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I've been learning about antennas recently and I have some questions about the halfwave dipole antenna.

  • My first question is about its resonance; as I understand it everything that oscillates has a certain resonant frequency but how would it apply to a dipole antenna which is an open circuit? Dipole Antenna
  • Why do we only take into account only the length of the red part of the antenna? enter image description here
  • Why wouldn't something like this work when monopole antennas exist?

Thanks in advance.

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    $\begingroup$ open circuit? That's a DC concept. If it is resonant, it's not an open circuit, because it forms a field between the (quarter wave) halves of the dipole that completes the circuit. Energy is lost from the antenna, and the loss is measured as radiation resistance. $\endgroup$
    – user10489
    Commented Jun 1, 2022 at 11:09
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    $\begingroup$ Monopoles are dipoles where half of the dipole is an image. $\endgroup$
    – user10489
    Commented Jun 1, 2022 at 11:11
  • $\begingroup$ I suppose this could be the basis for canonical Q&A on the subject, but this is really well-trod subject matter easily found with even the most rudimentary research. At least the fundamental notions. $\endgroup$
    – user21417
    Commented Jun 1, 2022 at 14:25

2 Answers 2

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This is a dipole antenna with a feedpoint, to borrow your picture.

Dipole

Your diagram shows a dipole with a transmission line feeding it, which is a more complex system. In your exact case the transmission line doesn't radiate and can be analysed using comparatively simple transmission line equations, while the Dipole needs full treatment with Maxwell's equations.

As for why it resonates: that's "merely" the imaginary part of the feedpoint impedance going through zero when the length is about $\lambda/2$. It's one of the many properties of a dipole that can be derived from the basic physics. The half-wave antenna is interesting from an engineering point of view because it's most useful for communication etc, but dipoles of all lengths are antennas - they carry currents and these currents radiate.

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I think the answer to the question, "How is it possible for a halfwave dipole antenna to be resonant?", could be answered more simply; it's because any radiator that is any multiple of 1/4 wavelength (minus the velocity factor of the antenna material), has the reflected voltage wave, in-phase or 180 degrees out of phase with the incoming current and voltage signal at the feed-point, the only difference between in and out of phase would be the impedance at the antenna feed point; but as long as the antenna's radiating element is some multiple of a 1/4 wave length, the capacitive and inductive reactances are cancelled out leaving what looks like a pure resistive load to a particular radio frequency.

A halfwave dipole is simply two 1/4 wave radiators with a differential voltage applied, per the first drawing. Furthermore, the parallel feedline, feeding that dipole, will not radiate since the fields on each wire are equal and opposite; the lower picture, showing a differential voltage being applied, is still just a parallel feed-line, which should not radiate.

However, if voltage were on only one of those parallel lines, and the other was a "ground", like a partially removed coax shield, the element with voltage on it would radiate, but there would be a lot of loss with the "ground" being so close and parallel to the radiator.

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