signal
0 Class of Power Class of
Station (kW) Channel Used Dayb Night Dayb Nightc
A 10–50 Clear SC 100 SC 500 50% SW SC 5 SC 25
AC 500 AC 500 GW AC 250 AC 250
B 0.25–50 Clear 500 2000b 25 25
Regional AC 250 250
C 0.25–1 Local 500 Not precised SC 25 Not precise
D 0.25–50 Clear 500 Not precise SC 25 Not precise
Regional AC 250
Note: SC = same channel; AC = adjacent channel; SW = skywave; GW = groundwave; RSS = root of sum squares.
aWhen a station is already limited by interference from other stations to a contour of higher value than that normally
protected for its class, this higher-value contour shall be the established protection standard for such station. Changes
proposed by Class A and B stations shall be required to comply with the following restrictions. Those interferers that
contribute to another station’s RSS using the 50% exclusion method are required to reduce their contribution to that RSS
by 10%. Those lesser interferers that contribute to a station’s RSS using the 25% exclusionmethod but do not contribute
to that station’s RSS using the 50% exclusion method may make changes not to exceed their present contribution.
Interferers not included in a station’s RSS using the 25% exclusion method are permitted to increase radiation as long
as the 25% exclusion threshold is not equaled or exceeded. In no case will a reduction be required that would result in a
contributing value that is below the pertinent value specified in the table.
cSkywave field strength for 10% or more of the time. For Alaska, Class SC is limited to 5 μV/m.
dDuring nighttime hours, Class C stations in the contiguous 48 states may treat all Class B stations assigned to 1230,
1240, 1340, 1400, 1450, and 1490 kHz in Alaska, Hawaii, Puerto Rico and the U.S. Virgin Islands as if they were class C
Source: FCC Rules and Regulations, revised 1991; vol. III, pt. 73.182(a).

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