What's in a RF signal??

Discussion in 'General Electronics Chat' started by electronewb, May 23, 2012.

  1. electronewb

    Thread Starter Member

    Apr 24, 2012
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    I'm trying to understand what's in a RF signal!!! Is it actual electrons floating in the air just like electrons flow in a copper conductor?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    No, its alternating electric and magnetic fields.

    As a very poor analogy, consider gravity between the earth and the moon. Now, there is nothing flowing between the earth and the moon (let's keep the discussion out of esoteric theoretical physics concepts) but the presence of the earth results in a force on the moon through the earth's gravitational field. Now, imagine someone figured out a way to wiggle the earth so as to vary its distance from the moon according to a signal from a microphone and then they talked into it. Since the gravitational force on the moon is a function of the distance between them, someone on the moon could detect this movement, if it were big enough, and recover the variations and play the speech using a speaker.

    You can think of an RF signal the same way. The transmitter shakes electrons in the transmitting antenna in response to a signal, such as from a microphone. That shaking causes time-varying electromagnetic fields that then propagate through the air (or the vacuum of space) in exactly the same manner that light does (since light is, itself, an electromagnetic wave). When those time varying fields reach the receiver, they are able to cause minute shaking of electrons in the receiving antenna and the receiver is able to detect the tiny currents and voltages that result and amplify them so as to recover the signal that was used to cause the shaking in the first place and, if that is what is desired, send them to a speaker so that we can hear what the person at the transmitting microphone said.
     
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  3. electronewb

    Thread Starter Member

    Apr 24, 2012
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    So is it the same phenomena that happens in a transformer? Electromagnetic field between 2 inductors (2 antennae)? To go back to the light analogy wouldn't the photons be like electrons in a copper conductor?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Not quite. A transformer uses the magnetic field only. Magnetic and electric fields generally only appear near the source of the field and their intensity drops off rapidly away from the source.

    Maxwell calculated the interaction of a varying electric and magnetic field and determined that this would generate an electromagnetic wave that would propagate through space for great distances at the speed of light. He determined this years before an actual radio wave was generated and detected (other than light, of course).

    Electrons are particles that travel in conductors at a relatively slow rate, and always move below the speed of light. Photons are quantum particles of the electromagnetic wave that travel only at the speed of light.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    While there are considerable differences, you can think of a transformer along similar lines. The current in one winding causes a magnetic field and, if it is changing in time, the field is changing in time. If there is another winding within the reach of the field (which, as crutschow says, drops of very quickly with distance), then a changing field (caused either by the field changing or by the winding moving within the field, by the way), then a current will be induced in the second winding because the changing magnetic field will exert forces on the electrons in the other wire. While a pretty rough analogy, it can provide the basis for you to build a mental picture that is useful.
     
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  6. Papabravo

    Expert

    Feb 24, 2006
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    Electromagnetic waves will propagate in a vacuum. No medium or "ether" is required. Propagating waves will interact with real particles that happen to be in the vicinity of the waves. The nature of the interaction is to change their momentum (magnitude or direction or both). Momentum is a vector quantity just like the Electric and Magnetic fields themselves are vector fields.
     
  7. electronewb

    Thread Starter Member

    Apr 24, 2012
    260
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    Great thanks guys!!!
     
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