Antenna Design for a Wireless Sensor Network Node

Transcription

1 Antenna Design for a Wireless Sensor Network Node Tiago Parra, Nuno Pires and António A. Moreira Instituto de Telecomunicações, Instituto Superior Técnico Av. Rovisco Pais 1, Lisboa, Portugal tiago.parra@ist.utl.pt, nuno.pires@ist.utl.pt, antonio.moreira@l.it.pt Laboratoire d Electromagnétisme et d Acoustique, École Poltechnique Fédérale de Lausanne STI-IEL, Station 11, CH-115 Lausanne, Switzerland Abstract Previousl to this work, a wireless sensor network (WSN) prototpe node was manufactured including a commercial integrated chip antenna solution which could not establish a satisfactor transmission beond meters. A whip antenna solution was also implemented; however, it increases the overall cost of the node. This work presents a solution based on a low-cost printed Bowtie- Shaped Folded Dipole (BSFD) antenna built on a FR-4 substrate tuned to cover the.4 GHz ISM Band and capable of overcome the drawbacks of the previousl mentioned chip antenna solution. To measure the impedance of the proposed antenna, three baluns and a test fiture were designed. The designed BSFD antenna was simulated on a free space and a prototpe was built and measured, along with the before mentioned structures, in a laborator environment, leading to overall satisfactor results. The BSDF antenna accomplished a better performance than the commercial chip antenna solution, thus making this antenna a candidate for a commerciall viable solution. Inde Terms Wireles Sensor Networks; printed antennas; printed baluns; differential impedance measurements I. INTRODUCTION Recentl there has been an increasing interest in WSNs with interesting applications such as militar, environmental, health, and industrial. As epected, each of these ma demand several application-level requirements such as mechanical dimensions of its nodes. For this reason, antennas for WSNs are in constant development to fulfill these requirements. This work presents an integrated solution of an antenna for an environmental wireless sensor network, namel agricultural, tuned to cover the.4 GHz ISM band. Several project requirements and constrains had to be met in order to present a valid solution. The micro-controller of interest in this work includes a radio-transceiver featuring a RF output port with 1 + j Ω differential impedance. Although this specification immediatel suggests a balanced fed antenna, it is also possible to implement a unbalanced fed antenna using a integrated balun solution. However, one of the man requirements of this project is to avoid etra lumped circuit elements making unbalanced fed antennas not usable. Other requirement is to have an omni-directional radiation pattern. II. ANTENNA DESIGN AND PERFORMANCE Several omni-directional balanced antenna concepts were studied. A new re-designed bowtie antenna (figure 1) based on the concept of [1], a dual-band dipole [] (figure ) and a new designed Bowtie-Shaped Folded Dipole were simulated considering that the fitted all the requirements and manufacturing constraints of this work s project. All antennas were designed with a FR-4 substrate of.8 mm thickness and both BSFD and Bowtie antennas are fed b a coplanar stripline (CPS) while the other b a grounded coplanar stripline (GCPS). Fig. 1. Bowtie antenna Fig.. BSFD antenna Fig.. Dual band dipole (DBD) composed of a dipole and two open loops Table I resumes the overall performance estimation of the considered antennas taking into account several performance parameters. Due to better return loss and bandwidth, the BSFD was implemented on a prototpe node, as illustrated in 6. The simulated reflection coefficient S 11 and antenna magnitude impedance Z of the BSFD are shown in figure 5. The computed radiation patterns, at.45 GHz, for different cutting planes are shown in figure 4 (aes as in figure ). z

2 Ω Differential), namel baluns, were characterised and tested along with the BSFD antenna. Also, a test-fiture is presented and tested. The first designed printed balun (figure 7) based on the concept of [4], named Balun A, consists of five design parts: quarter-wavelength impedance transformer; a power divider; two branches, to introduce a 18 phase shift; a grounded coplanar strip line. These parts were designed according to theor and fine tuned via a CST MWS optimisation. Two characterisation test boards (figure 8) specificall designed to measure the insertion and return loss of this balun reveal a fair agreement with the simulation results as shown in figure9. Antennas Performance parameters Bowtie BFD DBD Ω 1.8 Ω 1 Ω Return Loss (db) Bandwidth (MHz) Peak gain (dbi) ' Efficienc Impedance (Ω) 1 Reference XXX values Values for.45 GHz TABLE I P ERFORMANCE ESTIMATION OF THE PRESENTED ANTENNAS (a) (b) z (c) z Fig. 4. BSFD radiation patterns (linear scaling), at.45 GHz, of different cutting-planes GHz Antenna impedance Z [Ω] Reflection Coefficient S 11 [db] 1.8 Ω Fig. 5. Bowtie-shaped dipole reflection coefficient and antenna magnitude impedance (a) Front View (b) Back view Fig. 7. Balun A (a) Reflection coefficient test board (b) Transmission coefficient test board Fig. 8. Characterisation test boards for Balun A Fig. 6. Node prototpe including the proposed bowtie-shaped folded dipole III. BALUNS DESIGN AND CHARACTERISATION In order to obtain a measurement of the antenna proposal impedance, several microstrip-cps transitions (5 Ω - 1 The second printed balun based on the concept of [5] (figure 1), named balun B, consists of a microstrip-cps transition accomplished through a via structure on the CPS. The 5 Ω microstrip line is supported b a ground plane till the middle of the balun and the other half b a fissured ground plane. The via structure inverts locall the phase of the electric field, hence establishing a differential mode independent of the ground, as shown in figure 11.

3 S 11 S d1 S 1d S dd. S-parameters [db] Reflection Coefficient S 11 [db] Transmission Coefficient S 1 [db] Eperimental Simulation Fig. 9. Balun A reflection coefficient and transmission coefficient Fig. 1. S-parameters: reflection and transmission coefficients. The third balun, a commercial integrated solution (JTI. 45BL15B1), was characterised as a -port device (figure 1) and as a -port device (figure 14). The eperimental results presented in figure 15 show that there is a fair agreement between the characterisation methods. However, the reflection coefficient curves S11 of both methods do not quite matched the datasheet curves. This suggest that the integrated balun solution ma not perform well at.45 GHz. (a) Front View (b) Back view (a) Reflection coefficient test board (b) Transmission coefficient test board Fig. 1. Balun B Fig. 1. -Port characterisation test boards for integrated solution balun (a) Phase: (b) Phase: 18 Fig. 11. E-field (Z-component) at the CPS - differential mode The simulation results of the reflection and transmission coefficients are presented in figure 1 (microstrip feeding point is represented b port 1 and CPS b port ) which illustrates a good agreement with an ideal balun at.45 GHz. No eperimental characterisation was performed in this particular balun. Fig Port characterisation test board for integrated solution balun

4 S-parameters [db] 1 P S 11 RL S 11 Datasheet S 11 P S d1 Datasheet S d1 P S dd (a) (b) z (c) z (d) z.1 Fig. 15. Datasheet, -port device and -port device s-parameters comparison SMA connetor Test fiture Port 1 Balun B1 Balun B Integrated balun Test fiture Antenna under test Test cable VNA Fig. 18. Struc Impedance magnitude Z11 [Ω] Finall a test-fiture (figure 16) based on the concept presented in [], is introduced. This structure provides a port s-parameters measurement result that can be converted to 1-differential-port impedance measurement using equation 1 converted to a 1-port measurement. Previousl to the mentioned measurement, an etension port calibration had to be performed in order to shift the calibration plane to the feeding point of the BSFD antenna. An illustration of the measurement mounting setup and considered calibration planes along with the BSFD is illustrated in Calibration plane Fig. 16. Test-fiture Zd = Port Fig. 19. Measured impedance Z11 Etended calibration plane Fig. 17. Measurement mounting setup Z (1 S11 S + S1 S1 S1 S1 ) (1 S11 )(1 S ) S1 S1 (1) IV. R ESULTS Figure 18 presents the impedance measurements curves of the baluns combined with the BSFD antenna. The impedance Z11 results for the balun A and B including the proposed antenna revealed to be quite fair at.45 GHz, showing an impedance of 57.1 Ω and 5.1 Ω, respectivel. The commercial balun solution showed an impedance of 4.5 Ω. The test-fiture including the antenna, ehibited a differential impedance of 14 Ω against the epected 1 Ω. The measured impedance curves Z11 are illustrated in 19. Table II compares the maimum received power and crosspolarisation values measured in an anechoic chamber between a receiving horn antenna and three node prototpes featuring different antennas which includes the proposed antenna (BSFD). Radiation patterns for theses structures were obtained eperimentall. The chip antenna demonstrated poor results concerning power and directional properties (patterns not shown). The radiation patterns of the BSFD and the whip antenna for the E-plane and H-plane are illustrated in figure. As shown, the (proposed) BSFD antenna has a better performance than the whip antenna. Note that the area shaded in the radiation patterns should not be considered as this is a known limitation of the anechoic chamber used. Antennas BSFD antenna Whip antenna Chip antenna Ma. received power1 Ma. cross-polarisation dbm -. dbm -. dbm db -1.1 db -5.1 db 1 Maimum values for both E-plane and H-plane TABLE II M AXIMUM RECEIVED POWER AND CROSS - POLARISATION FOR THE ANTENNAS MEASURED IN THE ANECHOIC CHAMBER

5 (a) E-plane (b) H-plane Fig.. Whip antenna and proposed antenna radiation pattern comparison (linear scaling). Normalised at the maimum of the proposed antenna. V. CONCLUSIONS This paper presents a comparison of differential fed antennas for an agricultural WSN application. The studied antennas fulfilled several design requirements and manufacturing constraints. Several microstrip-cps transitions needed to characterise the balanced antennas impedance have been presented and satisfactor eperimental results achieved, especiall for the Balun B. According to antenna performance parameters the designed Bowtie-Shaped Folded Dipole proved to be the best solution. A prototpe of this antenna incorporated in WSN node was built and tested. An eperimental comparison with other tested nodes showed that this solution leads to power enhancement and overcoming the range limitations of eisting nodes with chip antennas. REFERENCES [1] I. Rosu. Wideband printed bow-tie antenna. [] P. Nepa, G. Manara, S. Mugnaini, G. Tribellini, S. Cioci, G. Albasini, and E. Sacchi, Differential planar antennas for.4/5. ghz wlan applications, in Antennas and Propagation Societ International Smposium 6, IEEE, Jul 6, pp [] S. Kona, T. Sasamori, T. Tobana, and Y. Isota, Calibration of impedance measurement of a balanced antenna using the s-parameter method, in Proc. ISAP, 1. [4] Y. Qian and T. Itoh, A broadband uniplanar microstrip-to-cps transition, in Microwave Conference Proceedings, APMC 97, 1997 Asia-Pacific, Dec 1997, pp vol.. [5] H.-R. Chuang and L.-C. Kuo, -d fdtd design analsis of a.4- ghz polarization-diversit printed dipole antenna with integrated balun and polarization-switching circuit for wlan and wireless communication applications, Microwave Theor and Techniques, IEEE Transactions on, vol. 51, no., pp , Feb.