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RF Signals - Radio waves or electrical signals?

  1. Oct 5, 2012 #1
    Hi all

    Sorry if this is quite a basic concept, but I am confused about RF signals. The way I currently understand it is that radio waves in space are an oscillating electric and magnetic field at 90 degrees to eachother travelling through a medium. However, when you pass a radio wave into a conductor, in an ideal case, it would absorb all of the radio wave power and seize becoming an oscillating electric and magnetic field (radio wave). Instead it would induce an alternating AC current/voltage at the same frequency as the RF. Is this correct?

    In which case does this mean that an RF signal generator connected to a co-axial cable produces RF AC current/voltage and not pure radio waves? I have been confused by people telling me that the signal travelling down the co-axial cable is an EM radio wave (just an electric field/magnetic field) produced by the sig-gen and not an electrical signal with current and voltage.

    Thanks!

    Paul
     
  2. jcsd
  3. Oct 6, 2012 #2
    Time varying signal travel down the circuit as EM wave even at low frequency. The voltage and current you measure is only the consequence of the boundary condition of the EM wave with the conductor surface. The two conductors ( coax, signal trace on pcb, two wire electrical cable etc.) is actually a transmission line that forms a guided structure where EM wave propagates between them.

    But in electronics, it is too confusing to think in terms of EM wave propagation, so a set of voltage and current equations are derived from the EM wave equation into voltage and current equations. And this is further simplified for lower frequency to explain in simple current and voltage that you see in most text books. Within certain limits, voltage and current notations work just fine. Just bare in mind it is EM wave that propagates. Most of the simple current and voltage equations work for lower frequency where the wave length is much longer than the dimension of the physical circuit. But when the dimension of the physical circuit approach about 1/20 the wave length, the simple voltage and current equation start to fall apart and a more complicated type of equations need to be used. Current and voltage phasor type of equation is more valid in the RF world where the dimension of the circuit larger than 1/20 of the wave length.

    Speed of electron moving is very slow, you can inject an electron into one end of a wire, put a voltage across the two ends of the wire, then you can go get a cup of coffee and come back and wait before the same electron comes out from the other end of the wire!!! So signal definitely is not from electrons traveling.
     
    Last edited: Oct 6, 2012
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