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why frequency modulated signal covers small area as compare to AM signal ?

user25665
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    It doesn't do any such thing! – Leon Heller Jun 24 '13 at 19:00
  • See http://electronics.stackexchange.com/q/39423/2028 – JYelton Jun 24 '13 at 19:07
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    Please clarify your question. Are you referring to commercial radio stations in the US and the common observation that FM stations tend to cover less geographical area than AM stations? – JYelton Jun 24 '13 at 19:11
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    I think you've confused modulation method with the propagation of radio waves and transmitter power. FM tends to be used at high frequency because of the bandwidth requirements. But there are some powerful VHF stations that cover a wide area. – JIm Dearden Jun 24 '13 at 19:41
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    On what planet is the "duplicate" question a duplicate? *What is the basic difference between AM and FM radio?* discusses differences in the modulation technique. It says *nothing* about the useful range of the signals, which is what this question is about. – Phil Frost Jun 25 '13 at 00:02
  • @ jim : FM is also used the 10 meter band for repeaters. It is not on the aircraft bands uhf/vhf due to doppler shift issues – Old_Fossil May 18 '16 at 07:22

1 Answers1

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The difference has less to do with the modulation, and more with the frequencies used. FM broadcast radio in the United States operates from 87800 to 108000 kHz. The most common AM radio operates from 540 to 1610 kHz. There are other areas of spectrum reserved for AM frequencies globally, but they are all much lower in frequency than those used for FM broadcast.

These different frequencies of electromagnetic radiation react differently with Earth and its atmosphere. At low frequencies, radio waves propagate primarily by ground wave. This is a combination of waves propagating as a surface wave on the moderately conductive Earth, as well as refraction by the lower levels of the atmosphere bending the waves past the visible horizon.

At low frequencies, the higher, more ionized layers of the atmosphere attenuate radio waves beyond usefulness, so only ground waves are useful for communication. However, as frequencies become higher, this attenuation lessens, and skywave propagation becomes possible. This is caused by the ionosphere refracting waves headed for space back towards the Earth.

The frequency where skywave propagation begins to occur varies by solar weather and time of day. In the day, the D layer of the ionosphere is maximally ionized, limiting skywave propagation to higher bands. At night, the D layer dissipates, allowing lower frequencies to propagate by skywave. The AM broadcast region between 540 to 1610 kHz is low enough that although much of its energy propagates by ground wave, it can, at night, have significant skywave propagation as well.

As frequencies continue to increase, a point is reached where the ionosphere is no longer able to refract the waves back to Earth, and the propagate into space. This frequency depends on conditions, and is called the maximum usable frequency (MUF). The FM broadcast region from 87.8 to 108 MHz is well above the MUF, so communication is limited to line of sight propagation.

Olin Lathrop
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Phil Frost
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