Available at: https://digitalcommons.calpoly.edu/theses/1713
Date of Award
MS in Electrical Engineering
Radio Frequency Identification (RFID) systems use electromagnetic signals to wirelessly identify and track RFID-tagged objects. A reader transmits a carrier wave request signal to an RFID tag, which then transmits a unique identification signal back to the reader. Applications include supply chain inventory management, automated toll booth fee systems, sports event timing, restricted access control, pet monitoring and retail theft prevention. An RFID tag includes an antenna connected to a Radio Frequency Integrated Circuit (RFIC). RFID tags in the ultra-high frequency (UHF), industrial, scientific and medical (ISM) 902-928MHz band and global Electronic Product Code (EPC) 860‑960MHz band are powered passively (power extracted from carrier wave) and cost less than 15 cents per tag. Low cost UHF ISM RFID tags are an effective solution for tracking large inventories. UHF ISM tag antennas are typically planar dipoles printed onto a plastic dielectric substrate (inlay). Power exchange and transmit range is maximized when a tag antenna’s input impedance is conjugate matched to the RFIC input impedance. Since RFIC input impedance includes capacitive reactance, optimized antenna input impedance includes compensating inductive reactance.
The T-match network adds inductive matching microstrips to conjugate match the RFIC. Narrowband (±1.5% of center frequency) and broadband (±5% of center frequency) lumped element designs also use inductive matching strips. Narrowband, lumped element design is accomplished through Smith Chart matching assuming lumped antenna elements. The broadband lumped element design is accomplished through a circuit transformation to an equivalent network and tuning the transformed circuit to resonate from 865MHz to 955MHz, with a center frequency of 910MHz.
This thesis demonstrates a start-to-finish design process for narrow (±1.5% of center frequency) and broadband (±5% of center frequency) RFID tag antennas . Furthermore, antenna matching element geometries are parametrically swept to characterize input impedance frequency response. Thesis accomplishments include (a) narrow and broadband antenna designs, (b) Keysight’s Advanced Design System (ADS) Momentum simulations, (c) antenna fabrication, and (d) differential probe impedance setup and antenna impedance measurements. Additional items include (e) impedance adjustments (f) tag range testing and (g) narrow vs. broadband matching technique comparisons. Antennas were fabricated in Cal Poly’s Graphic Communication Department by silk-screening silver conductive ink onto DuPont Melinix Polyethylene Terephthalate (PET) plastic. Impedance simulations are compared to fabricated antenna impedance measurements and range testing results.