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Why can't a constant electric field be used for transmission of signl?

  1. Aug 8, 2007 #1
    Just one question,
    I understand that for an antenna to transmit an EM signal, a time varying Electric or magnetic field needs to be applied to it, why is that so? I mean what's the physics behind it? why can't a constant non varying electric field be used for the purpose? Can you help me out? I would really appreciate it... :smile:
  2. jcsd
  3. Aug 8, 2007 #2


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    Can you show anything that can transmit information with a constant, non-varying signal?

    Last edited: Aug 8, 2007
  4. Aug 8, 2007 #3


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    To transmit a signal, you need at least 2 conditions that are detectably different. Otherwise it's like writing in black ink on black paper.
    Surely it's obvious that something which has only one state can't be used to convey information ?

    Maybe I've misunderstood your question but this is not physics, it's common sense.

    [simultaneous with Zz]
  5. Aug 8, 2007 #4
    because maxwell's equations say so. a changing Electric field creates a Changing Magnetic field. someone else can you fill you in with more detail than that because i don't know anything about QFT

    hmmm, <strikes a thinking pose>
    Last edited: Aug 8, 2007
  6. Aug 8, 2007 #5


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    I have to agree, though one could refer to such arcana as the "Shannon channel limit", see for instance http://en.wikipedia.org/w/index.php?title=Shannon–Hartley_theorem&oldid=147195906

    But there is no real need to get that complicated to make the point - if one will excuse a bit of american centrism, imagine Paul Revere saying "one if by land, and one if by sea" :-).
  7. Aug 10, 2007 #6
    ah... yeah... sorry for the vague way in which I put it... I now realize that I muddled it up and put up 2 questions together... What I meant for the first question was... why can't a digital signal be used to transmit information on an antenna... that also has 2 levels... So... you would get 2 spikes when the states change... anyway... That was explained to me by my teacher that its not impossible, but the simple thing was that an analog signal would consume less bandwidth...

    Another of my question was.. why does there need to be change in the field to send out an EM wave perpendicularly to the source, apart from the answer that Maxwell's equations require them to... I was wondering why does not a simple constant electric field generate the same kind of waves, and was wondering if there were any explanations for this... thats all...
  8. Aug 10, 2007 #7


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    You're asking: can you transmit digital information by transmitting a wave of a specific frequency, but with two different amplitude states? The answer is yes. You can use a low amplitude to represent 0 and a high amplitude to represent 1. The resulting modulation would be called "amplitude keying." This modulation is extremely inefficient, however, and has very very poor noise immunity. It's theoretically a perfectly valid way to transmit information, but, in the real world, it underperforms compared to the many other options.

    A constant electric field does not create any waves. It's rather like thinking about the surface of a very still pond in winter. If the field is constant, it cannot be conveying any useful information. Of course, you can use the presence or absence of the field to transmit information, but that's an extremely poor way of doing things.

    - Warren
  9. Aug 13, 2007 #8

    Claude Bile

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    Since you brought up the topic of bandwidth, I'll add my own 2c.

    A series of "highs" and "lows" still constitutes a wave, albeit one with many sinusoidal components. The shorter the duration of each "high" and "low" period (and hence the more bits transmitted per second), the more frequency components it contains, thus the more bandwidth is required to maintain that rate of information transfer.

    This is a fairly crude way to put it, but I think it conveys the general relationship between data transmission rate and bandwidth.

  10. Aug 14, 2007 #9
    yeah, that answers my question about an antenna and the transmission of a digital signal...

    Okay... about the physics part of it... lets say I am a mile away, and one person applies a constant, non-varying electric field across a wire, would I be able to detect an EM wave of that as long as the signal is applied? or would the amplitude need to change for me to detect the signal?

    I understand maxwell's equations require a change in amplitude for the transmission of a signal, I want to know if there is a reason for this... or is it just one of those laws of physics that are not to be questioned...
  11. Aug 14, 2007 #10


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    Yeah, there would be only one wave front though, when the field goes from 0 to S (S being whatever the magnitude of E is that they sent you). So effectively, the amplitude is changing from your point of view as they turn it on. After that, there would be no more waves unless he turned it off again and it went back to 0... then you'd get one more wave front.
  12. Aug 14, 2007 #11
    exactly... which is what I want to know... why is that? Is there a reason for it?
  13. Aug 14, 2007 #12


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    Well, that's kind of redundant: Constant is constant because it is constant. Or, you don't detect any change in a constant signal because there is no change in a constant signal.

    I'm not exactly sure what the problem is here because it doesn't look any deeper than that to me.
  14. Aug 14, 2007 #13
    It seems like the OP just can't grasp that a square wave counts as a wave.

    Sound analogy: Push a speaker diaphragm out, and nearby air molecules are pushed out (and in turn, push against further air molecules, in a process called an "air compression wave"), continuing until all the air molecules are (on average) slightly further displaced from the speaker. Or pull the diaphragm in, and the reverse effect (also called an air compression wave) propagates. In principle you could perhaps set up a receiver to inspect the distant air and determine which (constant) position the diaphragm is in. But unless you change the diaphragm position, the output from the receiver will stay constant. If you want to send a message to the receiver, you need to move the diaphragm to a different position (so that the receiver outputs the information that composes your desired signal/message). But *changing* the diaphragm from one (previously constant) position to a *different* (albeit constant from now on) position nonetheless constitutes producing a sound wave. It might be a more boring sound wave then the ones you usually listen to, but it isn't "constant", because you deliberately changed it. So no, a field (in this case, the air pressure field) isn't constant if you transmit information with it.
  15. Aug 15, 2007 #14


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    I'm not sure what you're asking. Questions like "why" and concepts like "reason" are really only used for explaining human motivation.

    It's just the way it is when it comes to the physical nature of things.
  16. Aug 16, 2007 #15
    okay... yeah... thats all I wanted to know... if there was a reason for it, or was it just one of those things you have to accept...
  17. Aug 16, 2007 #16


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    I'm still not sure you are getting it. There is a reson for it: It's the definition of a "signal". The question was redundant. Basically what you asked is the same as asking why no water flows when a valve is closed - it's because the valve is closed.
  18. Aug 17, 2007 #17
    yeah, see... all I was trying to say is that, one there's a difference between a 0 signal and a constant signal, second its okay to use the water analogy to the rf signals being generated, but both are not the same phenomena- first of all, the water waves are not a fundamental effect of nature-its something that arises because of the fundamental effects; the rf signals although are a fundamental effect of nature... so I was just contemplating on those differences...
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