Forum for Electromagnetic Research Methods and Application Technologies (FERMAT) Magneto-dielectric Substrate Influence on the Efficiency of a Reconfigurable Patch Antenna E. Andreou 1,2, T. Zervos 1, E. Varouti 3, M. Pissas 3, C. Christides 4, A.A. Alexandridis 1, G. Fikioris 2 1 Institute of Informatics & Telecommunications, National Centre for Scientific Research Demokritos, Athens, Greece 2 School of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece 3 Institute of Nanoscience & Nanotechnology, National Centre for Scientific Research Demokritos, Athens, Greece 4 Department of Computer Engineering & Informatics, University of Patras, Rion, Greece e-mail: eandre@iit.demokritos.gr
Copyright The use of this work is restricted solely for academic purposes. The authors of this work owns the copyright and no reproduction in any form is permitted without written permission by the authors. 2
Biography Evmorfili Andreou received the B.Sc. degree in Physics from the Aristotle University of Thessaloniki in 2010 and the Master degree in Electronic Physics (Radioelectrology) with specialization in Electronic Telecommunications Technology from the same university in 2013. Currently, she is a Ph.D. student at the School of Electrical and Computer Engineering of the National Technical University of Athens (NTUA). She is working toward the Ph.D. in collaboration with the Wireless Communications Laboratory of the Institute of Informatics & Telecommunications of NCSR Demokritos. Her current research interests include the design, implementation and evaluation of reconfigurable antenna systems, antenna measurements and channel modeling. She has four publications in conference proceedings. 3
Overview Abstract Background Objectives Antenna design and operation Effect of dielectric losses in OFF State Effect of dielectric and magnetic losses in ON State Feasibility study of fabricating YIG-Epoxy composite substrate with improved properties Antenna Prototyping Measurements Conclusions 4
Abstract We investigate the influence of dielectric and magnetic losses of an Epoxy bonded Yttrium Iron Garnet (YIG-Epoxy) composite substrate on the performance of a reconfigurable microstrip patch antenna. The antenna under investigation is a microstrip patch antenna printed on a 60/40 w/w% YIG-Epoxy composite substrate and controlled by an externally applied DC magnetic field. It is observed that high dielectric losses of the composite substrate can dramatically deteriorate the radiation efficiency of the proposed antenna in the demagnetized state, while in the magnetized state the magnitude of the resonance linewidth (ΔΗ) of the composite substrate is the main parameter responsible for the reduction or improvement of the radiation efficiency. Considerable effort has been made to fabricate a YIG-Epoxy substrate with improved properties regarding dielectric loss tangent and magnetic resonance linewidth. The prototyping and characterization of a reconfigurable patch antenna on the improved YIG-Epoxy substrate is also presented. Keywords: YIG-Epoxy Compound, Dielectric Losses, Resonance Linewidth, Antenna Radiation Efficiency, Reconfigurability. 5
Background The design of compact, reconfigurable and efficient antennas has gained significant interest over the last years. Among the various proposed techniques in literature, ferrite materials seem to be a promising solution. The basic idea behind magnetized ferrites, is that by properly tuning the external applied magnetic field, the frequency dependent permeability tensor of the ferrite varies and thus the antenna s characteristics can be altered. In our previous work, we have proposed several antenna configurations with controllable characteristics (frequency, polarization, pattern). For instance, in [1]-[3], the reconfigurable properties are attained when a bulk YIG is incorporated in the antenna design. In order to overcome difficulties that arise during the prototyping process, we alternatively proposed the fabrication of a reconfigurable antenna printed on a YIG-Epoxy composite substrate [4]. By mixing YIG powder and Epoxy resin, it is feasible to fabricate a magnetic composite with significantly low relative permittivity (ε r equal to 3.65) compared to pure ceramic ferrite materials which typically have high relative permittivity ε r (within the range 10 to 20). However, the main drawback of the proposed reconfigurable antenna [4] is its low radiation efficiency. According to measurement results the antenna radiation efficiency in the demagnetized state ( OFF state) is 50%, while in the magnetized state ( ON state) is only 30%. 6
Objectives Examination of the critical parameters that deteriorate the radiation efficiency of the proposed reconfigurable antenna through simulations and measurements Investigation of the influence of dielectric and magnetic losses of Epoxy bonded Yttrium Iron Garnet (YIG- Epoxy) composite substrate on the performance of reconfigurable microstrip patch antenna Feasibility study of fabricating YIG-Epoxy composite substrate with improved properties, regarding dielectric loss tangent (tanδ) and magnetic resonance linewidth (ΔΗ) Prototyping and evaluation measurements of reconfigurable patch antenna printed on the improved composite substrate 7
Antenna design and operation Antenna design and simulation was carried out using the EM simulation software package, CST MW Studio. The patch is printed on a 1.82mm thick 60/40ww% YIG-Epoxy substrate with the following properties: o Dielectric: εr=3.65, tanδ=0.024 o Magnetic: 4πMs=316.14 G, ΔΗ=400 Oe Reconfigurable operation of antenna o Magnetic unbiased state - OFF state: No application of external magnetic field (H o =0 Oe) o Magnetized state - ON state: Application of external magnetic field in the z-direction (H o >0 Oe) The appropriate modeling of the OFF and ON state of the antenna, requires the use of different permeability dispersion models o OFF state: Schloemann model [6] o ON state: Gyrotropic model [7] 8
OFF state- Effect of dielectric losses (1) Dielectric losses are described by the dielectric loss tangent (tanδ). We study the influence of tanδ on antenna s performance in the OFF state by changing its value. Three cases are studied: a) tanδ = 24 10-3 (dielectric losses of Composite_1 used in the initial design [4]) b) tanδ = 24 10-4 (one order of magnitude lower than 24 10-3 ) and c) tanδ = 5 10-2 Variations of S 11 as tanδ changes ( OFF state) 9
OFF state - Effect of dielectric losses (2) Effect of tanδ on simulated antenna properties ( OFF state) Dielectric Loss Tangent tanδ f res (GHz) Radiation Efficiency η rad (%) Polar. type 24 10-4 5.30 90 LP 24 10-3 5.40 55 LP 5 10-2 5.39 38 LP Antenna s radiation efficiency at resonance frequency is improved as dielectric losses (tanδ) of YIG-Epoxy composite substrate become smaller, and vice versa. The polarization type of the antenna is kept linear (LP) and the operating frequency is around 5.4 GHz in all the studied cases. When tanδ=24 10-4, the impedance matching of the antenna can be improved by properly changing the dimensions of the feed line inset. 10
ON state - Effect of dielectric losses (1) The next step of our study is to examine the feasibility of improving the antenna s performance in the ON state. The external magnetic field is applied perpendicular substrate (+z axis). to the antenna We set the external magnetic field H o equal to 1300 Oe and vary the value of tanδ; the same procedure as in the OFF state. Variations of S 11 as tanδ changes ( ON state) 11
ON state - Effect of dielectric losses (2) Effect of tanδ on simulated antenna properties ( ON state) Dielectric Loss Tangent tanδ f res (GHz) Radiation Efficiency η rad (%) AR at f res (db) 24 10-4 4.82 36 5.6 24 10-3 4.82 29 5.2 5 10-2 4.80 23 4.7 As tanδ lowers, η rad increases. Also, η rad is reduced compared to the corresponding in the OFF state. This additional reduction is assumed to be a result of the high magnetic losses of the fabricated Composite_1 YIG- Epoxy substrate (ΔΗ=400 Oe). 12
ON state - Effect of magnetic losses Magnetic losses are described by the resonance linewidth (ΔΗ). In ceramic materials, such as ferrites, the value of ΔΗ depends on the material preparation and can practically take values within the range 10 up to few hundred Oe. Materials with low ΔΗ are preferable. Effect of ΔΗ on simulated antenna properties ( ON state) Resonance Linewidth ΔΗ f res (GHz) Radiation Efficiency η rad (%) AR at f res (db) 100 4.8 63 5.8 200 4.81 50 5.7 300 4.82 42 5.7 400 4.82 36 5.6 As ΔΗ lowers, η rad is improved. Also, the polarization type of the antenna becomes elliptical approaching circular (the axial ratio (AR) at the resonant frequency is approximately 5dB). 13
Feasibility Study of Fabricating YIG-Epoxy Substrate with Improved Properties (1) Our main objective is to control the critical properties (tanδ, ΔΗ) of the YIG- Epoxy composite substrate during the manufacturing process. An improved version of YIG-Epoxy composite can practically be obtained by choosing and mixing YIG powder and epoxy resin with desirable properties (low losses). Based on literature, the majority of commercial epoxy resins have high dielectric losses (tanδ order of magnitude: 10-2 ). After experimental tests, we decided to use an epoxy resin produced by BISON company, which is slightly better than the epoxy resin of the Struers company used in our previous work in Composite_1 [8]. BISON epoxy resin is mixed with the powder of commercially available YIG garnet of Temex company (YIG_2). YIG_2 [9] was chosen due to its lower values of tanδ and ΔΗ (tanδ=10-4, ΔΗ=18) compared to the corresponding values of YIG powder (YIG_1) used in the initial fabricated YIG-Epoxy Composite_1 [8]. 14
Feasibility Study of Fabricating YIG-Epoxy Substrate with Improved Properties (2) tanδ of Bison and Struers Epoxy, YIG_1, YIG_2 and both fabricated composite substrates The values of tanδ were deduced by measurement of the samples with an Agilent E498A precision LCR meter at 300 K and low frequency band (1kHz-2MHz). Τhe difference between tanδ of Composite_1 and Composite_2 is almost one order of magnitude. Based on our experience, this trend will be also observed in higher frequency bands. 15
Feasibility Study of Fabricating YIG-Epoxy Substrate with Improved Properties (3) Magnetic properties of materials under investigation Material Resonance linewidth ΔΗ (Oe) Saturation Magnetization 4πΜ s (G) YIG_1 104 1832 YIG_2 (Temex) 18 1820 Composite_1 400 316.14 Composite_2 370 220 ΔΗ of Composite_2 substrate is almost 20 times higher than the pure YIG garnet of Temex company (YIG_2) due to the mixing procedure of YIG powder and Epoxy resin. It can be explained by a similar mechanism as the one that describes the behavior of ceramic materials with different porosity. 16
Antenna Prototyping Measurements Patch printed on Composite_2 substrate Patch printed on Composite_1 substrate The proposed antenna is printed on the new fabricated YIG-Epoxy substrate (Composite_2). The antenna prototype printed on Composite_1 substrate is presented in our previous work [4]. We used both prototypes inside an anechoic chamber to conduct comparative measurements of the reflection coefficient (S 11 ) and the axial ratio (AR) at resonance frequency in OFF and ON state. 17
Antenna Measurements S 11 measurements ( OFF and ON state) Verification of antenna s reconfigurable operation o OFF state : the antenna operates at 5.26 GHz and is linearly polarized, o ON state: two resonances appear at 4.625 GHz and 5.385 GHz. The measured axial ratio at f res =4.625 GHz is approximately 4dB, which indicates that the proposed antenna s polarization changes from linear to almost circular Preliminary 2D far field transmission measurements (S 21 ) in anechoic chamber present an increase in the radiated power. Based on this, improvement of the radiation efficiency is expected to be proved through extensive 3D radiation diagram measurements. 18
Conclusions A detailed study on the influence of the dielectric and the magnetic losses of the composite substrate in the proposed antenna s radiation efficiency was presented. Different loss mechanisms are responsible for the deterioration of antenna s radiation efficiency in the OFF and ON state. o OFF state: dielectric losses (tanδ) mainly affect the antenna s performance, o ON state: the magnitude of the magnetic losses (ΔΗ) determines the reduction level in the radiation efficiency. Interesting attempt to fabricate a YIG-Epoxy composite substrate with improved characteristics. Preliminary 2D far field transmission measurements (S 21 ) in anechoic chamber present an increase in the radiated power. Based on this, improvement of the radiation efficiency is expected to be proved through extensive 3D radiation diagram measurements. 19
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