DUAL FREQUENCY FLEXIBLE ANTENNA FOR COSPAS SARSAT ESA/ESTEC, NOORDWIJK, THE NETHERLANDS 3-5 OCTOBER 2012 Yiannis (J). Vardaxoglou (1, 2), P. DeMaagt (3), W. G. Whittow (4, 5), A. Chauraya (6), and R. D. Seager (7) (1, 5) Antrum Ltd, Loughborough Innovation Centre, Loughborough University Science & Enterprise Park, Ashby Rd, Loughborough, LE11 3AQ, UK. yiannisv@antrum.co.uk (2) School of Electronic, Electrical and Systems Engineering, Loughborough University, LE11 3TU, UK. j.c.vardaxoglou@lboro.ac.uk (3) Antenna and Submillimetre Wave Section, European Space Agency, The Netherlands. Peter.de.Maagt@esa.int (4) School of Electronic, Electrical and Systems Engineering, Loughborough University, LE11 3TU, UK. w.g.whittow@lboro.ac.uk (6) School of Electronic, Electrical and Systems Engineering, Loughborough University, LE11 3TU, UK. a.chauraya@lboro.ac.uk (7) School of Electronic, Electrical and Systems Engineering, Loughborough University, LE11 3TU, UK. r.d.seager@lboro.ac.uk ABSTRACT Current search and rescue systems require to operate at two main frequency bands, 121.5 MHz and 406 MHz. The low band is used for local telemetry and search by helicopter and/or boat, while the high band is specified for the satellite network. The paper will outline current and future solutions of flexible antennas operating simultaneously in both bands and mounted, for example on survival jackets. The low band has less stringent antenna requirements but both are required to operate near an often unsettled and hostile water environment. As well as encapsulating the low band the high band is primarily required to operate Circular Polarisation (CP). The paper will discuss the dual band antenna, and report on its matching bandwidth, return loss, radiation patterns and CP properties at 406 MHz. interfering with each other, and this is one of the novel features about this design. At the time of writing, we report both the monopole and spiral slotted ground plane fabricated on thin flexible laminates. In future, all these structures will be realised from fabric conductive materials, such as conductive yarns or flexible metal sheets [9, 10]. 1. INTRODUCTION In our previous work, we reported an antenna design integrated into fabric [1-3]. The antenna consisted of a grounded 8 element spiral topology featuring a crossed dipole (turnstile) design, and operated at 406 MHz. This antenna can be considered as an alternative to some reported wearable antennas [4-6], and was tested for its suitability in search and rescue applications, see Figure 1. We now report a fabric based antenna exhibiting dual band response at 121 MHz and 406 MHz, one of the important requirements for a COSPAS-SARSAT system [7]. An Inverted F Antenna (IFA) [8], was added to the 3 sides of a spiral slotted ground plane, which was excited by a monopole (walking stick), and separated by a Denim fabric, 1.6mm thick. Both the IFA and walking stick share the same feed, and both designed such that the 121 MHz and 406 MHz are matched without (c) Figure 1. Pictures of fabric antenna on a survival jacket with person inside water, outside water, and (c) close to a human body.
2. DUAL BAND FABRIC BASED SYSTEM DESIGN The fabricated and measured prototype is shown in Figure 2. Figure 2 shows a measurement set-up comprising of an Anritsu 37397D Vector Network Analyser, and the dual band antenna tested on a flat polystyrene surface. The spiral slotted ground plane is shown in Figure 2. Predicted and Measured S11 (db) 5 0-5 -10-15 -20-25 Predicted Measured -30-35 0.0 0.1 0.2 0.3 0.4 0.5 Frequency (GHz) Figure 3. Comparison between predicted and measured reflection coefficients of the dual band antenna. 5 0 Predicted and Measured S11 (db) -5-10 -15-20 -25 Predicted Measured -30-35 0.05 0.08 0.11 0.14 0.17 0.20 Frequency (GHz) Figure 3. The enlarged low band S11 performance in Figure 3 above. Figure 2. Prototype of the dual band antenna showing both the 121MHz and 406MHz band structures on the surface, and spiral slotted ground plane. 3.2 Measured Impedance 3. RESULTS 3.1 Predicted and Measured S11 In Figure 3, we show both the predicted and measured return loss results of the dual band antenna. The focus of the design was to achieve match better than -10dB level across the two bands, and the low band was as low as -24dB, whilst the worst case in the 406 MHz band was about -11dB. The measured prototype operated at 121 MHz and 406 MHz, and covered the -10dB level fractional bandwidth of 1.7% and 9.5% respectively. It is envisaged that such wide bandwidth can mitigate the detuning effects arising from harsh water environments and the proximity to human body. Figure 4. Measured impedance response between 100 MHz and 140 MHz. Generally, it is challenging to achieve a good match for both the 121 MHz and 406 MHz bands when fed from
the same port. Here we adopted to feed the IFA from the side of the monopole, and the shape and dimensions of the three sides were optimised using simulations. Designs were modelled using a full-wave Electromagnetic (EM) solver, Empire XCcel TM. The measured impedance in terms of real and imaginary parts show that there exists resonance and the match of about 50 Ω at both 121 MHz and 406 MHz. The measured impedance characteristics at 121 MHz and 406 MHz are shown in Figure 4 and 5, respectively. in Figure 7 and 8, in the 2-D and 3-D format respectively. These patterns are similar to conventional monopole plots, and the peak predicted gain at 121 MHz is -4.74dBi. Figure 5. Measured impedance characteristics from 380MHz to 420MHz. Figure 7 2-D Radiation plots in the principal planes at 121 MHz. 3.3 Simulated surface currents and radiation plots It can be noted that simulations of the surface currents show that high currents are concentrated on the on the IFA and spiral at 121 MHz and 406 MHz. The pictures of the surface current plots are shown in Figure 6. Figure 6. Screen snap shots of surface currents at 121 MHz, and 406 MHz The simulated radiation patterns at 121 MHz are shown Figure 8. Predicted 3 D Radiation plots at f = 121 MHz : EPhi and ETheta all scan.
At 406 MHz, the patterns are fairly CP and have a maximum gain of 2.4dBi, see Figure 9. Theoretical predictions suggest that the gain value can be improved by increasing the ground plane size, and in real life situations, the surface of water can be considered as an infinite ground plane size. level fractional bandwidth at 406 MHz is as high as 9.5%, and depending upon the application, the suggested fabric antenna system is less likely to suffer adversely from detuning caused by environmental effects. Also depending on the application, this proposed flexible antenna can be easily integrated and/or retrofitted into clothing and existing products. 5. Acknowledgement The authors wish to thank ESA/ESTEC, The Netherlands, and Antrum Ltd, UK for funding and supporting this work. Figure 9. Predicted 2-D and 3-D Radiation plots LHCP at 406 MHz 4. CONCLUSIONS Monopole antennas are commonly used in COSPAS- SARSAT systems, and are ultimately limited for applications where bending and safety are required. This paper has presented an alternative antenna system, that is flexible and fabric based. The proposed antenna system exhibits dual band performance at 121 MHz and 406 MHz. The EM simulated results are in good agreement with measured results. The measured 10dB 6. REFERENCES 1. R. D. Seager, A. Chauraya, J. C. Vardaxoglou, and P. de Maagt, "Towards a compact low frequency woven antenna," presented at Antennas and Propagation Society International Symposium, 2009. APS/URSI '09., 2009. 2. W. G. Whittow, A. Chauraya,, R. D. Seager, J. C. Vardaxoglou, P. de Maagt, Wearable Circularly Polarised VHF Antennas for Emergency Applications. In 33rd ESA Workshop on Challenges for Space Antenna Systems, 18-21 Oct, 2011, Netherlands. 3. T. Acti, S. Zhang, A. Chauraya, W. G. Whittow, R. Seager, T. Dias, and Y. Vardaxoglou, "High Performance Flexible Fabric Electronics for Megahertz Frequency Communications," presented at 2011 Loughborough Antennas & Propagation Conference (LAPC), Loughborough, UK, 2011. 4. J. C. G. Matthews, B. P. Pirollo, A. J. Tyler, and G. Pettitt, "Wide-band Body Wearable Antennas," presented at Wideband, Multiband Antennas and Arrays for Defence or Civil Applications, 2008 Institution of Engineering and Technology Seminar on, 2008. 5. B. Sanz-Izquierdo, J. C. Batchelor, and M. I. Sobhy, "Compact UWB Wearable Antenna," presented at Antennas and Propagation Conference, 2007. LAPC 2007. Loughborough, 2007. 6. Z. Shaozhen and R. Langley, "Dual-Band Wearable Textile Antenna on an EBG Substrate," Antennas and Propagation, IEEE Transactions on, vol. 57, pp. 926-935, 2009. 7. COSPAS-SARSAT, "COSPAS-SARSAT 406MHz Distress Beacon Approval Type Standard. C/S T.007. Issue 4 Revision 5," vol. Oct, 2010. 8. B. Qiang, and R. Langley, "Crumpling of PIFA
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