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How can an antenna capture + recieve EM Waves ?

  1. Jun 14, 2008 #1

    JPC

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    Hi

    I was wondering how can antennas capture + receive Electromagnetic waves
    I was thinking, in particular of FM Transmitters Antennas, and Fm receiver Antennas.
    And also, how come metals act as a shield to EM Waves

    I know that antennas have a resistance, and that they consume Energy by creating heat and light (like a Light Bulb)
    And since E = hv (v as frequency)
    the lower Energy 'E' created by the current passing through the antenna would just like a light bulb, emit photons of v=E/h. But since here E is a lot lower, it will emit photons of frequencies falling into the 'radio waves' band ?

    But now, how does current passing through a material where the resistance 'R' is (R>0) emmit photons ? Where and how are the photons formed ?
     
  2. jcsd
  3. Jun 14, 2008 #2

    Dale

    Staff: Mentor

    For the RF band it is pretty pointless to think in terms of photons. You may as well just think in terms of Maxwell's equations and waves. The frequency is so low that even the most sensitive antennas don't detect single photons in the RF range.

    So, from a classical Maxwell perspective, an antenna is a conductor, so for an externally applied EM field the charge carriers are free to move around. This motion is detected to recieve the signal. The same thing is the reason why it shields the EM waves, in a large piece of metal the motion of the charge carriers creates fields that act in opposition to the externally applied field.
     
  4. Jun 14, 2008 #3

    JPC

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    But i know there is a relation between the size of the antenna and the frequency we want to capture
    Why is it so ?
     
  5. Jun 14, 2008 #4

    Dale

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    Because the efficiency of the energy transfer between the EM wave and the antenna is a function of the ratio between the wavelength and the antenna length. This is a general wave property, not specific to light. This is the same reason that organ pipes and piano strings also have a characteristic length that is good for a specific note.
     
  6. Jun 14, 2008 #5

    rbj

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    more specifically, a simple antenna element (which is a conductive, usually metal, material with free electrons swimming around the outer shells of each metal atom) is around 1/2 wavelength to be long enough for charge (or the distubance of charge that the onset EM wave creates) to slosh from one side of the antenna element to the other side in time for the EM wave to reverse polarity and start sloshing back the other direction.
     
  7. Jun 14, 2008 #6

    JPC

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    Well for waves, i have already studied with a string :
    where '2L = n * h' for stationary waves
    and if 'F = n * F0', there are 'n' knots (un-moving points)

    But i dont understand how you put it in relation with a conducting metal rod and electromagnetic waves
    Im trying to understand your answers, but apparently there are some missing resources i havent for this...
    any diagrams on internet which explain what you are saying ?
     
  8. Jun 14, 2008 #7

    Dale

    Staff: Mentor

    Sorry if the connection was confusing. I just meant that resonance is a common phenomenon and not simply limited to electromagnetism. It occurs almost any time there is energy transfer, some frequencies will transfer more efficiently than others and the characteristic frequency at which the energy transfer is most efficient is the resonance frequency. That is all an antenna is, a structure that is resonant at the desired frequency so that it efficiently collects the energy from the signal.
     
  9. Jun 14, 2008 #8
    Antennas interact with photons coherently. Suppose you connect an antenna to an LC circuit tuned to some frequency [tex]\omega[/tex]. Then the energy levels of the LC circuit are given by:

    [tex](n + 1/2)\hbar\omega[/tex]

    The coupled antenna-LC circuit can absorb one photon of frequency [tex]\omega[/tex] and jump from energy level n to level n + 1.

    The current and voltages inside the circuit are not classical objects but they are operators that satisfy certain commutation relations, just like momentum and position do not commute. So, you cannot picture currents moving through the LC circuit in a classical way. Only in the classical limit is such a picture possible.
     
  10. Jun 14, 2008 #9

    JPC

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    Oh, now I understand.
    We could draw a sort of gauss function which amplifies fequencies around f0 , and which reduces the amplitude of those distant from f0?
     
  11. Jun 14, 2008 #10

    Dale

    Staff: Mentor

    Yes, exactly.
     
  12. Jun 15, 2008 #11

    JPC

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    I know LC circuits have a resonance given by F = 1 / (2 * Pi * sqroot(LC))
    But, does the antenna alone have a a resonance in function of its size ?
     
  13. Jun 15, 2008 #12

    Dale

    Staff: Mentor

    Yes. If you measure it carefully an antenna alone has its own (small) inductance, capacitance, and resistance.
     
  14. Jun 16, 2008 #13

    JPC

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    And does that have a big effect on the wavelenghts it can capture ?

    i mea does that proper resonance to the antena explain why we need different size antenas for different wavebands
     
  15. Jun 16, 2008 #14
    Not really. If you connect an antenna of the wrong size to a transmitter, then what happens is that the signal from the transmitter will be reflected back into the transmitter. One can deal with this problem by inserting a special circuit consisting of coils and capacitors inbetween the transmitter and the antenna.

    Now, it is true that antennas that work at very high frequencies, say a few gigaherz for mobile phones are completely different from the antennas that are used for short wave or medium wave.
     
  16. Jun 17, 2008 #15

    JPC

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    gigahert is high frequency ?
    if i remember well microwave oven is 2.4 Ghz
     
  17. Jun 17, 2008 #16

    Dale

    Staff: Mentor

    Yes.

    Note that what Iblis said is also correct. In practice, the same antenna will capture a range of frequencies for most designs. In other words, the resonance peak is generally not a single sharp spike but rather a fairly broad peak, so although it may have a specific resonance frequencies it will also absorb most of the energy at close frequencies also.
     
  18. Jun 19, 2008 #17

    JPC

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    ok

    but, how can the message be reflected back into the transistor if the antena is not big enough ?
     
  19. Jun 19, 2008 #18

    Dale

    Staff: Mentor

    reflection is another phenomenom that applies to all waves and not just electromagnetic waves. Any time a wave of any sort goes from one medium to another some of the energy can be reflected. The more dissimilar the two media the more energy is reflected. Most transmitters are 50 Ohms impedance, so you match your antenna to also be 50 Ohms at the frequency you want to broadcast.
     
  20. Dec 16, 2011 #19
    As a ham radio operator, i have come to understand there is a phenomenon known as "capture effect" where a low HF frequency wire antenna (80 meters) with a very small cross sectional capture area compared the the frequency, captures a tremendous amount of signal.

    Does anyone have more information on this effect?
     
  21. Dec 16, 2011 #20

    Drakkith

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    Staff: Mentor

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