![]() In 1993, the bandpass expanded to include frequencies up to 1700 kHz. The pioneer AM broadcast service started operation on the low frequencies it still uses, 540 to 1600 kilohertz (kHz). This schematic shows the ionosphere reflecting the waves, though actually they refract along curved arcs when passing through the ionosphere.Īmplitude modulation (“AM”) is the oldest system of commercial broadcast transmission. At night, electrons in the ionosphere's F2 layer can refract radio waves broadcast by AM stations, allowing them to be picked up by receivers many hundreds of miles away. Likewise, on the way back down, the D layer’s usual obstructions are gone, so the waves reach the ground in a well-preserved state - often many hundreds of miles from the transmitting station. So at night, radio waves easily reach the F2 layer, where they are refracted back toward the ground. But at a height of some 30 to 60 miles is the D layer, whose impact on radio propagation, especially at lower frequencies, is essentially negative: it absorbs energy from radio signals passing through it to the F2 layer and weakens them.īut as sunset approaches the D layer rapidly loses ionization and essentially vanishes radio waves are therefore absorbed (depleted) only during the day. The ionosphere’s main refraction layer in the ionosphere, called the F2 layer, is about 180 miles (290 km) above Earth’s surface and is present both day and night. They can either absorb the waves, thus reducing their intensity and reducing signal strength, or they can refract the waves, changing their direction conceptually this is akin to a radio-wave "mirror". The ions present in the ionosphere interact with radio waves in two ways. The ionosphere is composed of a set of tenuous, electrically conductive layers that consist of both neutral and charged particles, extending from altitudes of approximately 30 miles (50 km) to more than 250 miles (400 km). We can thank Earth’s ionosphere for natural long-distance radio reception at night. Note that, at altitudes below about 170 km, the count essentially drops to zero.ĭieter Bilitza et al / Journal of Geodesy (2011) The density of electrons in the ionosphere varies dramatically between day and night. Distant radio stations along and near to the path of totality might briefly experience enhanced propagation, thus making long-distance reception possible during a solar eclipse unlike any other time. Its effect on sunlight’s local intensity is remarkably similar to what happens at sunrise and sunset. Yet, cases like this happen every night.Ī total solar eclipse produces a broad, round area of darkness and greatly reduced sunlight that travels across Earth’s surface in a relatively narrow path during the daytime. This might seem odd if you are listening from Albany, New York, more than 700 miles (1,100 km) from the Windy City. Perhaps you’ve had the experience of driving in your car at night, listening to some program on the AM dial, when the announcer will identify the station as WBBM in Chicago. Should overcast skies prevail over your location on eclipse day, you can still make some interesting observations using an AM radio.ĭramatic changes can take place in radio reception when day changes into night and vice versa. Solar eclipses are more than remarkable visual astronomical phenomena they’re pretty interesting from a radio viewpoint too. ![]() on August 21st, you'll have have a chance to hear the eclipse as it happens. When the Moon's shadow glides across the U.S. ![]()
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