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An RF question

Discussion in 'Electronic Basics' started by Eric R Snow, Feb 10, 2007.

  1. Eric R Snow

    Eric R Snow Guest

    After listening to the RF interference radiated from a PDA on a
    portable radio it got me to thinking. When pulses are sent down a wire
    the wire will radiate RF. A lamp cord plugged into the wall radiates
    RF. But does a wire carrying DC doesn't radiate RF, does it? What is
    it about a rising and falling voltage that causes a conductor to
    radiate RF?
    Thanks,
    Eric
     
  2. Phil Allison

    Phil Allison Guest

    "Eric R Snow"


    ** It has a frequency.

    DC has none.



    ......... Phil
     
  3. Because the electric field falls off with the square of the distance but the
    electromagnetic field does not. A magnetic field is created by accelerating
    electric charges(so when the electrons move in a loop or because of a
    changing current) which can reinforce the electric field so that they
    propagate through space much more easily than the electric field by itself.

    With DC you do not get this propagation except when the dc changes(when you
    start a device that uses DC power it will still create an EM wave
    initially). But alternating fields you get an EM wave.

    Now a wire is not a great antenna but it is an antenna. When wires are
    configured to transport power they do a good job of keeping the EM field
    inside the conductor(where its much more conductive than outside the wire)
    but when configured as an antenna they do a much better job at radiating the
    power as an EM field.

    To know the *exact* reason you have to look at quantum mechanics but even
    that isn't going to explain it all.

    You'll just have to take it as an emperical fact that a changing electric
    field produces a magnetic field and vice versa. This doesn't happen to any
    extent with DC except for isolated cases(else it wouldn't be DC).

    In steady state conditions DC will produce an electric and magnetic field
    but neither is changing(else its not steady DC). Once the DC changes then
    you get a changing electric field which produces a changing magnetic field
    which produces a changing magnetic field etc. Ofcourse when this happens you
    get EM radiation. It can happen with DC as long as the DC is theoretically
    DC, if not you do get some EM radiation.

    Note that if you have AC then you always get radiation and it depends on how
    the wire that contains the AC is acting like an antenna.

    Take a wire and put 60VAC through it with a few amps and coil it into a
    circle. Now you have an antenna. (there are better antennas and you don't
    need so much current but...)

    If you do the same with another coil of wire but don't put any voltage on it
    and just messure the voltage off it you will pick up the signal produced by
    the other coil. Its not so good at 60 hz but you can play around with that
    idea if you want. (this is essentially what Faraday did)

    Anyways, hopefully that clears up some things,
    Jon
     
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