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EM waves

  1. Sep 13, 2007 #1
    Sorry if its a simple question but I was wondering what EM waves are composed of. Is it like an electron beam travelling through the air and the magnetic field a consequence of the moving electrons?

    Another thing thats been really confusing me and no matter how many radio circuit tutorials I look at I can't find the answer to this question. How do the EM waves travel? Lets say I broadcast radio waves of a specific frequency from Portugal and I need a radio station in New Zealand to receive my radio waves. The electrons leave the antenna and enter the air. From what I gather they start off in a straight line then spread out as they get further away from the antenna. Where do they go before the antenna in New Zealand receives them? What if I need one radio station in New Zealand and another one in Venezuela?

    Do these radio signals enter the atmosphere or what? In that case are long distance radio waves scattered all over to every single part of the world?
  2. jcsd
  3. Sep 13, 2007 #2
    EM waves are not composed of electrons. They are just time-varying electric and magnetic fields which cause induction of each other, and thus propagate. Maxwell's equations are the best starting point to understanding EM waves -- and can be found in many textbooks.
  4. Sep 13, 2007 #3
    Thanks alot. I'll look up Maxwells equations.
  5. Sep 13, 2007 #4


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    They would go in straight lines in empty space, but there are several effects that make them propagate round the surface of the earth because of refraction and/or reflection.

    Google for "radio propagation", "ground waves", "sky waves", "ionosphere".
  6. Sep 13, 2007 #5
    As an Engineer, and I'm assuming your are one since you asked your question in the engineering section not in the physics one, there is a very simple answer to your question we don't know and we don't care, see the structure of EM waves can not be explained to us fully because we don't have the essential background neither in quantum mechanics nor in quantum filed theory, the best I've reached to modelling EM waves was to think of them consisting of a photon-like particles such as light which is an EM wave itself.

    As far as your second question, it depends on the type of the antenna the length of the antenna, the frequency of broadcasting and many other conditions.

    Also I find it worth mentioning that most of the time the "Look" of the EM wave itself is irrelevant mostly it is the power that it carries is the important factor which would normally look something like a spear, sphere, donut...etc.
  7. Sep 14, 2007 #6
    Thanks for the answers. No I'm not an engineer abdo. Not yet anyway. I'm trying to get myself a base understanding of electronic theory before I get into the simpler practical applications of electronics.

    I knew the studying the composition of EM waves goes into quantum physics but I thought there was a scientific model for explaining what exactly makes up the different frequencies of EM waves. I suppose you'd have to find out what electrical energy and magnetic energy is composed of first.

    Thats how I pictured EM waves travelling. Spreading out from the antenna in the shape of a donut. What was confusing me is the fact that if the signal is powerful enough it can be picked up by radio stations anywhere in the world which makes me suspect the waves are propagated and spread out to every part of the globe.

    Radio waves must dissipate quickly since the same signals aren't picked up over and over again.
  8. Sep 14, 2007 #7


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    If you haven't already done it, please do some reading at wikipedia.org or arrl.org about antennas and radiation patterns -- I think you will enjoy the info. To be honest, I've never gotten comfortable with the self-sustaining magnetic/electric field oscillation thing for EM waves, but I do know from every-day experience that they do work and do exist, so fine. I do have an intuitive feel for how most antennas create an oscillating E&M field in the near field, based on the antenna textbooks that I've studied.

    And for the distance transmission portion of your question (DX = distance transmission in HAM terms), you will always have a channel budget to consider -- receivers are not infinitely sensitive, and your receiver's bandpass filtering and low-noise RX amp will limit how small of a signal you can detect. The signal-to-noise ratio (SNR) of the channel and electronics will determine how far you can go (i.e., how much attenuation you can tolerate).
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