An electromagnetic field propagates outward from the interrogator’s antenna, and a small proportion of that field (reduced by free-space attenuation) reaches the tag’s antenna. The power is supplied to the antenna connections as high-frequency voltage, and after rectification by diodes it can be used to power the tag or activate or deactivate the tag. Some proportion of the incoming RF energy is reflected by the antenna and reradiated outward into free space. The amount of energy reflected depends on how well the antenna couples to the electromagnetic wave. RFID tags that use backscatter to reply to their interrogators have antennas that are designed to resonate well with the carrier signal emitted by the interrogator. The reflection characteristics of the antenna, its effective cross-section, can be influenced by altering the load connected to the antenna. To transmit data from the tag to the interrogator, a load resistor connected in parallel with the antenna is switched on and off in time with the data stream to be transmitted. By changing resonant properties of its antenna, the tag makes itself a good or poor reflector. This varies the strength of the signal reflected from the tag, creating a pattern that is detected at the interrogator as data. This technique is referred to as modulated backscatter.
Before the backscattered signal arrives at the interrogator antenna, it goes through forward and backward path loss, many types of interferences in both the directions, and absorption by the tag to power it. The reflected signal also travels into the antenna connection of the interrogator in the reverse direction from the original signal. It is decoupled using a directional coupler and transferred to the receiver input of the interrogator. The forward signal of the transmitter is to a large degree suppressed by the directional coupler.