In theory, radio-frequency-identification (RFID) tags provide the means of adding wireless labels to clothing or other merchandise almost anywhere and at any time, if those RFID tags can be made inexpensively enough to be practical. To find out, a research study took a look at four different ways to prototype flexible UHF RFID tags: based on ink-jet printing, wax-based ink deposition, cutting plotter shaping, and screen printing. An RFID chip from manufacturer Impinj (Seattle, Wash.) was used with the antennas as part of the evaluation.
The antennas were fabricated on layers of polyimide (PI) Kapton HN material from Dupont (Wilmington, DE) with all four fabrication processes used to form antennas. One of the antennas was a design recommended by the RFID chip manufacturer; the other antenna was designed just for the research project. A total of eight different RFID antennas were fabricated for the research project, two of each antenna design for each fabrication process, to evaluate the nuances of the different fabrication processes. Antenna designs described as thin propeller (TP), as recommended by the RFID chip manufacturer, and trapezoidal meandered (TM), as created for this research project, were fabricated with each process to study the effects of the process on each antenna’s basic performance parameters.
The different RFID tags were evaluated for tag sensitivity and radiation pattern, which is useful when analyzing tag behavior when varying its angular position with respect to the RFID reader antenna. Measurements on the tags representing the four different fabrication processes were performed at both the European RFID standard frequency of 866 MHz and the U.S. standard frequency of 915 MHz. The metal traces for both antennas are 1 mm wide. The gaps between adjacent metal lines ranges from 1 to 2 mm in the TP antenna and always 0.76 mm for the TM antenna case.
In addition to sheet resistance measurements of the metal conductors, a commercial model E5071C vector network analyzer (VNA) from Keysight Technologies was used to measure the scattering (S) parameters of the antenna circuits. A special differential test fixture and custom calibration kit were used with the VNA to make measurements on the eight different printed-circuit RFID antennas.
During the study, all of the fabrication processes were found to be suitable for constructing RFID antennas with acceptable performance for the U.S. and European RFID tag standards. The processing approaches in the number of circuit layers fabricated and thus the amount of printing materials required to assemble a given RFID tag antenna, so the cost of the different fabrication approaches will differ. But, as the researchers note, parameters other than electrical performance, such as cost, processing time, and robustness can be used as differentiators among the fabrication processes to determine which process makes the most sense for a particular RFID (or other) antenna application.
See “Comparison of Fabrication Techniques for Flexible UHF RFID Tag Antennas,” IEEE Antennas & Propagation Magazine, Vol. 59, No. 5, October 2017, p. 159.