How does RFID technology work and what are the RF design considerations for a reader antenna?

RFID (Radio Frequency Identification) technology works by using electromagnetic energy from a reader to communicate with and identify passive or active tags attached to objects. In passive RFID (the most common type), the reader transmits a continuous RF carrier signal that powers the tag through electromagnetic coupling (inducting energy into the tag's antenna), and the tag responds by modulating its antenna impedance (backscatter modulation), which creates a reflected signal that the reader detects and decodes to extract the tag's unique identifier and stored data. RFID operates in three main frequency bands: LF (125-134 kHz, near-field magnetic coupling, read range 1-10 cm), HF (13.56 MHz, near-field magnetic coupling, read range 10-100 cm, including NFC), and UHF (860-960 MHz, far-field electromagnetic propagation, read range 1-15 meters). The reader antenna design considerations for UHF RFID include: antenna gain (6-9 dBic typical for a circularly polarized patch antenna), polarization (circular polarization preferred to read tags in any orientation), VSWR below 1.5:1 across the UHF RFID band, front-to-back ratio greater than 20 dB to minimize reflections from behind the reader, and beamwidth matching the desired interrogation zone (typically 60-90 degree half-power beamwidth). The reader transmit power is limited by regulation (typically 1-4 W EIRP depending on country), and the read range is determined by the forward link (power to activate the tag) and the return link (ability to detect the weak backscattered signal).