Basic antenna working doubt

Discussion in 'Wireless & RF Design' started by surya137, May 8, 2014.

  1. surya137

    Thread Starter New Member

    Sep 16, 2012
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    Antennas are supposed to radiate EMF waves.
    Magnetic field exists around a closed current carrying conductor.
    An antenna connected to a transmitter does not form a closed circuit, how is it then possible for EMF waves to be radiated by an antenna when you don't even have magnetic field.
    I mean only when magnetic and electric field exists perpendicular to each other we have EMF waves ..
    This question is bothering me for long, please help.
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    an antenna at resonance has current circulating bck and forth from end to end. also, an antenna out of resonance has rf voltage across it, any changing voltage thorugh a conductor will create a magnetic field. see works by Maxwell and Lorentz.
     
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  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    It is a closed circuit but the circuit is the entire universe. When charge (and the matter associated with that charge) at one place in space changes its relationship with the rest of the other charges in space that information is transfered by something we call force and that force energy in 'motion' called work. That force can be gravity , the EM force or many other types of 'forces'. One of the first things you need to do to really understand antennas is to see how the electric and magnetic fields are only parts of the EM force in difference modes.

    The electrons in a conductor (antenna) don't move at the speed of light but the electrical potential applied to it does so if we have a electrical potential that varies in time (frequency with a free space wavelength) on that wire it's possible to have potential difference between the charges (electrons) on the opposite ends (and places between) of that wire. If it's electrical length is a good fraction of the wavelength of the signal changes we now have a wire with voltage (electric field) across it, the charges (free electrons) move slightly to neutralize the charge imbalance inside the wire causing a current. (magnetic field) These two fields are the components of the dual entity of electromagnetic energy in space.

    At this point we have a wire (dipole antenna) with a spherical wavefront of fields that move into space as spherical somewhat reactive waves first and become classic EM plane waves of EM energy as they enter free space away from the antenna. (I'm skipping how.)

    [​IMG]

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    Last edited: May 8, 2014
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  4. jjw

    Member

    Dec 24, 2013
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  5. nsaspook

    AAC Fanatic!

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    Last edited: May 9, 2014
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  6. nsaspook

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  7. surya137

    Thread Starter New Member

    Sep 16, 2012
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    @nsaspook: thnx a lot, I dint get any mail so I actually thought no one has answered to my thread, accidently took a look at my older posts.. phew those links and illustrations were really helpful.. :) :)

    @alphacliff and jjw: thnk u as well.. :) :)
     
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