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Radio Waves
Radio is actually a form of light that exists below the visible spectrum. Like visible light, radio travels as a wave. Imaging the shape of a wave. The highest point that the wave ever reaches is called the crest and the lowest is called the trough. The term "amplitude" refers to the distant between a wave's crest and trough. "Frequency" is a measurement of the number of times a wave's crest and trough would pass a fixed point in a certain amount of time. So, a graph of a radio wave with a higher frequency would look like a series of small waves packed together, while a low frequency wave would have relatively smooth contours.
AM Vs. FM
AM and FM radio transmissions send information by altering the way a radio wave is shaped. AM works by adjusting, or modulating, the amplitude of the radio wave. Frequency Modulation instead adjusts the waves frequency. Devices that receive the signals interpret and reproduce them as sound.
FM History
When broadcast radio technology was originally developed, engineers favored AM radio because it was too difficult to produce clear sound via frequency modulation. At the beginning of the 20th century, Edwin Armstrong made significant improvements to FM radio, mainly by adjusting the bandwidth it used. This new, clear FM radio offered many advantages over AM.
FM Advantages
FM radio is both less expensive to broadcast and easier for mobile receivers to reproduce accurately. The main reason for these advantages is that FM radio can weather changes in signal strength without signal distortion. As a signal becomes stronger or weaker, its amplitude changes but the frequency remains the same. This means that an FM receiver is more capable of producing a stable sound when moving than an AM receiver.