Why AM radio stations goes far away in the nigiht?
February 14, 2010 - 3:37 pm
During the day I can listen only to local AM radio stations. During the night even radio from other countries.
Why?
Note.
I am asking about medium wave radio not short wave radio.
There are a couple of different layers of the ionosphere in play here. The lower layer, called the D layer, tends of absorb radio signals during the day. At night, the Sun is gone and the ionization of this layer ceases. The D layer disappears and signals can pass higher up to layers that refract, rather than absorb signals like the E or F layers.
During the day, MF sky wave prorogation is absorbed and you can only get signals via ground wave propagation. At night, you get the sky wave.
February 14th, 2010 at 8:53 pm
DZMM 630 and DZBB 590
References :
February 14th, 2010 at 9:04 pm
Longer-wave stations tend to be reflected, not only by the earth, but also by the ionosphere (roughly 85 km up to 600 km above the surface of Earth). During the day, the solar radiation tends to disrupt the ionosphere a bit, but at night, things smooth out, and the reflection can happen much more easily. In fact, AM station licenses often require AM stations to reduce their power at night, so they don’t swamp other stations. See http://en.wikipedia.org/wiki/AM_broadcasting#Operation
References :
February 14th, 2010 at 9:17 pm
Amplitude modulation ( AM ) radio signals originate from their station antenna and "slam" down along a ground field ( a checkerboard of copper wiring spread along the AM antenna base area, buried just below the ground surface–much like re-bar wiring for a concrete pour area ) and then skip along the atmosphere.
During set daytime hours as per the FCC, AM stations operate with their transmitters going at FULL power, allowing a fixed broadcast radius. At set PM hours, as per the FCC, the AM station’s transmitters operate at HALF or LOW power, causing their same AM frequency to skip along higher atmospheres and in some cases, travel greater distances. Why exactly the FCC demands this be done escapes me.
The FCC was created to strictly regulate broadcast signals, allowing for better communication ability–especially in the event of an emergency. In fact, the FCC’s first director, as a young boy in New York, was one of the first to pick up the wireless Morse distress messages from the Titanic moments after the ship hit the iceberg. He later claimed that too much unregulated radio traffic interfered with the SOS signals, something that lent a hand in how the disaster was handled.
References :
It’s a link for kids–but I think it’ll help better answer further AM radio signal questions: http://www.fcc.gov/cgb/kidszone/faqs_radio.html
February 14th, 2010 at 9:30 pm
The reason why you are sometimes able to pick up a distant AM radio station is a phenomenon called "skip." This occurs after sundown, when AM stations are required to reduce power in accordance with an FCC regulation. As you may know, radio waves are part of the electromagnetic spectrum and travel in a straight line, or "line of sight" manner from the transmitting antenna. Well, when atmospheric conditions are right, the radio wave will bounce off the ionosphere (above the tropospere — where our weather happens, the stratosphere, the mesosphere) and back down toward the ground.
I, too have experienced the effects of "skip" when I was young. We (me, my mom, and my brother) were on our way home in one city from our grandmother’s house in another city about 50 miles away, and were accidentally able to listen to a radio station from near the Gulf (of Mexico) Coast. We thought that was pretty cool.
Long-haul truckers also are able to talk to distant truckers over their CB radios due to skip, sometimes.
References :
http://en.wikipedia.org/wiki/Ionosphere
February 14th, 2010 at 10:20 pm
There are a couple of different layers of the ionosphere in play here. The lower layer, called the D layer, tends of absorb radio signals during the day. At night, the Sun is gone and the ionization of this layer ceases. The D layer disappears and signals can pass higher up to layers that refract, rather than absorb signals like the E or F layers.
During the day, MF sky wave prorogation is absorbed and you can only get signals via ground wave propagation. At night, you get the sky wave.
References :
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