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From my very limited knowledge of EM, I believed that anything tied to earth can absorb radio waves, so if an antenna was to be placed above a ground plane, why does the ground plane not just absorb the nearby signals? Some people’s answers online are to do with reflecting the em waves, but surely a random ground plane is not tuned to emit specific gps waves?

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  • \$\begingroup\$ By that line of reasoning, every antenna on the planet shouldn't work because they are installed over the earth. Think about your old fashioned outdoor TV antenna, for example. \$\endgroup\$
    – SteveSh
    Nov 20, 2023 at 1:28
  • \$\begingroup\$ And reflecting is not the same as emitting (or receiving). Heck, any 'ol piece of metal (any conductor) will, to some extent, reflect an EM wave. \$\endgroup\$
    – SteveSh
    Nov 20, 2023 at 1:29

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A patch antenna looks more like a capacitor divider, with the top plate coming in from free space (incident electric field), the middle plate being the patch itself, and the ground plane being, well, ground.

But that's not a very good model, because voltage varies across the patch itself; it has some inductance and capacitance distributed around. And ambient fields are a lot more nuanced than nodes and wires are. A full analysis would of course require a full EM description, but that wouldn't be very helpful here, so you'll have to take it for granted that there's more going on than a simple equivalent circuit can capture, and obviously, yes, they work as designed.

As for waves and fields, yes, reflection is relevant. Metals absorb fields, but how much depends on their relative (bulk) impedance compared to the space around them. Impedance, is one way to express index of refraction, at the level of local fields (and thus voltages and currents in wires, and impedance is critical for circuit analysis).

As it happens, the effective index for metals can be very large (thousands), so it doesn't take much material thickness to mostly reflect incident waves. Hence a ground plane makes a good mirror, at most any frequency.

Mirror-ness doesn't really matter much for the antenna structure, because the antenna itself is comparably sized to the wavelength, diffraction matters, and we can't apply concepts from geometric optics.

Suffice it to say, waves give the full description, between these two extremes: how a capacitive divider, made from pieces of mirrors, can act as an antenna.

Antennas themselves, deserve some definition. They act as a transducer between forms of waves; unlike, say an acoustic transducer like a speaker, the waves they transduce aren't even different kinds, but it's electrical waves (in space) between electrical waves (in wires). Understand that, because fields in space are much more complex than circuits (multi-dimensional, rather than zero-dimensional), we need some way to express what waves, and how much, are able to couple into wires in this way. Well, the antenna provides this coupling, and what directions and shapes of waves, depends on its design.

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