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Antenna Theory

  1. Dec 17, 2009 #1
    Hi All,

    I'm currently wrestling with trying to understand the theory behind radiation from a dipole antenna. Little by little I'm putting the pieces together but I need verification regarding a conclusion that I have drawn although I have not read it.

    An antenna has a near field and a far field. The near field is due to the fact that all practical antennas have reactive elements even at resonance which do not completely cancel out. The far field is due to resonance and in a perfect world with no reactance, the transmission line to an antenna at resonance would see only a pure resistor with no reactive components.

    Am I correct so far? If so, is it fair to say that an ideal antenna operating at resonance with no reactive component would not have a near or inductive field?

    In trying to tackle this issue, I am probably going to be asking further questions one at a time at they come up.

    Thank you,

    Bob
     
  2. jcsd
  3. Dec 17, 2009 #2

    marcusl

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    I think this is close but not quite. On resonance the antenna looks resistive from the terminals, but that doesn't mean it lacks reactive components. An analogy might help: even though an LCR tank circuit is purely resistive at resonance, energy is stored alternately in the L and C parts. In fact, that stored energy is maximal and it decreases away from resonance. In the antenna, capacitive and inductive reactance similarly balance and cancel at resonance at the terminals, but substantial energy is still stored in the electric and magnetic fields in the near field. This happens regardless of how ideal the antenna is.
     
  4. Dec 17, 2009 #3

    Born2bwire

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    I would also add that one of the simplest definitions that I have come across for an antenna is that an antenna is a transformer that changes a guided wave into a free-space propagating wave and vice-versa. In this manner, an antenna acts as an impedance transformer, trying to change the guided wave in a waveguide of an impedance of X ohms to the free wave in a medium of 377 ohms (if free space). There is a little more to it than just an impedance transformer since we are going from guided to propagating but it gives a way of imagining how the antenna behaves. A poor antenna will provide a bad impedance matching, causing energy to be reflected back to the generator in the waveguide and back off the antenna in free space. Any reactive properties (not necessarily from reactive elements but from physical characterstics that give rise to reactive behavior) will cause energy to be trapped and deemed useless. As marcus states, since we do not necessarily have a physical reactive circuit element, the energy gets trapped in the near-fields around the antenna. They are caught between the guided wave and propagating wave modes. Since any antenna will have these reactive properties, we have to provide a suitable matching circuit to maximize the transmitted power and while this removes any net reactive impedeance it does not prevent a portion of the energy from being trapped as reactive power.
     
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