AM Basics

     AM transmitters modulate (or vary) a carrier wave (the basic signal used to "carry" the sidebands that contain the program information) by its amplitude (loudness) rather than its frequency, and do so many thousands of times per second. Seen diagrammatically, AM waves vary in height, indicating power changes in accordance with the signal being transmitted, rather than frequency, as in FM radio. Electronic static, most of which is amplitude modulated in its natural state, cannot be separated from the desired signal, though engineers spent years attempting to do so.

     In the United States, AM channels are 10 kHz wide, whereas in much of the rest of the world by the 1990s, stations were licensed to use 9 kHz (a move to do the same in the United States was defeated by industry pressure in the early 1980s). With careful monitoring, an AM station can transmit from 5,000 to 7,000 cycles per second, which is sufficient for voice and some music, but misses the overtones of true high fidelity sound. On the other hand, AM channels, being narrower, allow far more stations to be accommodated per kHz than is the case with FM.

     In most countries, AM operates on the medium wave frequencies (in the United States, 535 to 1705 kHz). Such a spectrum location means that signals are propagated along and sometimes just beneath the ground (day and night), and by sky waves bouncing off the ionosphere (nighttime only). This process has its benefits and drawbacks. The former comes from the extreme distances a powerful AM signal may travel on a cold, clear, winter evening-1,000 miles and often more. Unfortunately, such transmissions can never be exactly predicted; this leads to frustration in tuning distant stations, and more importantly, interference with other outlets, even though they may be closer to the listener. Further, ground waves and sky waves arrive at the same tuning (listening) point at differ­ ent times (the sky waves having traveled much greater distances), which also causes interference.

     Because of the sky wave problem, more than half of all U.S. AM stations are licensed for daytime operation only (the FCC stopped issuing new daytime-only licenses in 1987). Virtually all remaining stations reduce power in the evening hours as a condition of their licenses. This greatly reduces, but does not eliminate, the sky wave problem. Because so many AM stations were crowded on the air in the half century after World War II, most now must use directional antennas to "steer" their signals away from other stations.

     All of these issues make AM engineering very complex. Because of this situation and the fact that more than 500 stations were on the air when effective regulation was established in 1927, there are no allotted channels in AM, as there are with FM and television services. When applying for a license, a new station must convince the FCC that it will not cause intolerable interference to others in the same or nearby markets-a very difficult and expensive thing to prove.

     In an attempt to reduce interference and add stability to licensing, the Federal Radio Commission (FRC) in 1928 established three different types of AM radio channels: clear, regional, and local (in descending order of power and coverage area). There were few clear and regional channels and hundreds of local ones. Powerful (50,000 watts) clear channel stations were designed to serve large rural regions about 750 miles across and were located at great distances from one another (at first there were no other stations on clear channels at night-hence the term "clear"). Regional stations were less powerful and covered smaller areas; these stations naturally increased as channels were reused by stations in different areas. Finally, local stations reached only 10 or 20 miles, were separated by a few hundred miles, and soon became the most common type of AM outlet, often using lower power at night, with many operating on the same channel in different areas. The system was simplified in the 1980s.

Short History

     AM radio broadcasting originated from the early 20th-century radio telephony experiments of Frank Conrad, Lee de Forest, Reginald Fessenden, Charles Herrold, and others. Early radio broadcast transmitters in the 1920s were manufactured by hand, were hard to adjust and maintain, and delivered uneven and sometimes unpredictable performance. This led to considerable detail in the regulatory requirements established by the FRC after 1927. Stations required a full-time engineer for all the modifications and monitoring required. By 1941 the quality of AM technical equipment was considerably improved.

     Until 1941 all U.S. commercial stations-about 600 to 700 at any one time-operated only with AM transmission and competed only with other AM outlets. The radio industry at this time was small and friendly. While by 1941 the largest cities had a dozen stations, many smaller towns had but one or two and large parts of the country had no local radio service at night. Radio networks dominated programming and advertising with relatively few stations surviving as independent operations. By 1941 there were only a handful of educational stations on the air.

     AM faced its first competition when the Federal Communications Commission (FCC) approved the creation of FM stations at the beginning of 1941; television arrived by the middle of the same year. The U.S. involvement in World War II limited growth of the new services so that most people could only tune in to AM for the duration of the war. After the war, AM grew from about 900 stations in 1945 to some 2,500 by the early 195os-a frenetic rate of growth that illustrated public interest in and demand for more local radio service. By the 1960s, the FCC initiated two different freezes on further AM licenses, steering new applicants to the FM band instead. Any town of any size had a full complement of AM stations with no room for more.

     AM stations dominated the industry through these four decades: there were more of them, they earned the vast majority of radio industry income, and they reached most of the audience most of the time. Most program developments were focused on AM stations, especially the arrival in the 1950s of top-40 rock stations and their many spin-offs in the years that followed. AM station owners controlled broadcast trade associations and saw FM as merely an expansion of what they already offered.

     By 1980, however, AM's competitive situation had changed dramatically. The year before, more people tuned to FM stations for the first time, and the gap continued to widen for the next two decades as FM's higher fidelity won over listeners. FM was expanding faster as well. By the turn of the century, only a quarter to a third of the radio audience regularly tuned to AM outlets. In many cases, AM stations shifted to news and talk formats, abandoning music to higher-fidelity FM. Long dominant, AM had become a minor partner in a still expanding radio business. The number of AM outlets was actually in decline by the 1990s as stations left the air unable to attract sufficient listeners and thus to make a case to advertisers. The AM business was in trouble.

Improving AM

      Faced with signs of this decline, the industry manufacturers and the FCC pressed for relief by improving AM's limited technical capabilities. The first debate concerned stereo transmission, which many in the business thought held great potential for competing with FM. From 1977 to 1980 the AM industry sought to develop an agreed-upon standard for such a service that could be recommended to the FCC. Unable to make a decision among a half dozen mutually incompatible systems by as many companies, in March 1982 the FCC announced that ​​AM broadcasters were expected to establish their own technical standard, although anti-trust laws made it impossible for the industry to overtly collaborate on such a decision. Given this confused situation, few manufacturers built receivers, few stations installed AM stereo transmission capability (about Io percent of all AM stations on the air), and listeners were never given a reasonable opportunity to accept or reject the technology. By 1992 the AM stereo "experiment" was clearly a failure, with two systems (Kahn and Motorola) still contending to be the final choice. Under a congressional mandate to finally make a choice, the FCC in late 1993 picked the Motorola system as a de facto standard. By then it was largely too late-too few stations (and fewer listeners) cared.

     That AM still needed technical improvement in order to compete remained obvious. The FCC asked the National Radio Systems Committee (NRSC), an industry group acting in an advisory capacity to the commission, to aid in the effort, admitting it was "dealing with no less an issue than the survival of the AM service." Heavily criticized for the AM stereo debacle, the FCC appeared eager to demonstrate a real commitment to AM. In a series of decisions, it adopted NRSC-1 and NRSC-2 standards, which would help to reduce AM interference and encourage manufacturers to create improved receivers.

     One AM problem had always been insufficient spectrum space. Despite post-World War II growth, the AM band remained unchanged since 1952. After the International Telecommunication Union approved a recommendation for Western Hemisphere nations to add to their AM radio bands in 1979, the FCC began to shift existing services out of the affected frequencies, and in 1990 began to actively plan for AM station use of the new band (1605-1705 kHz). Most of the new band was allocated to the United States, although both Mexico and Canada received use of some frequencies as well. After deciding not to license new stations (which would merely exacerbate existing interference), in a series of decisions the FCC selected existing AM outlets (based largely on how much interference they caused or received) to shift from lower frequencies into the new space. Assignments began in 1997 and the first station, WCMZ in Miami, was operating in the new frequencies by late that year. However, the newly relocated stations often reached far smaller audiences as old receivers could not tune the new frequencies. Only time and new sets would slowly change this problem.

     At the same time the United States did not adopt another proposal-to narrow AM channels from Io kHz to 9 kHz, a decision that would have allowed hundreds of new stations on the air. Touted in the late 1970s to help bring U.S. standards in line with those of the rest of the world, the idea was shot down by industry arguments that such spacing would increase interference and would make most digital receivers obsolete, as they were calibrated for Io kHz channels. Not as overtly stated was a strong industry belief that the last thing AM radio needed was more competing stations.

     When all was said and done, however, the fact remained that AM was an older technology with an inferior sound to FM and could not accommodate developing digital formats. It could not compete on an equal basis with its newer competitors, and had not been able to for the last two decades of the 20th century, although the seeds of its decline dated back further. To a considerable degree, however, all of these technical "fixes" were band-aid approaches to a fundamental problem that could not be viably addressed without converting AM stations into digital operations.

AM Outside the U.S.

     AM radio appeared in other industrial nations in the 1920s and early 1930s, and somewhat later in several of their colonies. Through World War II, AM and shortwave were the primary radio transmission choices available for broadcasting (there were some longwave stations in operation). Most nations had far fewer stations than the United States because they were smaller and didn't need as many to cover a more limited area, because they lacked advertising revenues to develop and support further stations, and because they often allocated fewer frequencies for broadcasting.

     In tropical regions especially, static (often from thunderstorms) proved a substantial challenge as there was no way to carve the noise away from the desired voice or music signal. Radio stations were often clustered in the national capital and other major cities, with little radio service available in rural regions. In colonial areas, service was aimed primarily at the expatriates of the mother country rather than at indigenous peoples. Indeed, radio was used primarily to help tie the colony to the mother country.

     Well into the 1970s, most foreign radio systems were typically of the public-service type, operated directly by governments or a governmentally supported independent operation (such as Britain's British Broadcasting Corporation (BBC), and supported by fees assessed on receiver sales or ownership. Few carried advertising, and thus entertainment programming was less dominant than in the United States. On the other hand, cultural, experimental, and educational programs of all kinds were far more common on most non-U.S. radio systems.

     Since the 1980s, AM's star has dimmed in Europe for the same reasons the medium became less important in the United States. The benefits of FM (called VHF in most other nations because of the spectrum it uses) radio became more widely understood and adopted, allowing foreign systems to experience greater diversity of content and far more entertainment (typically popular music) formats than had been possible with a handful of AM outlets. Expansion of FM radio and the much greater cost of television so increased overall system costs that most nations modified their AM-based public service systems to allow advertising (either on separate stations, parallel radio systems, or the formerly government-operated outlets). For all of these reasons, radio in other countries sounded far more like radio in the United States by the beginning of the 21st century than it had just a few decades earlier.