AM Broadcasting - Operation

Operation

AM radio technology is simpler than frequency modulated (FM) radio, Digital Audio Broadcasting (DAB), satellite radio or HD (digital) radio. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification.

In North American broadcasting practice, transmitter power input to the antenna for commercial AM stations ranges from about 250 to 50,000 watts. Experimental licenses were issued for up to 500,000 watts radiated power, for stations intended for wide-area communication during disasters including Cincinnati station WLW, which used such power on occasion before World War II. WLW's superpower transmitter still exists at the station's suburban transmitter site, but it was decommissioned in the early 1940s and no current commercial broadcaster in the U.S. or Canada is authorized for such power levels. Some other countries do authorize higher power operation (for example the Mexican station XERF formerly operated at 250,000 watts). Antenna design must consider the coverage desired and stations may be required, based on the terms of their license, to directionalize their transmitted signal to avoid interfering with other stations operating on the same frequency.

Medium-wave (medium frequency, MF) and short-wave (high frequency, HF) radio signals act differently during daytime and nighttime. During the day, MF signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred miles (or kilometers) from the signal transmitter. However, after sunset, changes in the ionosphere cause MF signals to travel by skywave, enabling radio stations to be heard much farther from their point of origin than is normal during the day. This phenomenon can be easily observed by scanning the medium wave radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. Such stations are commonly referred to as daytimers. In Australia medium wave stations are not required to reduce their power at night and consequently stations such as the 50,000-watt 3LO can be heard in some parts of New Zealand at night.

In the United States and Canada, some radio stations are granted clear channel status, meaning that they broadcast on frequencies with few other stations allocated, allowing an extended coverage area when skywave propagation takes over at night, starting at or near local sunset. Relatively few stations enjoy clear-channel status. The vast majority of local MW stations rely on ground-wave coverage only, limiting their target market to their own local area. Non-clear channel stations typically have reduced coverage at night, due to noise and a mish-mash of other stations propagating in via skywave after dark. The area covered by a local station at night without significant skywave interference is known as the nighttime interference-free (NIF) contour, and is typically specified in mV/m (signal strength). The higher the NIF value, the stronger the local signal must be to override nighttime interference, resulting in a smaller coverage area and fewer listeners able to hear the station without interference.

The hobby of listening to long distance signals is known as DX or DX'ing, from an old telegraph abbreviation for "distance". Several nonprofit hobbyist clubs are devoted exclusively to DXing the AM broadcast band, including the National Radio Club and International Radio Club of America. Similarly, people listening to short wave transmissions are SWLing.