<P> The signal is subject to interference from electrical storms (lightning) and other electromagnetic interference (EMI). One advantage of AM radio signal is that it can be detected (turned into sound) with simple equipment . If a signal is strong enough, not even a power source is needed; building an unpowered crystal radio receiver was a common childhood project in the early decades of AM broadcasting . </P> <P> AM broadcasts occur on North American airwaves in the medium wave frequency range of 525 to 1705 kHz (known as the "standard broadcast band"). The band was expanded in the 1990s by adding nine channels from 1605 to 1705 kHz . Channels are spaced every 10 kHz in the Americas, and generally every 9 kHz everywhere else . </P> <P> AM transmissions cannot be ionospherically propagated during the day due to strong absorption in the D - layer of the ionosphere . In a crowded channel environment, this means that the power of regional channels which share a frequency must be reduced at night or directionally beamed in order to avoid interference, which reduces the potential nighttime audience . Some stations have frequencies unshared with other stations in North America; these are called clear - channel stations . Many of them can be heard across much of the country at night . During the night, absorption largely disappears and permits signals to travel to much more distant locations via ionospheric reflections . However, fading of the signal can be severe at night . </P> <P> AM radio transmitters can transmit audio frequencies up to 15 kHz (now limited to 10 kHz in the US due to FCC rules designed to reduce interference), but most receivers are only capable of reproducing frequencies up to 5 kHz or less . At the time that AM broadcasting began in the 1920s, this provided adequate fidelity for existing microphones, 78 rpm recordings, and loudspeakers . The fidelity of sound equipment subsequently improved considerably, but the receivers did not . Reducing the bandwidth of the receivers reduces the cost of manufacturing and makes them less prone to interference . AM stations are never assigned adjacent channels in the same service area . This prevents the sideband power generated by two stations from interfering with each other . Bob Carver created an AM stereo tuner employing notch filtering that demonstrated that an AM broadcast can meet or exceed the 15 kHz baseband bandwidth allotted to FM stations without objectionable interference . After several years, the tuner was discontinued . Bob Carver had left the company and the Carver Corporation later cut the number of models produced before discontinuing production completely . </P>

What are the mechanics in operating a radio station