Feasibility of a triple mode, low SAR material coated antenna for mobile handsets

Transcription

1 Loughboough Univesity Institutional Repositoy Feasibility of a tiple mode, low SAR mateial coated antenna fo mobile handsets This item was submitted to Loughboough Univesity's Institutional Repositoy by the/an autho. Citation: KITRA, M.I.... et al (25). Feasibility of a tiple mode, low SAR mateial coated antenna fo mobile handsets. IN: Poceedings of Loughboough Antennas and Popagation Confeence, Loughboough Univesity, Loughboough, Apil Additional Infomation: This is a confeence pape Metadata Recod: Publishe: c Loughboough Univesity Please cite the published vesion.

2 This item was submitted to Loughboough s Institutional Repositoy by the autho and is made available unde the following Ceative Commons Licence conditions. Fo the full text of this licence, please go to:

3 FEASIBILITY OF A TRIPLE MODE, LOW-SAR MATERIAL COATED ANTENNA FOR MOBILE HANDSETS M.I. Kita, C.J. Panagamuwa, P. McEvoy, J.C. Vadaxoglou, J.R. James CMCR, Electonic & Electical Engineeing, Loughboough Univesity, UK Abstact It has been peviously established that mateial loaded monopoles having equal pemittivity and pemeability values, give inceased bandwidth and adiation efficiency fo a given size eduction. Vey low Specific Absoption Rates (SAR) have been obtained by othogonally positioning the antenna towads the head. Recent esults obtained fom a TLM simulato indicate the antenna s capability to opeate in dual mode. In this pesent pape vaious designs ae eseached, in ode to peseve and tanslate the antenna featues into a moe compact embedded vesion. A final design solution is pesented with the addition of metal stips, confiming the ealisation of a small, tiple-band, low-sar handset antenna. 1. Intoduction The apid development of wieless communications has esulted in the escalating need of handset manufactues to cove seveal opeational bands with a single antenna. At the same time, thee is continued inteest in futhe educing the size of the antenna and its Specific Absoption Rate (SAR) whilst maintaining a good efficiency. The use of ceamic mateials povides a useful means of compacting the antenna but the bandwidth and efficiency can be significantly educed. It has ecently been demonstated [1] that the inclusion of feite mateial can ovecome some of the afoementioned constaints. Boad band vesions of mateial loaded antennas have been investigated [4] and the effect of the aspect atio has been detemined. In addition, vey low SAR values have been achieved [2] by oienting the antenna othogonal to the head. In this pesent pape, an attempt is made to incopoate the above design methodologies into a mobile handset and at the same time ceate a multi-band antenna, coveing the GSM18, GSM19 and Bluetooth bands. The antenna height is constained to 1 cm and the vaious design options involving coupled metal stips ae examined. In conclusion, an optimum design is identified and the implementation is discussed. 2. Efficiency and bandwidth optimisation An analytical model based on a ceamic spheical esonato excited by a small wie pobe o loop [1] has initially established the pefomance benefits of intoducing pemeability into dielectic esonato antennas. The geneality of the benefits [2, 3] has been confimed by extensive simulation of ectangula and othe iegula shapes using Flomeics Micostipes TLM. The efficiency and bandwidth ae maximised fo any paticula mode when the pemeability and pemittivity ae of compaable values. The additional design feedom intoduced with μ povides moe contol ove the fequency sepaation between adjacent modes and hence allows wideband dual mode opeation by coupling togethe two adjacent modes. The antenna SAR is mainly

4 dependent on the positioning of the antenna and educes by an ode when the axis of symmety of the antenna is othogonal to the head. The above findings ae shown in Figues 1 and 2, which display the efficiency, bandwidth and SAR vaiations fo a cube antenna (side=3mm) souced by a wie-fed dipole. Fo each pemittivity value, μ and the monopole length wee adjusted to achieve esonance at 1.8GHz. The pemeability fo maximum efficiency is 3.3. Realistic pemeability loss tangents ae included which ae seen to educe the efficiency. Apat fom the intoduction of pemeability, anothe facto that was found to significantly influence mode coupling was the width to height atio of the antenna [4]. A ectangula antenna with dimensions 4x2x2mm and a width to height atio of.5 was modelled and compaed to the peviously simulated 3x3x3mm cube antenna (width to height atio of 1). As the aspect atio deceases, the -1dB bandwidth inceases by 5%. The efficiency and 1gSAR values ae also affected, with the ectangula antenna exhibiting 3.7% incease in efficiency and 12% decease in 1gSAR. It is concluded that a eduction in the width to height atio enhances the wideband dual opeation and geneal pefomance of the antenna but this situation is constained by the height constaint imposed by the handset space available. 3. Handset size constaints The next step involved the integation of the antenna into the handset, tying to peseve its attibutes of high efficiency, boad bandwidth and low-sar. The necessay dimension eductions pesented a lot of constaints, egading efficiency and the application of boad-banding techniques such as the width to height atio. As a stating point to the investigation, it was decided to model a 2x2x1mm ectangula antenna, mounted on a 4x4mm gound plane and fed by a 4.5mm monopole. The antenna was simulated fo vaious atios of pemittivity and pemeability and its efficiency and bandwidth esponse esembled the one of the cube esonato (shown in Figue 1) howeve thei maxima had dopped by 74% and 6% espectively due to the size eduction. In ode to detemine the effect of the length of the block, two moe ectangula antennas wee modelled with dimensions 15x2x1mm and 25x2x1mm. The esults fo all thee esonatos ae shown in Figue 3. It was concluded that inceasing the length esulted in inceased efficiency and a slightly deceased bandwidth. Also, fo a highe length to height atio, the minimum Q shifted to loweε, μ values, favouing antenna ealisation. A simila investigation was made fo the width of the block and a simila conclusion as above was eached. Having detemined the effect of alteing two of the block dimensions, a second mode had to be intoduced to get close to the ultimate aim, of achieving a multi-band handset antenna. Keeping the height and width of the antenna constant at 1mm and 2mm espectively, a set of simulations was un, with 2mm steps fom 15mm to 4mm, to find the length of the antenna that would intoduce a second mode nea 2.4GHz. It was decided to optimise an antenna with dimensions 32x2x1mm.

5 4. Tiple band antenna The initial taget fequency bands fo the handset antenna wee the GSM18 and Bluetooth. The 32x2x1mm block antenna was e-modelled with vaying atios of ε and μ, to detemine values of pemittivity and pemeability that would maintain esonance at 1.8GHz and 2.45GHz and also maximum bandwidth at 1dB. Those values wee found to be ε = 6 and μ = 8. 5 fo tanδ ε, μ =. 3. This esulted in the modes being at the equied fequency, howeve neithe of them coveed the necessay bandwidth. In ode to achieve that, seveal design vaiations wee attempted. It was concluded that the addition of wap-aound metallic stips was the most advantageous. Alteing the thickness and distance sepaating the two ings, the esonance of a new mode could be contolled and added a new band. The final antenna design is shown in Figue 4. By tuningε, μ and the monopole length, the antenna coveed the GSM18, GSM19 and Bluetooth fequency bands at 6dB. The S11 and adiation patten ae shown in Figue 5 and Figue 6 espectively. 5. Specific Absoption Rate The adiation patten of the antenna at 18MHz in Figue 6 is dipole like, as expected. The patten null is diected towads the head and pevious eseach has indicated that this is the main cause behind minimum adiation absoption by the head. When the antenna was modelled against a 75mm spheical phantom ( ε = 41, σ = Ohm/m), the bandwidth emained unchanged, the installed efficiency was 13.9% and the 1g and 1gSAR values wee.25w/kg and.176w/kg espectively. Compaed to the SAR values obtained fom the cube esonato (Figue 2), they had futhe educed. This is mainly attibuted to the deceased efficiency of the antenna (12.19%) compaed to the efficiency of the cube esonato (5%). 6. Conclusions * Design methodologies based on peviously investigated boad band vesions of mateial loaded antennas, evealed the optimum dimensions fo a compact embedded vesion of the antenna. * The addition of metal stips and tuning of the antenna paametes led to the ealisation of a tiple band antenna, coveing the GSM18, GSM19 and Bluetooth bands. The SAR emains at vey low values, due to the pesevation of the antenna null towads the head. * Installation is likely to equie some small design paamete adjustments compatible with a given handset gound plane envionment. 7. Refeences [1] Kita, M.I., McEvoy, P., Vadaxoglou, J.C., James, J.R.: A Theoetical and Simulation Study of Dielectically Loaded Antennas and thei Contibution Towads Low-SAR, Intenational ITG Confeence on Antennas (INICA), Septembe 23, pp [2] Kita, M.I., McEvoy, P., Vadaxoglou, J.C., James, J.R.: A Theoetical and Expeimental Study of Dielectically Loaded Antennas and thei Contibution

6 Towads Low-SAR, 24 IEEE AP-S Intenational Symposium on Antennas and Popagation and USNC/URSI National Radio Science Meeting, June 24,pp [3] Kita, M.I., McEvoy, P., Vadaxoglou, J.C., James, J.R.: Mateial Loaded Antennas and thei Contibution Towads Low-SAR, 24 IEE Antennas Measuements and SAR Confeence, Loughboough Univesity, UK, May 24. [4] Kita, M.I., McEvoy, P., Vadaxoglou, J.C., James, J.R.: Investigation into the ealisation of a low-sar, dual mode mateial coated antenna fo mobile handsets, 25 IEEE Intenational Wokshop on Antenna Technology: Small Antennas and Novel Metamateials, Singapoe, Mach 25, to be published. The authos acknowledge the contibution of Flomeics and the use of thei MicoStipes TLM modelling tool. This wok was suppoted by the UK EPSRC, Gant numbe GR/R94596/1. Efficiency (%) efficiency tand=.1 efficiency tand=.3 bandw idth tand=.1 bandw idth tand= pemittivity Figue1 Efficiency and BW fo cube esonato dB Bandwidth (%) Dipole nomal 1gSAR (W/Kg) Dipole nomal tand=.1 Dipole nomal tand=.3 Dipole paallel tand=.1 Dipole paallel tand= pemittivity Figue 2 1gSAR vaiation fo cube esonato Dipole paallel 1gSAR (W/Kg) Efficiency (%) and -1 db Bandwidth (%) Efficiency W1=15 Efficiency W1=2 Efficiency W1=25 BW W1=15 BW W1=2 BW W1= pemittivity Figue 3- Efficiency and BW fo diff. lengths Figue 4 Ti-band Antenna design Figue 5 S11 gaph of ti-band antenna Figue 6 Patten cuts of ti-band antenna