RADIOENGINEERING, VOL. 26, NO. 3, SEPTEMBER 2017 735 DOI: 10.13164/re.2017.0735 ELECTROMAGNETICS Passive UHF RFID Tags with Specific Printed Antennas for Dielectric and Metallic Objects Applications Katherine SIAKAVARA, Sotirios GOUDOS, Argiris THEOPOULOS, John SAHALOS Dept. of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece skv@auth.gr, ssoti@physics.auth.gr, atheo@physics.auth.gr, sahalos@auth.gr Submitted November 27, 2016 / Accepted July 15, 2017 Abstract. Design process and respective results for the synthesis of specific Radiofrequency Identification (RFID) tag antennas, suitable for dielectric and metallic objects, are presented. The antennas were designed for the UHF (865 MHz-869 MHz) band and their basic configuration is that of the printed spiral type. Six modification steps to the classical spiral layout are proposed and it was proved that they can lead to tags with high readability and reading distances up to 10 m when designed for dielectric object and up to 7 m in the case of metallic objects. The results of the measurements of the fabricated tags are explained via theoretical evaluations which take into account reflection phenomena, which are present in a real environment at which the tags are used. Keywords Printed antennas, RFID tags, spiral antennas 1. Introduction The concept of communicating by means of reflected electromagnetic power is not new as can be traced back to more than six decades of years. Over this time period, the idea has evolved and been realized as modern, efficient and thus worldwide applied technology which exploits the capability of receiving and storing data by RF modulated and backscattered waves and in this way providing auto- matic wireless identification and tracking capability of things and personnel. Nowadays, applications of RFID systems span from inventory tracking, security and ticket- ing to electronic payment. Frequencies inside various bands of electromagnetic spectrum as the Low Frequency (LF) band, the High Frequency (HF), the Ultra High Fre- quency (UHF) and Microwave Frequency (MW) (2.4 GHz to 2.4835 GHz and 5.8 GHz) bands are dedicated to RFID applications. Among these, the UHF band is preferred for most of applications since UHF passive tags are capable of offering relatively fast reading speeds and can be read over long distances [1], [2]. The main grades of an RFID system are: a) the reader (interrogator) supplied with an antenna, b) the tags (tran- sponders) which are microchips combined with an antenna in a compact package, c) a host computer, and d) a mid- dleware including software and data base. Concisely, a passive RFID system operates in the following way: RFID reader transmits a modulated RF signal towards the RFID tag. The chip receives power from the tag's antenna and responds by varying its input impedance modulating, in this way, the backscattered signal. Modulation type, often used in RFIDs, is Amplitude Shift Keying (ASK). At ASK the chip impedance switches between two states: at the one of them it is matched to the antenna (chip collects power in that state) and at the other one it is strongly mis- matched, backscattering the incident power [1], [3]. Basic requirement of the RFID forward-backward communication, is the ensuring of sufficient power at the input of the tag's IC in order that the IC to work. Two are the key factors for this to be obtained. The first one is the use of a suitable antenna capable of gathering satisfactory amount of power from the environment, as at a passive tag its antenna is the sole source of energy for the chip. The second key is the transfer of a high, as possible, fraction of this power to the IC [4]. The power selected via the tag's antenna depends inversely from the distance between reader and tag, the antenna's directive gain and these pa- rameters along with the IC's sensitivity determine the maximum permitted distance between reader and tag at which the tag can be read. So, the designer of a tag has to solve two main problems: To design an antenna small in size but with high gain and to match the antenna to the IC. The majority of the commercial chips have complex input impedance with small real part and imaginary part of high capacitive value. Consequently in order the conjugate matching between antenna and IC to be obtained, the an- tenna has to operate out of resonance having an inductive input reactance. These specific requirements make the antenna's design and generally of the whole tag not an easy task. In accordance to the literature, various configurations have been employed for the design of passive tag antennas, which aim at both satisfactory gain and matching to the IC be achieved: antennas, of simple dipole or monopole type [5], [6], of meandered line shape [7–10], of fractal shape [11–14], of metamaterial-inspired type [15–17] or based on