Section outline

  • General

    Collapse all Expand all

  • 无线电波

    A boy enjoys music from a radio, dancing with a joyful expression.

    William can’t help rocking out when his favorite song starts playing on the radio. Do you like to listen to the radio? Did you ever wonder how music travels from a radio station to a radio receiver? How do the sounds travel through air and buildings and everything else in between you and the station? The answer is by .  
    ::当他最喜欢的歌曲开始在电台播放时,威廉不能不摇摆。你喜欢听电台吗?你有没有想过音乐如何从电台传到收音机?这些声音如何通过空气和建筑物以及你和电台之间的一切?答案是:

    A Spectrum of Waves
    ::波浪的光谱

    Electromagnetic waves consist of vibrating electric and magnetic fields. They transfer across space as well as through matter. Electromagnetic waves vary in their and frequencies, and higher-frequency waves have more energy. The full range of wavelengths of electromagnetic waves is called the . It is outlined in the following Figure  .
    ::电磁波由振动电磁场和磁场组成,在空间和物质之间转移。电磁波的频率和频率各不相同,高频波的能量也更多。电磁波的波长全部称为.如下图所示。

    Visual representation of the electromagnetic spectrum with various wave types and corresponding wavelengths.

    Introducing Radio Waves
    ::采用无线电波

    Electromagnetic waves on the left side of the Figure  are called radio waves. Radio waves are electromagnetic waves with the longest wavelengths. They may have wavelengths longer than a soccer field. They are also the electromagnetic waves with the lowest frequencies. With their low frequencies, they have the least energy of all electromagnetic waves. Nonetheless, radio waves are very useful. They are used for radio and television broadcasts and many other purposes. 
    ::图左侧的电磁波称为无线电波,无线电波是波长最长的电磁波,波长可能比足球场长,也是频率最低的电磁波,由于频率低,无线电波在所有电磁波中的能量最小,但无线电波非常有用,用于无线电和电视广播以及许多其他用途。

     

     

    Q: Based on the electromagnetic spectrum Figure  , what is the range of frequencies of radio waves?
    ::问题:根据电磁频谱图,无线电波频率的范围是多少?

    A: The range of frequencies of radio waves is between 105 and 1012 Hz, or waves per second.
    ::A:无线电波频率范围在105至1012赫兹之间,或每秒波。

    AM and FM Radio
    ::调调电台和调频电台

    In radio broadcasts, sounds are encoded in radio waves, and then the waves are sent out through the atmosphere from a radio tower. A radio receiver detects the waves and changes them back to sounds. You may have listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts.
    ::在无线电广播中,声音在无线电波中编码,然后波浪从无线电塔通过大气层发出,无线电接收器探测波浪,将其改变为声音,你可能已经听到调频电台和调频电台。无线电波中的声音编码方式与调频和调频广播不同。

    • AM stands for a mplitude m odulation. In AM broadcasts, sound signals are encoded by changing the amplitude , or maximum height, of radio waves. AM broadcasts use longer wavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in the Figure  . Because the waves are reflected, they can reach radio receivers that are very far away from the radio tower.
      ::AM 代表振幅调控。 在 AM 广播中, 音频信号是通过改变无线电波的振幅或最大高度来编码的。 AM 广播使用比调频广播更长波长的无线电波。 由于其波长, AM 波反射出一个叫做电离层的高层大气层。 您可以在图中看到这种情况是如何发生的。 由于波被反映, 它们可以到达离无线电塔很远的无线电接收器 。
    • FM stands for f requency m odulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually produce clearer sounds than AM broadcasts. However, the relatively short wavelengths of FM waves means that they don’t reflect off the ionosphere as AM waves do. Instead, FM waves pass through the ionosphere and out into space. This is also shown in the Figure  . As a result, FM waves cannot reach very distant receivers.
      ::在调频广播中,调频信号通过改变无线电波的频率来编码。调频调频使调频波能够比振幅调控来编码更多的信息,因此调频广播通常比调频广播更清晰。然而,调频波的相对短波长意味着它们不会像调频波那样从电离层反射出来。相反,调频波穿过电离层进入空间。图中也显示了这一点。因此,调频波无法到达非常远的接收器。

    AM and FM radio waves reflecting off the ionosphere and their paths.

    Q: The composition of the ionosphere changes somewhat from day to night. The changes make the nighttime ionosphere even better at reflecting AM radio waves. How do you think this might affect the AM radio waves travel at night?
    ::Q: 电离层的构成从白天到晚上有所变化。 这些变化使得夜间电离层更能反映AM无线电波。 你认为这会如何影响AM无线电波在晚上的运行?

    A: With greater off the ionosphere, AM waves can travel even farther at night than they can during the day. Radio receivers can often pick up radio broadcasts at night from cities that are hundreds of miles away.
    ::A:随着电离层的扩大,AM波可以在夜间比白天更远的距离更远。 无线电接收器常常可以在晚上从数百英里外的城市接收无线电广播。

    Television
    ::电视电视电视电视电视

    Television broadcasts also use radio waves (see Figure  ). For TV broadcasts, sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded waves are broadcast from a TV tower. When the waves are received by television sets, they are decoded and changed back to sounds and pictures.
    ::电视广播也使用无线电波(见图 ) : 在电视广播中,声音以频率调控编码,图片以振幅调控编码,电波由电视塔广播,波浪由电视机接收,波浪被解码并改成声音和图片。

    Summary
    ::摘要

    • Electromagnetic waves vary in their wavelengths, frequencies, and energy levels. The full range of electromagnetic waves makes up the electromagnetic spectrum.
      ::电磁波的波长、频率和能量水平各不相同。电磁波构成电磁波的频谱。
    • Radio waves are electromagnetic waves with the longest wavelengths, lowest frequencies, and least amount of energy. They are used for radio and television broadcasts and many other purposes.
      ::无线电波是电磁波,波长最长,频率最低,能量最少,用于无线电和电视广播及许多其他目的。
    • In radio broadcasts, sounds are encoded in radio waves by changing either the amplitude (AM) or frequency (FM) of the waves. The encoded waves are broadcast from a tower and changed back to sounds by radio receivers.
      ::在电台广播中,声音通过改变波浪的振幅或频率(FM)在无线电波中编码,由塔台广播,由无线电接收器转回声音。
    • In television broadcasts, sounds and pictures are encoded in radio waves, broadcast from a tower, and changed back to sounds and pictures by television sets.
      ::在电视广播中,声音和图片以无线电波编码,从塔台广播,并通过电视机改成声音和图片。

    Review
    ::回顾

    1. What are radio waves?
      ::什么是无线电波?
    2. Compare and contrast AM and FM radio broadcasts.
      ::比较和对比调幅和调频无线电广播。
    3. Explain how radio waves are used to encode sounds and pictures in television broadcasts.
      ::解释如何利用无线电波在电视广播中编码声音和图片。