Editing Medium wave (section)

== Use in North America ==
{{See also|North American Regional Broadcasting Agreement}}
Initially, broadcasting in the United States was restricted to two wavelengths: "entertainment" was broadcast at 360 meters (833&nbsp;kHz), with stations required to switch to 485 meters (619&nbsp;kHz) when broadcasting weather forecasts, crop price reports and other government reports.<ref name="erh">{{cite web|url=https://earlyradiohistory.us/buildbcb.htm |title=Building the Broadcast Band |publisher=Earlyradiohistory.us |access-date=2010-05-07}}</ref> This arrangement had numerous practical difficulties. Early transmitters were technically crude and virtually impossible to set accurately on their intended frequency and if (as frequently happened) two (or more) stations in the same part of the country broadcast simultaneously the resultant interference meant that usually neither could be heard clearly. The Commerce Department rarely intervened in such cases but left it up to stations to enter into voluntary timesharing agreements amongst themselves. The addition of a third  "entertainment" wavelength, 400 meters,<ref name="erh" /> did little to solve this overcrowding.

In 1923, the [[United States Department of Commerce|Commerce Department]] realized that as more and more stations were applying for commercial licenses, it was not practical to have every station broadcast on the same three wavelengths. On 15&nbsp;May 1923, Commerce Secretary [[Herbert Hoover]] announced a new bandplan which set aside 81 frequencies, in 10&nbsp;kHz steps, from 550&nbsp;kHz to 1350&nbsp;kHz (extended to 1500, then 1600 and ultimately 1700&nbsp;kHz in later years). Each station would be assigned one frequency (albeit usually shared with stations in other parts of the country and/or abroad), no longer having to broadcast [[Weather forecasting|weather]] and government reports on a different frequency than entertainment. Class A and B stations were segregated into sub-bands.<ref>{{cite book|title=Stay tuned: a history of American broadcasting|year=2002|publisher=Psychology Press|isbn=0-8058-2624-6|author=Christopher H. Sterling |author2=John M. Kittross|page=95}}</ref>

In the US and Canada the maximum transmitter power is restricted to 50&nbsp;kilowatts, while in [[Europe]] there are medium wave stations with transmitter power up to 2&nbsp;megawatts daytime.<ref>{{cite web|url=http://www.mwlist.org/mwlist_quick_and_easy.php?area=1&kHz=540|title=MWLIST quick and easy: Europe, Africa and Middle East|access-date=11 December 2015}}</ref>

Most United States [[AM broadcasting|AM radio]] stations are required by the [[Federal Communications Commission]] (FCC) to shut down, reduce power, or employ a directional antenna array at night in order to avoid interference with each other due to night-time only long-distance [[skywave]] [[Radio propagation|propagation]] (sometimes loosely called ‘skip’).{{citation needed|date=March 2025}} Those stations which shut down completely at night are often known as "daytimers". Similar regulations are in force for Canadian stations, administered by [[Industry Canada]]; however, daytimers no longer exist in Canada, the [[CJDL-FM|last station]] having signed off in 2013, after migrating to the [[FM broadcasting|FM band]].{{citation needed|date=March 2025}}