IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 59, NO. 7, JULY 2011 2619 Parametric Design of Compact Dual-Frequency Antennas for Wireless Sensor Networks Simone Genovesi, Member, IEEE, Sergio Saponara, and Agostino Monorchio, Senior Member, IEEE Abstract—A parametric study for the design of a planar com- pact dual-frequency antenna is presented. The proposed geometry and the suggested tailoring procedure provide a useful template for generating a single antenna able to serve as bidirectional node for a generic Wireless Sensor Network within the UHF and mi- crowave frequency bands. The constructive parameters can be set to adapt the radiating device for different unlicensed bands and to comply with the international radiation power regulations. A careful design procedure will be described to tune the antenna tem- plate for working at the required frequencies. In order to prove the effectiveness of the suggested approach as well as the reliability of the adopted technique, comparisons between simulations and mea- surements of realized prototypes will be reported. Index Terms—Dual-frequency antenna, loop antenna, printed antenna, wireless sensor network. I. INTRODUCTION W IRELESS SENSOR networks (WSN) are receiving a growing interest for several applications such as logistic, home automation, healthcare, structural monitoring, security/safety systems, intelligent transport systems [1]–[12]. They are typically [6] constituted by several client modules, dis- tributed in the environment to be monitored. They are realized as compact printed circuit board (PCB) hosting one or more sensors, a battery or energy harvesting unit, a programmable IC for signal conditioning [12], [13] and a RF transmitter for com- municating towards a server module. In many cases, the client’s RF unit can be conﬁgured so to support different modulations schemes (ASK, OOK, BPSK, ), frequencies (usually ISM unlicensed bands in the UHF range) and transmitted power. The conﬁgurability of the RF part is important also to cope with different national regulations. As an example, we mention tire pressure monitoring sys- tems (TPMS) [1]–[3] where a wireless client hosting pressure and temperature sensors is mounted on each wheel’s rim trans- mitting tire status to a centralized receiver mounted on the car chassis. Usually, the network covers a local range and the max- imum client RF transmitted power amounts to few tens of mW. A key issue for sensor networks is the availability of compact antennas at the wireless client side, possibly requiring only con- ventional PCB technologies for their production. Manuscript received November 15, 2009; revised November 12, 2010; ac- cepted December 02, 2010. Date of publication May 10, 2011; date of current version July 07, 2011. The authors are with the Dipartimento di Ingegneria dell’Informazione, Uni- versity of Pisa, 56122 Pisa, Italy (e-mail: simone.genovesi@iet.unipi.it; sergio. saponara@iet.unipi.it; a.monorchio@ieee.org). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TAP.2011.2152313 In WSN a bidirectional communication scheme has to be pre- ferred between client and server units. In addition to the wireless transmission of data from a client sensor to a server unit, a wire- less link is needed from the server towards the clients for a wide range of tasks. For example, it can be necessary to acknowledge the safe receipt of data or to request their re-transmission in case of errors. Moreover, it may be useful to wake-up the clients and enable data transmission in wireless networks where the clients are normally in idle mode for power optimization or to conﬁgure the clients. Finally, two-way communication can be requested to exchange security codes for identiﬁcation or for encrypted data transfer. To this aim, most systems known in literature adopt two separate antennas at the client side [1]–[3], [14] operating at dis- tinct frequencies, thus increasing module size and cost. Typical solutions adopt a printed antenna at UHF together with a LF coil and its relevant demodulator (this solution is adopted for the TPMS in [3] or for safe entry system in [11]) or two separate antennas printed on opposite sides of the board and operating at distinct UHF frequencies (e.g., 433 MHz and 868 MHz for the TPMS in [2]). To exploit all added features of bidirectional communication schemes in WSN while avoiding the use of two separate an- tennas, this work proposes the design of a novel and compact double-loop antenna, patent ﬁled [15], resonating at multiple frequencies in the UHF range, whose realization complies with standard low-cost PCB technologies. Another distinct characteristic of this work is that the antenna design is parametric; while the external loop is ﬁxed to limit the overall size, the other elements (inner loop, tuner) are parame- ters that can be conﬁgured to allow the synthesis of different an- tennas. Therefore the antenna design is not ﬁxed and customized only for a speciﬁc application, as in [7], [8]. Different antennas can be easily generated from the same architectural template having the same topology and the same general characteristics but resonating at different speciﬁc frequencies that can be tuned to meet the typical requirements of different national authori- ties. In fact, it is worth noting that 433 MHz and 868 MHz are the typical frequencies used in Western Europe, 315 MHz, 450 MHz and 915 MHz in US, 315 MHz and 915 MHz in South America, 433 MHz and 915 MHz in Australia [16]–[18]. Hereafter in Section II the parametric antenna design is proposed and the effects of the sizing of its building elements (the tuning element, the inner and outer loops, the matching network) on the resonating frequencies are discussed. Some tuning heuristic rules are derived in Section III allowing to generate and realize the desired antenna conﬁguration for a given target of dual-band working frequencies, starting from the parametric antenna template. The antenna design and the con- ﬁguration process are described in Section III, while Section IV 0018-926X/$26.00 © 2011 IEEE