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How does antenna transmits electromagnetic waves?

  1. How does antenna transmits electromagnetic waves? For example radio antenna. How does the antenna produces moving electromagnetic waves?
     
  2. jcsd
  3. DaleSpam

    Staff: Mentor

    The antenna is a conductor, so charges are free to move inside the antenna. You apply a voltage, charges move, you apply an opposite voltage, charges move the other way.
     
  4. But what makes them travel?
     
  5. The simple fact that charges in (accelerated) motion *do* radiate electromagnetic waves. Oscilating motion between two ends of a copper wire is such motion.

    This is a feature not easily quantified and many textbooks have whole chapters dedicated to describing this effect alone. However I was able to find this wiki-article on a radiating dipole what mostly cover the important conclusions and features a nice animation of the electric field
     
  6. If I move faster the dipole, will I create bigger frequency? (which is logical, I think so)
     
  7. the antenna is connected to a transmitter which is designed to output a voltage as a function of time. this voltage is an e.m.f. that forces free charge in the conductive element of the transmitting antenna to slosh back and forth along the element. in the receiving antenna, there is free charge in the conductive element that are affected (because like-signed charges repel and unlike-signed charges attrack) by the movement of charge that is happening in the transmitting antenna. because of the usual large distance between the transmitting antenna and the receiving antenna, that movement of charge in the receiving antenna is minute, much smaller than the quantity of charge and movement in the transmitting antenna. but that is what receivers and RF amplifiers are for; making that small movement of charge control a much larger movement of charge (that eventually finds its way to your radio loudspeakers).

    so you have the motion of charge at one location affecting the movement of charge at another location. since the reaction of charge in the receiving antenna is not an instantaneous reaction (from the POV of an observer that is equal distant between the two antennae), what is it that is in between the two antennae that, after a finite period of time, forces the charge in the receiving antenna to move?
     
    Last edited: Jan 7, 2008
  8. A pretty simplified way to look at it is this:
    When you close a circuit you have an electrical field E,E create a current in the antena.
    Now as you know a wire that have a current flowing in it creates a magnetic field B.

    P = E X B , and thats the radiation you recieve.
     
  9. It is similar somewhat to the fact that you tap you hand on the water surface periodically and you make waves.
     
  10. Ok, I have another question connected with antennas. I found a text from wikipedia, and also found this quote:
    Why the electromagnetic waves are partially reflected? When they encounter an interface, I think that there is process called absorption, so the electrons release the excess of energy in form of radiant energy, shouldn't that all energy be reflected?
     

  11. if you have a wave traveling along a "string" and somewhere in the middle of that taut string, it changes to 1/4 inch nylon rope (the two are spliced together, then is pulled tight). when the wave is incident upon the splice, some of it will transmit through and some will be reflected.

    now replace the 1/4 inch nylon rope with a massive brick wall. here, very little is transmitted and all of it is reflected.

    it's because of a change of characteristic impedance of medium.
     
  12. In the sentence it says like, touching no matter what interface, it will partially reflect and partially transmit the waves. So it matter, what is the material of the interface, right?
     

  13. the stuff on the other side of the interface has to be different. a different density and/or a different stiffness or compressibility. if you have an interface of some kinda Jello on one side and it's the same Jello on the other side, all of the incident wave will be transmitted.
     
  14. I understand, thank you very much.
     
  15. Can you tell me how dipole antenna works?
    Here is picture.
    What the dipole antenna is it for?
     
  16. I'd like to ask a related question. I understand that the transmitter outputs a voltage as a function of time (which creates an electric field) thus causing a flow of charge or current (creating the magnetic field) and these two fields travel down the antenna, but why don't they stop once they reached the end of the antenna. How do they get blasted into space?

    Any insight would be greatly appreciated!
     
  17. "Blasted"? Maybe if you think of it in classical terms (waves) rather than quantistically (photons) it will not require you to see anything material actually exiting the antenna, but just the energy...

    Think of a large canvas, such as a bed blanket, covering your bed with edges unbound. With a hand, you pick up the middle point and you start pulling it up/down. You'll see waves flowing outwards the blanket, but there is nothing material really blasted out, you don't shoot around pieces of fabric ;)

    The effect is similar with the antenna. By vibrating the dipole, you change the EM field around the antenna, but the variation is not instantaneous everywhere, instead it propagates with a finite speed (c, the speed of light). It is the constant attempt at changing the whole EM field coupled with the finite speed that will make the field "look" like it's moving outwards.

    It's more than a look of course, since you can say that there is an energy associated to that "spinning" of the EM field.
     
  18. Actually antenna propagation is based on the fact of insetia of electric and magenetic field. As we know that the moving charges produce magetic field and static charge produce elctric field. So when an antenna is excisted by time varying voltage the elctrons moves forth and back. And the field is generated. For +ve cycle of voltage the electric field direction is opposite as compared to the -ve cycle. When the rate of change of this is very high the filed will take some time to get changed. And the inertia of this will come in picture. The field which is generated by the +ve cycle will reflect the -ve cycle generated field. And there is not transmission of charges in the free space only the filelds is getting transmitted in the antenna process. Thats is why the low frequency signal cannot be transmitted.
    Let me know if anybody can explain any other idea.
    Thanks and Regards
    Komal Kumar Dhote
     
  19. The transmitter voltage does not create the electric field. The transmitter outputs current and voltage simultaneously, as well as an E and H field simulataneously. E, H, I, & V are all in unison for a resistive t-line (Z0 = real). There is a finite impedance value for the transmission line, Z0. The current and voltage are simultaneously present and Ohm's law is always upheld. For a 300 ohm Z0, the transmitter connected at the input outputs a V and an I, say 600 mV and 2.0 mA. The I and V waveforms travel along the t-line and when the end is reached the E and H fields radiate power into space.

    The reason is that an antenna only works for high enough frequencies. The current here is displacement current. No closed path is reuired for such. The t-line ends abruptly in mid-air, yet current exists. The E and H fields are present in between the conductors of the t-line. Then at the end where it ends, the fields continue to propogate into space.

    Have I helped or made matters worse?

    Claude
     
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