# How is it possible for a halfwave dipole antenna to be resonant?

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?
• Why do we only take into account only the length of the red part of the antenna?
• Why wouldn't something like this work when monopole antennas exist?

• 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. Jun 1 at 11:09
• Monopoles are dipoles where half of the dipole is an image. Jun 1 at 11:11
• 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.
– user21417
Jun 1 at 14:25

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.