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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?

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2 Answers 2

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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.

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  • \$\begingroup\$ FYI, most of the .svg files on Wikimedia have several .png renderings that you can use for raster images (that's where I got this one) \$\endgroup\$
    – Greg d'Eon
    Commented Apr 5, 2015 at 20:33
  • \$\begingroup\$ Where do you find those? I have been resorting to google image search for something similar whenever I find an SVG. It's rather disappointing that stack exchange does not support SVG directly as using a rasterized png is inferior for people (such as myself) with high DPI monitors. \$\endgroup\$
    – alex.forencich
    Commented Apr 5, 2015 at 20:36
  • \$\begingroup\$ For an example, I found this one here: commons.wikimedia.org/wiki/File:Monopole_and_image_antenna.svg - you can probably find a similar page for any Wikimedia image. \$\endgroup\$
    – Greg d'Eon
    Commented Apr 5, 2015 at 20:37
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    \$\begingroup\$ Oh, I see, you have to click on 'other resolutions'. That's a bit counter-intuitive as those are returning the image in a completely different format than the original. \$\endgroup\$
    – alex.forencich
    Commented Apr 5, 2015 at 20:39
  • \$\begingroup\$ So from top-top-bottom, is it: top of the antenna, resistor, input terminal at the bottom of the antenna, output transmitter and the ground?? \$\endgroup\$
    – PeterH
    Commented Apr 5, 2015 at 21:49
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An antenna converts electrical energy to electro magnetic energy so, without two wires you can't transfer electrical energy.

The antenna projects back to the driving source the impedance of free space. This happens to be resistive at about 377 ohms however, the antenna, depending on type and complexity transforms this value to typically a few tens of ohms and quite often this has a complex value that needs to be taken into account to get maximum power transfer.

A dipole antenna produces an e field on one limb that is the same amplitude but opposite sign to the e field on the other limb. This means that a plane, centralized at the antenna's midpoint has zero e field as far as the eye can see, so to speak. This is exactly equivalent to ground.

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