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Simplified rendition of the ionosphere.
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The D layer is dense during daylight and vanishes at night. This makes long-distance sky-wave propagation poor during the daytime and excellent at night on frequencies below about 7 MHz. The D layer s disappearance at dusk is responsible for the dramatic change in the behavior of the AM broadcast band (535 kHz to 1. 605 MHz) that occurs after sunset. The E layer varies in density with the 11-year sunspot cycle. The greatest ionization is during peak sunspot years (1979, 1990, 2001, etc.) and the least is during slack years. Solar flares cause the E layer to form dense regions that return radio waves to earth at frequencies as high as 100 MHz. The E layer can produce communications over distances of hundreds or even thousands of miles. The F layer returns radio waves at all frequencies up to a certain maximum, called the maximum usable frequency (MUF). The MUF varies depending on the sunspot cycle and also on day-to-day solar activity. Reliable communications can usually be had up to about 7 MHz during the night and 15 MHz during the day when sunspot activity is minimum; this increases to perhaps 20 MHz at night and 50 MHz during the day when sunspot numbers are maximum. All the ionospheric modes are called sky-wave propagation. This is the mode responsible for worldwide shortwave communications and for nighttime AM broadcast reception over thousands of miles.
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Tropospheric EM propagation
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At frequencies above about 30 MHz, the lower atmosphere bends radio waves towards the surface (Fig. 27-3). Tropospheric banding or tropo occurs because the index of refraction of air, with respect to EM waves, decreases with altitude. The effect is similar to sound waves hugging the surface of a lake in the early morning, letting you hear a conversation a mile away. Tropo makes it possible to communicate for hundreds of miles when the ionosphere will not return waves to the earth.
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27-3 The lower atmosphere bends EM waves toward the surface.
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Another type of tropospheric propagation is called ducting. It takes place when EM waves are trapped in a layer of cool, dense air sandwiched between two layers of warmer air. Like bending, ducting occurs almost entirely at frequencies above 30 MHz. Still another tropospheric-propagation mode is troposcatter. This takes place because air molecules, dust grains, and water droplets scatter some of the EM field at very high and ultra-high frequencies (above 30 MHz).
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Radio waves can bounce off the aurora (northern and southern lights). This is auroral propagation, and it occurs at frequencies from roughly 15 to 250 MHz. It can take place between stations separated by up to about 2000 miles. Meteors entering the upper atmosphere produce ionized trails that persist for several seconds up to about a minute; these ions reflect EM waves and cause meteor-scatter propagation. This mode allows communication for hundreds of miles at frequencies from 20 to 150 MHz. The moon, like the earth, reflects EM fields. This makes it possible to communicate via earth-moon-earth (EME), also called moonbounce. High-powered transmitters, sophisticated antenna systems, and sensitive receivers are needed for EME. Most EME is done by radio hams at frequencies from 50 MHz to over 2 GHz.
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Any communications receiver, whether analog or digital, audio or video, must do certain basic things well.
Receiver specifications 503
Sensitivity
The sensitivity of a receiver is its ability to recover weak signals and process them into readable data. The most common way to express receiver sensitivity is to state the number of ac signal microvolts at the antenna, needed to produce a given signal-to-noise (S/N) ratio. Sometimes, the signal-plus-noise-to-noise ratio, abbreviated S+N/N, is given. When you look at the specifications table for a radio receiver, you might see, for example, better than 0.3 V for 10 dB S/N. This means that a signal of 0.3 V or less, at the antenna terminals, will result in a S/N ratio of 10 dB. This figure is given as an example only and not as any sort of standard separating the good from the bad. Technological advancements are always improving the sensitivity figures for communications receivers. Besides that, a poor figure for one application, or on one frequency band, might be fine for some other intended use, or on some other frequency band. The front end, or first RF amplifier stage, of a receiver is the most important stage with regard to sensitivity. Sensitivity is directly related to the gain of this stage; but the amount of noise it generates is even more significant. A good front end should produce the best possible S/N or S+N/N ratio at its output. All subsequent stages will amplify the front-end noise output as well as the front-end signal output.
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