I know that AM signals tend to behave more powerfully at night. I’m curious to know whether AM stations in the same time zone are required by the FCC to turn down their wattage at a specified time. Or is it up to the discretion of the individual stations.
The reason I ask is because I listen to a particular AM station on the way home from work every night, and I can always tell when they turn down their power(around 4:45 PM, this time of year).
The time that is mandated by the FCC is (roughly) sunset (with resumption of full power at sunrise). For simplicity of scheduling, most stations set a schedule that changes through the year, but only shifts every few weeks, rather than using an ephemeris to calculate each day’s sunset/sunrise and then watching the clock each day to avoid upsetting the FCC.
(The FCC has published a guidebook, by month, for their sunrise and sunset times, by region, and I suspect that the radio stations use it to avoid trouble.)
The FCC maintains a page describing the basic rules at
http://www.fcc.gov/mb/audio/bickel/daytime.html
From that page, you can link to the AM Query page that lets you look up various radio stations by license, call sign, regions, or other search arguments. Once you have found the one-line response to the query, clicking on the hypertext call sign opens that station’s complete information page–including the FCC-determined daylight and nighttime hours of operation.
Thank you Tom.
[related hijack]
Why do these stations turn down their power?
Sorry!! Should have read that link. Its all explained there.
I must read links before posting.
I must read links before posting.
I must read links before posting.
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Non-U.S. stations, of course, aren’t subject to the jurisdiction of the FCC, and some keep pumping out the watts all night. As a kid growing up in NY, I recall picking up AM stations from places like West Africa in the early evening!
Or Wolfman Jack broadcasting out of Mexico at a jillion watts being heard all the way to Canada.
Rock and roll, people. Rock and roll.
In the U.S., each month’s sunset and sunrise times create 15-minute wide time bands which go across the country. “DXer’s” – hobbyists who try to pick up distant stations – have put together maps that show the sunrise and sunset bands for each month. I have put of copy of the January map at: http://www.ipass.net/~whitetho/lss.gif (On this map, the solid lines show sunset times, and the dashed lines sunrise.)
This explains why the sacramento AM station I listen to in the bay area is really only listenable during the daytime.
Some AM stations also broadcast in a directional pattern, using multiple antenna arrays. They may also be required to have different patterns for day and night, again, to reduce the chance of interference with other stations.
I worked at one AM station where the pattern was so “tight” there was a dead spot in reception barely a mile from the antenna.
