This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available

Craddock, IJ

Dean, M

Luximon, A

Navarro, E

Paul, DL

English

Enrique A. Navarro

Avinash Luximon

Ian J. Craddock

Dominique L. Paul

Michael Dean

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Multilayer and conformal antennas using synthetic dielectric substrates

Multilayer antennas - for example, stacked patches - often employ low permittivity substrates, such as foams. These foams, however, tend to be fragile, difficult to glue, and difficult to bend. In a previous contribution, the authors described how a Duroid substrate may be drilled in order to obtain a low permittivity synthetic dielectric. This paper shows, by means of both practical and numerical experiments, the results of using such a substrate in a range of multilayer antennas.Multilayer antennas - for example, stacked patches - often employ low permittivity substrates, such as foams. These foams, however, tend to be fragile, difficult to glue, and difficult to bend. In a previous contribution, the authors described how a Duroid substrate may be drilled in order to obtain a low permittivity synthetic dielectric. This paper shows, by means of both practical and numerical experiments, the results of using such a substrate in a range of multilayer antennas

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                          Navarro, E., Luximon, A., Craddock, I. J., Paul, D. L., & Dean, M. (2003).Multilayer and conformal antennas using synthetic dielectric substrates.IEEE Transactions on  Antennas and Propagation, 51(4), 905 - 908. [Issue 4].10.1109/TAP.2003.811106Link to published version (if available):10.1109/TAP.2003.811106Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 4, APRIL 2003 905Multilayer and Conformal Antennas Using SyntheticDielectric SubstratesEnrique A. Navarro, Avinash Luximon, Ian J. Craddock,Dominique L. Paul, and Michael DeanAbstract—Multilayer antennas—for example, stacked patches—oftenemploy low permittivity substrates, such as foams. These foams, however,tend to be fragile, difficult to glue, and difficult to bend. In a previous con-tribution, the authors described how a Duroid substrate may be drilled inorder to obtain a low permittivity synthetic dielectric. This paper shows,by means of both practical and numerical experiments, the results of usingsuch a substrate in a range of multilayer antennas.Index Terms—Conformal, finite-difference time-domain (FDTD), multi-layer antennas, synthetic dielectric.I. INTRODUCTIONStacked patch antennas commonly employ three layers of substrate,with the central layer having a relative permittivity close to unity. Thislayer is often made from foam (in [1], for example, a low dielectricfoam with "r = 1:07 is recommended for optimum bandwidth), butthis is usually fragile, difficult to cut, bend, and glue, and unable tosupport a metallization An air-gap is an alternative, but this requiresspacing elements [2] that complicate manufacture and reduce strength.A previous investigation by the authors [3] demonstrated how a lowdielectric constant material can instead be obtained by drilling a latticeof holes in a standard Duroid substrate ("r = 2:2). This is a practicaltechnique for small antennas since numerically controlled machinescan be used to drill the holes. In [3], this “synthetic dielectric” wasemployed as the substrate of a simple single-layer patch—multilayeredantennas were not considered. The objective, however, was always toemploy the material in a multilayer patch.This paper therefore presents the application of the concept in thedesign of various stacked patch antennas. Numerical results from arange of stacked patch antennas using different synthetic substrates arepresented and their input and far-field responses compared to resultsfrom a stacked patch antenna using a central layer of low dielectricfoam. The drilled Duroid is especially useful for conformal designssince the substrate will bend (unlike many foam materials). This is alsodemonstrated here by presenting practical results for both conformaland planar stacked patches.II. METHODOLOGYTo a first approximation, a substrate with electrically small holesmight be expected to behave as though it were a synthetic dielectric—asolid substrate with an average dielectric constant (although it mightalso exhibit other behavior). The working frequency of the resultingantenna could then be obtained by assuming this dielectric constantand employing numerical, semianalytical, or empirical formulas [4].Manuscript received December 18, 2000; revised February 8, 2002.This work was supported by the Government of Valencia under ProjectGV-98-14-126.E. A. Navarro is with the Department of Applied Physics, Universitat de Va-lencia, 46100 Burjassot, Valencia, Spain.A. Luximon, I. J. Craddock, and D. L. Paul are with the Department of Elec-trical and Electronic Engineering, University of Bristol, BS8 1UB Bristol, U.K.M. Dean is with DERA, WR14 3PS Malvern, U.K.Digital Object Identifier 10.1109/TAP.2003.811106Fig. 1. FDTD model of circular stacked patch design with drilled substrate.If a square lattice of circular holes is employed, the average dielectricconstant "r(av) would be"r(av) = "r  (d2    a2) +   a2d2whereas for an equilateral triangular lattice"r(av) = "r  (d2 sin60    a2) +   a2d2  sin60where d is the distance between holes in the square lattice, a is the ra-dius of each hole, and "r is the substrate relative permittivity (generally2.2 in the authors experiments). Obviously, for a hole radius a and dis-tance d between holes, the triangular lattice produces a lower dielectricconstant because of a higher filling factor of the holes.The major drawback of a microstrip patch antenna in its basic formis its inherently narrow bandwidth. The stacked patch antenna is de-signed with the idea that two coupled patches will resonate at differentfrequencies, close to each other, and this frequency response will pro-vide the desired bandwidth.In order to obtain a good input bandwidth, a low permittivity materialis often recommended as the central layer of the stacked packed patch[1]. However, low dielectric constant materials, such as foams, oftenhave disadvantages as described above. It was shown in [3] that a rangeof low dielectric constant materials can be achieved by drilling a latticeof holes in a standard Duroid substrate with "r = 2:2.Now some questions arise.1) Will the drilled Duroid be a satisfactory replacement for a truelow-dielectric foam in the stacked patch?2) The "r(av) will be dependent upon the filling factor of each unitcell. Thus, can we expect the same results for different dimen-sions that have the same "r(av), i.e., the same filling factor?These questions are answered in the following sections by means ofnumerical and practical experiment.III. NUMERICAL VALIDATIONAn finite-difference time-domain (FDTD) model of a three-layer cir-cular stacked patch design (Fig. 1) was initially specified. The lowerand upper substrates have "r = 2:2, separated by a low-permittivitysynthetic dielectric substrate. Thirteen models were used to evaluatethe effect of changing the radius of the holes and the dimensions of0018-926X/03$17.00 © 2003 IEEE906 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 4, APRIL 2003TABLE ICOMPARISON OF FREQUENCY BANDWIDTH OF CIRCULAR STACKED PATCH ANTENNA DESIGNS FOR SOLID AND VARIOUS DRILLED SUBSTRATE LAYERSthe square or triangular lattice. The results of this investigation are pre-sented in Table I—the goal being to obtain an antenna that has the sameperformances as the original stacked patch antenna with a solid centralsubstrate of "r = 1:2 (model 0).Models 3, 5, 7, and 10 have similar performance to model 0—thereturn loss of antennas 5, 7, and 10, matching almost exactly the returnlosses of the original antenna. However, a wider bandwidth is achievedby antenna 3, although the response is slightly shifted 0.1 GHz to higherfrequencies, probably as a result of the large holes (nine holes) onlylocated under the patch. With antenna models 12–13, we get an evenwider bandwidth, and shift in frequency, mainly because the syntheticdielectric is "r = 1:1. This is slightly lower than that of our originalantenna (number 0), which uses a foam having "r = 1:2.So far, in order to obtain the same return loss as the original antenna,we have to make holes in the substrate in order to obtain an average "rvalue that is the same as the desired antenna. As long as "r(av) approx-imates "r = 1:2 of the ideal antenna, the same return loss is obtained.By increasing the filling factor of the holes further, as is seen in cases 12and 13,"r(av) is further reduced, producing a slightly different behaviorwith a wider bandwidth. The dimensions of the square lattice and ra-dius of the holes do not seem to matter as long as the same "r(av) isachieved. Fig. 2 shows S11, for the original antenna with foam (model0), and for a selection of antennas with synthetic dielectrics (models 3,5, 7, and 10).There is also no appreciable influence in the radiation pattern (cal-culated at the center frequency of 9.65 GHz). The levels of cross-polar-ization are the same as those found in the original design. These resultsare presented in Fig. 3. Similar results were obtained for the other syn-thetic dielectric antennas having the same "r(av) and different lattice(square-triangular).IV. EXPERIMENTAL VALIDATIONAttention is now turned to experimental validation of the approach,this time (for ease of construction) using a larger, square version of theprevious modeled structure to work at around a center frequency of 4.9GHz. Each patch was 2 cm across; all three substrates were 0.635 mmin thickness. Versions were constructed using foam (this time with apermittivity of 1.1) as the central layer and using a drilled material with"r(av) = 1:588 (using a triangular lattice, d = 2 mm, a = 0:75 mm)instead.S11 results from the different versions of the antenna are shown inFig. 4. While there are some differences (probably largely attributableIEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 4, APRIL 2003 907Fig. 2. Numerical (FDTD) return losses in antenna number 0 (original with foam), 3, 5, 7, and 10: ( ) antenna 0-original, (.) antenna 3, () antenna 5, (x) antenna7, () antenna 10.Fig. 3. Radiation pattern of antenna 0 (original with foam), 5, and 7.Antenna 0:E-plane: Copolar, Cross-polar, H-plane: Cross-polar, Copolar Antenna5: E-plane:    Copolar,    Cross-polar, H-plane:    Cross-polar,   Copolar; Antenna 7: E-plane: xxx Copolar, xxx Cross-polar, H-plane: xxxCross-polar, xxx Copolar.to construction tolerances since such antennas are sensitive to feed po-sition) between the antennas built using foam and those using the per-forated substrate, the measured bandwidth is very similar.Finally, the individual substrates of the square stacked patch designusing the drilled substrate were heated and bent around a 6-cm-diam-eter cylinder. After gentle heating, the curved substrates—includingthe drilled material—retained their shape and could easily be glued to-gether to form the conformal stacked patch shown in Fig. 5. S11 datafor this antenna are also shown in Fig. 4.While none of these designs was in any way optimized for bandwidth(or input match), all offer similar and respectable (for a coaxial-fedstacked patch) figures of 13–16% for their 10-dB bandwidth.Drilling material from a substrate weakens it. However, it has beenfound perfectly feasible to produce material with relative permittivitiesas low as 1.383 from standard "r = 2:2 Duroid (triangular lattice withd = 1:5 mm, a = 0:65 mm). Provided that the drilled layer is ade-quately supported (as it is when glued to the surrounding layers), anantenna using such a material is rigid enough for most applications.V. CONCLUSIONThis paper has presented modeled and experimental results to sup-port the case that drilled material can be substituted for a layer offoam or air in a stacked antenna design. By using synthetic dielectrics,several stacked patch antennas with similar performances have beendemonstrated, some even better than the original foam design. Thedrilled material is well suited to multilayer planar antennas since it cansupport a metallization and can easily be cut and glued. As the materialcan easily be bent and retains its shape after heating, it is particularlyattractive for conformal designs.908 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 4, APRIL 2003Fig. 4. S results from measurements of various square stacked patches.Fig. 5. Conformal square stacked patch. The dimensions of both patches are20  20 mm . The substrates are 50  50 mm .ACKNOWLEDGMENTThe authors J. P. McGeehan and Prof. I. H. White for provision offacilities at the University of Bristol, U.K.REFERENCES[1] R. B. Waterhouse, J. T. Rowley, and K. H. Joyner, “Stacked shorted patchantenna,” Electron. Lett., vol. 34, no. 7, pp. 612–613, 1998.[2] R. Q. Lee, K. F. Lee, and J. Bobinchak, “Characteristics of a two-layerelectromagnetically coupled rectangular patch antenna,” ElectronicsLetters, vol. 23, no. 20, pp. 1070–1072, 1987.[3] E. A. Navarro, I. J. Craddock, and D. L. Paul, “Synthetic dielectrics forplanar antenna design,” Electronics Letters, vol. 36, no. 6, pp. 491–493,2000.[4] I. J. Bahl and P. Bhartia, Microstrip Antennas. Norwood, MA: Artech,1980.Broad-Band Linear Polarization Sensor Using aTravelling-Wave AntennaV. F. FuscoAbstract—This paper shows how the tilt angle associated with a lin-early polarized signal can be obtained by direct homodyne detection usinga broad-band dual-port travelling-wave circularly polarized antenna. Theresulting sensor would find applications in systems where polarization shiftdue to multipath reflections occurs and requires quantification and/or com-pensation. The operation of the sensor is demonstrated at 1.6–2.4 GHz anda simple analytical model given that establishes the principle of operation.Index Terms—Circular polarization, decomposition, RF sensor, trav-eling-wave antenna.I. INTRODUCTIONModern mobile communications systems use dual slant linear polar-ization in an effort to combat multipath fading [1]. In practice, polariza-tion rotation due to multiple rereflection of lossy dielectric and metallicreflecting surfaces [2] as well as the arbitrary orientation of the handsetantenna mean that signal strength reception based on a slant 45 lin-early polarized antenna can be much worse than if a circularly polar-ized antenna had been used. Reception of a linearly polarized signalreceived by a single-port circularly polarized (CP) antenna will resultin an attendant reduction of 3 dB in received signal strength.A simple method of extracting for a linearly polarized signal its tiltorientation , relative to the x axis, as defined in Fig. 1, is to usetwo orthogonal linearly polarized antennas. This approach relies onsignal-strength manipulation and thus requires element amplitude char-acterization. In addition, if simple linearly polarized elements such asdipoles are used, the arrangement is inherently narrowband. In thispaper, we show that by using a dual-port travelling-wave CP antennaManuscript received November 14, 2001; revised January 31, 2002. Thiswork was supported by EPSRC under GR/R15139.The author is with the School of Electrical and Electronic Engineering,High Frequency Laboratories, BT9 5AH Belfast, Northern Ireland (e-mail:v.fusco@ee.qub.ac.uk).Digital Object Identifier 10.1109/TAP.2003.8111110018-926X/03$17.00 © 2003 IEEE

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Multilayer and conformal antennas using synthetic dielectric substrates

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