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I'm having trouble understanding how RFID works on the technical side. I know an RF signal is sent to an antenna, modulated, and then backscattered back to the RFID reader. For example, Does this ON-OFF Keying system work?

schematic

simulate this circuit – Schematic created using CircuitLab

My logic is as follows: Then antenna receives the RF Signal.

The signal is absorbed through ground when the transistor is on and "acting" like a closed switch. This represents a zero.

When the transistor is off, it acts like an open circuit and reflects the RF Signal back to the antenna and the antenna radiates back to the reader. This represents a one.

What I see wrong with this is that the antenna cannot simultaneously transmit and receive the RF signal, or at least to my knowledge it can't.

Another example is this google image I found. How does the antenna receive, modulate, and send back a signal simultaneously. Are there two antennas that are "lumped" as one antenna?

enter image description here

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In these kinds of systems, "antenna" is a bit of a misnomer:

instead, consider the two coils of the reader and the tag as primary and secondary windings of a transformer. A simple measurement on how much power can be pushed through the transformer allows the reader to determine the "short-circuit-ness" of the tag side, to stay within your picture.

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  • \$\begingroup\$ Is this because RFID usually operates in the near field? Is it feasible to have far-field backscattering system? I'm tasked to design one for a senior project and have an antenna I want to use, but I'm lacking in my EM modeling skills. \$\endgroup\$
    – Aaron
    May 25 '21 at 6:01
  • \$\begingroup\$ Wouldn't call it backscatter, but passive RFID system. These exist. \$\endgroup\$
    – mmmm
    May 25 '21 at 6:44
  • \$\begingroup\$ If you look at RFID operating at UHF band frequencies, then antenna is not a misnomer, they are actually antennas, we're talking 868/915 MHz, you don't use coils for those and communication distances can go up to 10 meters, so we're talking about far-field communication. And the principle is still the same, it's backscatter, the tag modulates an incoming signal being transmitted by the reader. \$\endgroup\$ Oct 27 '21 at 21:10

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