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[تنبيه - عنوان غير مناسب : ]السلام عليكم

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AM && FM waves To understand some of the concepts we introduced in the last module, as well as some of the strange things that happen in broadcasting, we need to take a look at how radio works. First, let's look at the AM radio band (group of frequencies). AM stands for amplitude modulation, which will be explained later. AM radio ranges from 535 to 1705kHz (kilohertz, or thousands of cycles per-second of electromagnetic energy). These are the numbers you see on your AM radio dial. Note in the tan area at the left of the illustration below that AM radio waves are of a lower frequency than either FM radio or TV waves. Thus, as we will see, after being transmitted, they will behave differently.   Stations can theoretically be placed every 10kHz, along the AM band. This means that there are a total of 117 different channels available for AM radio stations. If it all stopped there, things would be rather simple; but, unfortunately, a lot of other factors come into play.   First, you can't put stations on the same frequency that are too close together in a geographic area. They will interfere with each other. And for the same reason you can't have two stations close together in frequency (close to each other on the radio dial) in the same area. So these are the first things that limit the number of radio stations in an area. The good news is that since the signals of stations tend to be limited in their range, you can use some of the frequencies many times — as long as the stations are far enough apart geographically. This is why the United States can have nearly 5,000 AM radio stations on only 117 different frequencies. .. How far an AM station's signal travels depends on such things as the station's frequency (channel), the power of the transmitter in watts, the nature of the transmitting antenna, how conductive the soil is around the antenna (damp soil is good; sand and rocks aren't), and, a thing called ionospheric refraction. The ionosphere (see illustration below) is a layer of heavily charged ion molecules above the earth's atmosphere. Are you still there? Okay, let's go on. Ionospheric refraction is a big issue, because AM radio waves can end up hundreds and even thousands of miles away from where they started, and in the process interfere with all other stations on the same frequency. But, as we'll see in a later module on international shortwave, ionospheric refraction can be good, because it makes possible long-distance communication. Here's how that works.   Note that for AM radio stations the ground wave (in light blue above) doesn't go very far. This means numerous stations can be put on the same frequency without interfering with each other — assuming they are far enough apart. (Keep in mind that this drawing can't be anywhere near close to scale and show these things.) The problem arises — if you want to see it as a problem — is the sky wave can end up in other states, provinces, or even in other countries. The ionosphere is much more effective in reflecting these radio waves at night. (Incidentally, technically, it's refracting, not reflecting, but the effect is somewhat the same.) That's why at sunset most AM radio stations in the U.S. have to: reduce power   directionalize their signal (send it more in some directions than others), or   go off the air (sign off until sunrise the next day) This may explain why your favorite AM radio station goes off the air at sunset, or becomes much harder to hear (because of reduced power). .. FM & TV waves FM (frequency modulated) radio and TV waves don't act in the same way as AM radio waves. For starters, ▲ they are on a higher frequency in the RF spectrum  (The name RF, for radio frequency, was obviously named for radio, but when TV came along they just stuck with the name). The FM radio band goes from 88 to 108 MHz (megahertz, or millions of cycles per second). Again, you can see these numbers on your FM radio dial. FM stations must be 200kHz apart at these frequencies, which means that there's room for 200 FM stations on the FM band. But, unlike AM radio stations, FM stations don't end up being assigned frequencies with nice round numbers like 820 or 1240. Thus, an FM station may be at 88.7 on the dial. You may have noticed that FM stations don't reduce power or sign off the air at sunset. Because of their higher frequency ionospheric refraction doesn't appreciably affect FM or TV signals.   For the most part, FM and TV signals are line-of-sight. Although this means that FM stations don't interfere with each other, this characteristic creates a couple of other problems. First, these waves go in a straight line and don't bend around the earth as AM ground waves do. Thus, they can quickly disappear into space. So, the farther away from the FM or TV station you are, the higher you have to have an antenna to receive the FM or TV signal. Note that the earth is round — we hope this doesn't come as a shock to anyone — and, therefore, these signals will literally leave the earth after 50 miles or so.   And, there's another problem. Since FM and TV signals are line-of-sight, they can be stopped or reflected by things like mountains and buildings. In the case of solid objects like buildings, reflections create ghost images in TV pictures and that "swishing sound" when you listen to FM radio while driving around tall structures. Of course, the higher the FM or TV transmitter antennas are the greater area they will cover — which explains why these antennas are commonly very tall, or placed on the top of mountains. AM radio doesn't need that kind of advantage, since, as we've seen, AM radio waves don't behave in the same way. Note also from the drawing above that FM and TV signals tend to go through the ionosphere rather than being refracted form it. Again, this means that no matter what the station's power, it's signal will at some point leave the earth. BASIC DIF. BETWEEN AM && FM We need to mention a couple of other things before we leave the discussion of how radio works. We've talked about AM and FM radio, but we haven't explained the real difference. In fact, there is a lot of difference — and not just a difference in the station numbers on your radio dial. The first type of radio service — the one we've been talking about in the last couple of modules — was AM radio. The term modulation refers to how sound is encoded on a radio wave called a carrier wave; or, more accurately, how the sound affects the carrier wave so that the original sound can later be detected by a radio receiver. In the top-left of this drawing the RF energy (carrier wave) is not modulated by any sound. There would be silence on your radio receiver. Sound transmitted by an AM radio station affects the carrier wave by changing the amplitude (height) of the carrier wave, as shown on the left. Unfortunately, this type of modulation is subject to static interference from such things as household appliances — and especially from lightening storms. AM also limits the loud-to-soft range of sounds that can be reproduced (called dynamic range) and the high-to-low sound frequency range (called frequency response, to be explained below).   FM radio, which came along in the 1930s, uses a different approach than AM. It's virtually immune to any type of external interference, it has a greater dynamic range, and it can handle sounds of higher and lower frequencies. This is why music, with its much greater frequency range than the human voice, sounds better on FM radio. Note on the left that when the carrier wave of FM radio is modulated with sound that the distance between the waves, or the frequency of the carrier wave, changes.   Thus, AM radio works by changing the amplitude of the carrier wave and FM radio works by changing the frequency of the carrier wave.   Why do all FM radio station in USA end in an odd number? FM radio stations all transmit in a band between 88 megahertz (millions of cycles per second) and 108 megahertz. This band of frequencies is completely arbitrary and is based mostly on history and whim. Inside that band, each station occupies a 200-kilohertz slice, and all of the slices start on odd number boundaries. So there can be a station at 88.1 megahertz, 88.3 megahertz, 88.5 megahertz, and so on. The 200-kilohertz spacing, and the fact that they all end on odd boundaries, is again completely arbitrary and was decided by the FCC. In Europe, the FM stations are spaced 100 kilohertz apart instead of 200 kilohertz apart, and they can end on even or odd numbers. Why do you hear some radio stations better at night than in the day? Radio waves naturally travel in straight lines, so you would naturally expect (because of the curvature of the earth) that no radio station would transmit farther than 30 or 40 miles. And that is exactly the case for ground-based (as opposed to satellite) TV transmissions. The curvature of the earth prevents ground-based TV transmissions from going much further than 40 miles (64 km).  Bruno Vincent/Getty Images Ground-based radio signals are influenced by the ionosphere. Certain radio stations, however, especially the short-wave and AM bands, can travel much farther. Short-wave can circle the globe, and AM stations transmit hundreds of miles at night. This extended transmission is possible is because of the ionosphere -- one of the layers of the atmosphere. It is called the ionosphere because when the sun's rays hit this layer, many of the atoms there lose electrons and turn into ions.  As it turns out, the ionosphere reflects certain frequencies of radio waves. So the waves bounce between the ground and the ionosphere and make their way around the planet. The composition of the ionosphere at night is different than during the day because of the presence or absence of the sun. You can pick up some radio stations better at night because the reflection characteristics of the ionosphere

الموضوع بالمرفقات ... وشكرا لكم

الموضووع بلمرفقات... وشكرا لكم

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