Electronic – Explanation for: A monopole antenna must contain a resistor (or equivalent) and therefore must have 2 terminals

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Could someone explain why this sentence makes sense:

A monopole antenna transfers energy from electrical domain to the electromagnetic domain, hence must contain (equivalently) a resistor, hence must have 2 or more terminals.

Why must it contain a resistor and how can you conclude that you need 2 terminals because of the resistor?

Also,

A monopole is half of a dipole with groundplane in its symmetry plane and the drive is between antenna-feedpoint and ground(plane)

What is the symmetry plane? What does drive mean here? The transmitter?

Best Answer

All antennas have 'radiation resistance'. Power 'consumed' by the radiation resistance is actually being radiated away by the antenna instead of being dissipated as heat in a normal resistor. Now, in order to dissipate power, you have to have both a voltage difference and a flow of current. You can't do either of these with only 1 terminal.

For a monopole antenna, one terminal is at the bottom of the antenna and the other one is on a ground plane. The figure on the wikipedia article on monopole antennas has a nice drawing of this that also answers your symmetry question:

Monopole and image antenna

The 'symmetry plane' is simply a plane that you can draw between the two halves of a dipole antenna so that whatever is on top of the plane is the same as what is on the bottom of the plane. A monopole antenna simply sticks a ground plane in there and deletes the bottom half. I suppose you can also draw a symmetry plane vertically through a dipole as well, but the electric field is not tangential to a vertical slice like that so you can't replace it with a ground plane.