What is an antenna, and why are NFC "antennas" weird?

So near field antennas like NFC and far field “transmissive” antennas like Bluetooth operate very differently and are mostly incompatible. This makes researching them very confusing because they’re both called antennas. The industry does that because they work on the same principal, just at different ends of the functional spectrum of what an antenna can do.

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So the way a transmissive antenna generates signals and sends them out is: a pulse of current is sent through the antenna, which generates a magnetic field. The shape/length of the antenna and the characteristics of nearby features like ground planes affect the frequency that the antenna operates most efficiently at. If the magnetic field pulse is sufficiently powerful and at the correct frequency to match the antenna characteristics, then the magnetic field will generate a predominatly electric field that propagates off into space as a transmission.

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Electricity and magnetism are interconnected though. Every electric field has a magnetic field and vice versa. Perturbing an electric field generates a magnetic field, and vice versa. The two fields combine to create an electromagnetic wave, in which the electric or magnetic field might dominate depending on the transmission medium and the thing they’re interacting with.

When a transmissive antenna receives a signal, the electromagnetic wave hits it and induces a magnetic field in the antenna, which generates current and is interpreted as a signal.

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That whole conversion process is terribly inefficient though. Very little actual power can be delivered through transmissive antennas unless you specifically tune the whole system for energy harvesting.

For near field antennas like NFC, they wanted more power delivery at the cost of range. So they decided what if we just do away with the whole transmissive electric field part of the antenna process, and just directly used the magnetic field produced by the pulse of current without converting it into an electromagnetic wave. It still needs to be tuned and everything for impedance matching, but we don’t really need to worry as much about the antenna geometry because we don’t need a long antenna segment to fling the field into space. We just need a closed loop back to ground for current to flow and generate a magnetic field. And then on the receiving antenna, we do away with all those transmissive elements and just add an unpowered coil that can have current induced in it by the field like what happens in a transformer.

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And to transmit data instead of generating and receiving signals, let’s just have the reader and tag modulate (draw current to raise and drop the level of) the shared magnetic field.

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