Editing Co-channel interference (section)

== Daytime vs nighttime ==
In the [[medium frequency]] portion of the radio spectrum where most [[AM broadcasting]] is allocated, signals propagate full-time via [[Ground wave propagation|groundwave]] and, at nighttime, via [[skywave]] as well.  This means that during the nighttime hours, co-channel interference exists on many AM radio frequencies due to the [[medium wave]]s reflecting off the ionosphere and being bounced back down to earth.  In the [[United States]], [[Canada]], [[Mexico]], and [[the Bahamas]], there are [[North American Regional Broadcasting Agreement|international agreements]] on certain frequencies which allocate "[[clear-channel station|clear-channel]]" broadcasting for certain stations to either have their respective frequencies to themselves at night, or to share their respective frequencies with other stations located over hundreds or even thousands of miles away.  On other frequencies, there are "Regional Channels" where most stations on these frequencies either reduce power or change to a [[phased array|directional antenna]] system at nighttime to help reduce co-channel interference to each other's signals.  In the [[United States]], there are six "Local Channel" frequencies, also known as "graveyarders" where nearly every station on those frequencies has the same power and antenna pattern both day and night and, as a result of skywave propagation, there is normally massive co-channel interference in rural areas on these frequencies, often making it difficult, if not impossible, to understand what's being said on the nearest local station on the respective channel, or the other distant stations which are bouncing on the same channel, during the nighttime hours. Skywave has been used for long distance AM radio reception since radio's inception and should not be construed as a negative aspect of AM radio. FCC deregulation allowed many new AM radio stations on the former clear and regional channel designations; this is the principal cause of overcrowding on the AM band at night. A new source of interference on the AM broadcast band is the new digital broadcast system called HD, any AM station that broadcasts HD superimposes digital "hash" on its adjacent channels. This is especially apparent at night as some stations, for example WBZ transmits its 30&nbsp;kHz wide signal for hundreds of miles at night causing documented interference and covering another station on an adjoining frequency (WYSL 1040) as far as 400 miles away.