AM/FM Radios

  1. Hey guys i was just doing a bit of poking around and i found this site. I was wondering if you lot could help me. You see i have to compare and contrast two types of similar communication. i chose AM/FM radio transmitions

    I know some of the bare basics like:
    They transmit by minipulating the radio waves (am by amplitude and fm by frequency)

    AM can have longer range becuase it can reflect off the ionosphere. And FM transmits in a straight line.

    FM has better sound quality then AM. but AM has more range.

    I was just curoius as to why FM and AM have different quality is it to do with the frequency range of the way the waves are minipulated? And what are some other advangates or disadvantages to using the different frequencies


    PS. this is the first time ive posted so if its in the wrong spot im sorry.
     
  2. jcsd
  3. chroot

    chroot 10,427
    Staff Emeritus
    Science Advisor
    Gold Member

    An AM signal is broadcasted with a single specific frequency, called the carrier frequency. The amplitude of this signal is varied (modulated) by a voice signal with a much lower range of frequencies.

    An FM signal is broadcasted with a specific amplitude, and a low-frequency voice signal is used to modulate the frequency.

    FM signals are more immune to noise, because the information is contained in the frequency of the received signal, rather than its amplitude. Amplitude is strongly affected by, for exampe, passing a building, whereas frequency is not.

    All of the other characteristics you mention -- reflectivity, line-of-sight, range, etc. actually have nothing to do with the modulation technique used (AM vs. FM). As it happens, in North America, spectrum in the 1 MHz range is used for commercial AM broadcasts, but spectrum in the 100 MHz range is used for commercial FM broadcasts. All of the propagation differences are due to the difference in frequency, not in the difference in modulation technique.

    Lower-frequency (longer-wavelength) waves can diffract around larger obstacles, while high-frequency waves are simply stopped by the same obstacles. This is why AM (~1 MHz, 300 m wavelength) signals can diffract around a building, still producing a usable signal on the other side, while FM (~100 MHz, 3 m wavelength) signals essentially require a line-of-sight path between transmitter and receiver.

    - Warren
     
  4. OK yeah that helps heaps.

    I was just wondering though what is it thats significant about the lower frequencies that casue them to defract around said buildings and objects?
     
  5. chroot

    chroot 10,427
    Staff Emeritus
    Science Advisor
    Gold Member

    Lower frequencies imply larger wavelength. Waves can diffract around objects approximately their own size. Buildings and other man-made structures have sizes on the order of tens or hundreds of meters, and 1 MHz waves can diffract around them. 100 MHz waves have wavelengths on the order of 3 meters, and cannot diffract around buildings that are many times that size.

    - Warren
     
  6. Lower frequencies can reach farther than high frequency waves. My example: Ship's low freq. foghorn.
     
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