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How are Electromagnetic signals specified?

  1. Mar 30, 2013 #1
    I mean, what all can you modify in an electromagnetic wave? Wavelength and amplitude right? So, for example, why does my garage opener open my garage door and not my neighbor's? Does every single wireless device have its own specified wavelength that is just fractions of meter off from others?
     
  2. jcsd
  3. Mar 30, 2013 #2

    mfb

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    Different types of devices usually have different frequencies (=different wavelengths). Different devices of the same type can have the same frequency: with amplitude modulations, you can transmit a lot of data (like a code to open a specific garage), so multiple devices at the same frequency can still work, if they don't send at the same time at the same location.
     
  4. Mar 30, 2013 #3

    sophiecentaur

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    It is rare to use a single frequency (CW, or Carrier Wave) transmission because it does not carry any information (other than the fact that the transmitter is switched on). It is normal to 'Modulate' a Carrier Wave with information - such as TV signals, Sound signals or data. There are many different ways of modulating a CW to carry such information. Amplitude and Frequency Modulation were all that was available until quite recently but nowadays, it is much more efficient use of spectrum space to use digital modulation and there are a myriad systems for this.
    At one time, in history, such devices would have been operated at a number of different frequencies but nowadays it is more common to use just a single frequency and a set of digital codes, transmitted on identical frequencies from all controllers but each code applies to just one target system. The codes are complicated enough not to risk chance operation by a different controller. Interference between controllers would not matter as they are only operated in short bursts.
     
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