em wave transmission

Discussion in 'Wireless & RF Design' started by saraswathi.95, Feb 14, 2014.

  1. saraswathi.95

    Thread Starter Member

    Apr 11, 2013
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    Hello,
    From maxwell's equations, electric field when oscillates magnetic field lines gets generated and also vice verse ,and this is the basic principle of em wave propagation,my question is that whenever alternate current flows through antenna magnetic field gets generated around it,then wheather electric lines gets generated from these lines for propagatioin or not?
    if so how could those magnetic flux lines could do that in air.
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
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  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The video is good but it skips over the reactive near field zone (immediate) stored energy at the antenna and jumps directly to far field zone in-phase propagation. The question of why is there EM radiation from moving charge is largely left unanswered.
    [​IMG]


    http://www.youtube.com/watch?v=9VpDO2iYpRU&list=PL7LVi7WA7Mkd7hWnabCUB8cXJWSRc4MY9&index=14
    http://accelconf.web.cern.ch/accelconf/e02/PAPERS/WEPRI038.pdf
    "Radiation2D" Program.
    http://www.shintakelab.com/en/enEducationalSoft.htm

    One way to look at the changing magnetic field component of an EM wave to see it as the changing electric field from a different point of reference caused by rotation in time and space. There is only one EM energy entity that can be viewed and transformed into many different facets.
    http://cosmology.auckland.ac.nz/relativistic-em-visualizations/
     
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  4. saraswathi.95

    Thread Starter Member

    Apr 11, 2013
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    Thank you,
    i want know that whether these lines of force are generating new wavefronts or else the source waves are pushing them in to free space in fraunhoffer region.
    if it is so how these waves could be directional as we discuss about directional antennas,i know that in the near field the fields in one direction are made maximum in effective and they were cancelled in other directions,will the waves do not spread apart widely as they move further in to the space.
     
  5. nsaspook

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    Aug 27, 2009
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    For waves to be pushed in some mechanical manner you normally assume there is a magical "luminiferous aether" or medium for waves to move in. That's a outdated theory with modern "Special Theory". You can look at the problem several ways but using moving charge with already existing static electric lines of force to generate EM energy waves might be more intuitive (without adding the complexity of QM or photons).
    http://web.mit.edu/jbelcher/www/java/plane/plane.pdf

    Direction antennas can by reflection and/or element phasing concentrate EM power to/from a planar beam wavefront so it's no longer a spherical wavefront spreading apart like from a point source.
    http://www.youtube.com/watch?v=785kRIZ7aeI
     
    Last edited: Feb 15, 2014
  6. saraswathi.95

    Thread Starter Member

    Apr 11, 2013
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    can you extend your concept b y considering more than one antenna placed close to each other,then those antennas also radiates and as a result waveforms gets spread in to free space waves in the space are nothing but force lines when we consider them in usual terms ,will they do not affected by waves from other antennas beside it?
    so many of them suggested me to know about superposition theorem,and had given me so many analogies i could understand it well,but i am unable relate them with this,can any one help me.
     
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I'm not sure what your question is. The superposition theorem is correct for EM waves in space. Let go back to your 'pushing/pressure' comment. When EM waves interact with matter in the near field (mainly by the movement of electrons) the (non-propagating) evanescent waves generated from that interaction (reflection/re-radiation) are not the same waves that caused it. Some of the wave energy is reabsorbed by the antenna and then re-emitted again when charges move in response to that potential. The force of the (propagating) traveling wave on the electrons in the antenna decreases as the space between it and the antenna elements increases, so you move from a zone of energy storage in fields that regenerate from existing fields by moving charge to a zone where moving fields recreate from only themselves and energy moves away from the antenna.
    http://en.wikipedia.org/wiki/Evanescent_wave_coupling#Evanescent-wave_coupling
    There is nothing really unusual about this, it happens when there is an impedance mismatch (from the antenna to free space in this case) that causes a partial reflection at the media boundary and is the basis with a few new twists like resonance for some of the 'new' wireless energy methods hype with fancy names like “WiTricity”.

    Further away but not in the far field we see mainly induction effects of the propagating energy on other conductors like a second antenna or an element that can cause them to absorb and emit waves that change the directionally of the energy from the original antenna.
     
    Last edited: Feb 16, 2014
  8. saraswathi.95

    Thread Starter Member

    Apr 11, 2013
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    Thank you so much for your patience reply,
    what my doubt is how all the antennas are able to receive such correct signals,without interruption from a huge number of field lines in the space just by maintaining different frequency to each signal and proper modulation.
    i am unable to understand it when i include all field analysis,kindly see the posts in the thread that i have posted by name doubt clarification,so many tried to explain me about the interference,from them i had cleared so many of my doubts but not able to relate it with fields.
     
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