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PBG-Assisted Gain Enhancement of Patch Antennas on High-Dielectric Constant SubstrateF.R. ang, R. Coccioli, Y. Qian, T. ItohElectrical Engineering Department, University of California at Lo s Angeles, CA 90095

Abstract: The recently developed Uniplanar Compact PBG (UC-PBG) substrate issuccessfully u s e d to reduce surface wave losses of a patch antenna on a high dielectricconstant substrate. The surface wave dispersion diagram of the UC-PBG substrate hasbeen numerically computed and found to have a complete bandgap in the range offrequency 10.9-13.5GHz. This feature is exploited to enhance both bandwidth andbroadside gain of a patch antenna working at the frequency 11.5GHz in the forbiddenbandgap. Measured esults show a 2.8dBhigher radiation in the broadside direction thanthat of a standard patch antenna built on the same high-permittivity grounded dielectricslab.I. IntroductionMicrostrip antennas are widely used in a broad range of military and commercialapplications mainly because of their advantageous features in t er m of lightweight, lowprofile, low cost, and easy manifacturability. The demand of ever increasing frequenciesand decreasing cost in many applications requires innovative design with high integrationlevel of active components, circuitry, and radiating elements. While compact circuitdesign is best achieved on high dielectric constant substrates, optimum performancepatch antennas are built on low permittivity substrate [11. To avoid using costly hybridtechnology, innovative design must be develop to successfully integrate antenna withcircuitry on high dielectric substrates. This novel RF system architecture reduces size,weight, losses and it is also suitable for integration with microelectromechanical systemsto realize reconfigurable circuits and antennas .

Two technologies have been mainly pursued so far to achieve microstripantennas on high-dielectric substrate with optimum performance. One is based onmicromachining technology [24] while the other makes use of the concept of photonicband-gap substrates [5-71. In the fust case, part of the substrate underneath the radiatingelement is removed to realize a low effective dielectric constant environment for theantenna. In this way, power losses due to surface wave excitation are reduced whilecoupling of radiated power to space waves is enhanced. The second approach relies onthe concept of photonic band-gap engineering: the high-permittivity substrate isperiodically loaded to create a so called electromagnetic crystal whose surface wavedispersion diagram presents a forbidden frequency range around the desired antennaoperative frequency. Because surface waves cannot propagate along the substrate, anincreased amount of radiated power couples to space waves reducing antenna losseswhile increasing its gain and bandwidth.Previously proposed PBG substrates were obtained by drilling a periodic patternof holes in the substrate [SI (this technique has also been used in [2] to synthesize lowdielectric-constant substrates) or by etching a periodic pattern of circles in the groundplane [8,9]. A more effective and compact approach, which makes use of a triangular or

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square lattice of metallic pads connected to ground with vias, has been recently proposedin [6,7]. Both computations and measurements have been presented for thi s family ofcrystals, showing the presence of a complete forbidden bandgap for surface waves (forboth TE and TM polarized waves as well as for each direction of propagation along thesubstrate) [6,7]. The structures presented in [6,7] are the first realizations of compactPBG subsuates with complete forbidden bandgap in the radic-frequency range.

Subsequent research efforts have lead to the realization of a Uniplanar CompactPBG substrate (UC-PBG) whose applications to planar slow-wave structures and low-leakage Conductor-Backed Coplanar Waveguide have been recently presented in [10,11].Advantageous features of this crystal substrate include simple, low cost manufacturing(novias are necessary) and compatibility with standard MMICs fabrication technology.In this work a theoretical characterization of the UC-PBG is presented and employed toenhance gain and bandwidth performance of a microstrip antenna on high-permittivitysubstrate.Measwed results confiim the expectation suggested by the numerical analysis.11.PBG substrate: numerical analysisThe recently developed Uniplanar Compact PBG substrate (UC-PBG) sketched in Fig. 1has been used in this work to enhance the efficiency of a patch antenna. The features ofthis substrate have been experimentally verified and exploited to realize a slow-waveplanar structure [IO] nd a low-loss Conductor Backed Coplanar Waveguide [ll]. TheUC-PBG configuration employed in this work is designed starting from a groundeddielectric slab 25mil thick with dielectric constant 10.2. The period of the structure is 120mil. Characteristic dimensions of the metallic pattern are given in Fig. 1.An in-housedeveloped three-dimensional FDTD code has been used to compute the dispersiondiagram of the first few surface wave modes and the computed dispersion diagram isshown in Fig. 2. Two complete forbidden bandgaps are seen in the frequency range 10.9-13.5GHz and 18-21.8GHz between the first and second mode, and third and fourth mode,respectively.

Further numerical analysis reveals that the fust mode is a predominantly TMmode with magnetic field circulating around the strips connecting the square pads andwith a significant component of longitudinal electric field. The second mode instead ispredominantly TE with a significant component of longitudinal magnetic field andtransverse electric field mainly confined within the gaps between metallic islands.

(a) (b )Fig. 1. - U) Schematic of UC-PBG: substrate thickness t=25mil, dielectric constant-10.2; (b )unit cell dimensions (inmil): a =120; w=10; 1=30; d=27.5; g=lO; gl=20.

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noFig. 2 -UC-PBG surface waves dispersion diagram. Fig. 3-E-plane CO-polarpatternsIII.Patch antenna measurementsA microstrip fed rectangular patch antenna with inset feed has been designed to work at11.5GH.z and etched on a 25mil thick substrate with -10.2. The UC-PBG metallicpattern is etched around the patch antenna to disrupt surface wave formation in thesubstrate at the antenna operative frequency. Because the computed results indicate acomplete bandgap, the relative orientation between the radiating edges of the antenna andthe UC-PBG latt ice s not an issue. However, for practical purposes the radiating edges ofthe patch have been aligned to the y-axis shown in Fig. l(a). It is worth noting that theUC-PBG is built at a distance from the antenna roughly equal to twice the substratethickness so to avoid perturbing the TMlo cavity mode and the f ringing field around thepatch. Also, the same distance is maintained between the microstrip line feeding theantenna and the UC-PBG pattern not to a lter the characteristic mpedance of the line.

As a reference, a patch antenna with exactly the same layout has been etched on agrounded dielectric slab with same thickness and dielectric constant as that used torealize the PBG-surrounded antenna. First, the return loss from both antennas has beenmeasured. The resonant frequency of the reference antenna is 11.27GHz with a minimumrenun loss of -15.7dB. The resonant frequency of the UC-PBG surrounded antenna isshifted at 11.51GHz, with a minimum return loss of -23.2dB.To determine the amount of energy coupled to space waves, we observed also theradiation pattern of the two patches at 11.43GHz. where the two antennas present thesame input reflection coefficient s l p l1.6dB.ig. 3shows the co-polarization radiationpattern of the patch antennas on the E-plane. The UC-PBG patch has a level of radiatedfield in the broadside direction 2.8dB higher than that measured in the case of thestandard patch. Also, as expected, a reduction (2.5dB) of field radiated along thedirection 9=rd2, which is associated to surface waves, is observed. Suppression ofradiation in the direction 8 =4 2 s not observed because only one period of UC-PBGwas etched around the antenna along that direction.This was done to avoid altering thecharacteristic impedance of the microstrip feeding line.The radiation pattern in the H-plane does not show significant differencesbetween the two antennas. This is also expected because a low leve l of surface waves islaunched along that plane.

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IV.ConclusionsA patch antenna on a novel Uniplanar Compact Photonic Bandgap (UC-PBG) substrate,realized by etching a metallic pattern on the upper surface of a grounded dielectric slab,has been designed, built and tested. Experimental results agree with theoreticalpredictions that show a complete bandgap for surface wave propagation in the range 10.9to 13GHz. The suppression of surface waves produce bandwidth and gain enhancementof a patch antenna designed to work at 11.5GHz, as well as reduction of the side lobelevel in the E-plane. The proposed UC-PBG is not only effective in suppressing surfacewave, it is also compact, easy to fabricate, and do not need the presenceof vias.AcknowledgmentsThis work was supported by U.S.Army MURI under contract DAAH04-96-1-0389 andDAAH04-96-1-0005.V.References..D.M. Pozar, Considerations for millimeter wave printed antennas, IEEETransactions onAntennas and Propagation, vol. AP-33, no. 5 , pp. 740-747, Sept.1983.G.P. Gauthier, A. Courtay, G.M. Rebeii, Microstrip antennas on synthesized low&electric-constant substrates, IEEE Transactions on Antennas and Propagation,vo1.45,no. 8,pp.1310-1314,Aug. 1997.I. Papapolymerou, R. Franklin Drayton, L.P.B. Katehi, Micromachined patchantennas, IEEE Transactions on Antennas and Propagation, vol. 46, no. 2, pp.215-283, Feb. 1998.V.M. Lubecke, K. Miwno, G.M. Rebek, Micromac&ing for mahertzapplications, IEEE Transactions onMicrowave Th eory and Techniques, vol. 46,no. 11 ,pt. 2, pp.1821-1831,Nov. 1998.E.R. rown, C.D. Parker, E. Yablonovitch, Radiation propenies of a planarantenna on a photonic-crystal substrate, Journal of rhe Optical Society ofAmerica B, vol. 10, no. 2, pp. 404-407, Feb. 1993.Y. Qian, R. Coccioli, D. Sievenpiper, V. Radisic, E. Yablonovitch, T. Itoh,Microstrip Patch Antenna Using Novel Photonic Band-Gap Structures, toappear, Microwave Journal, January 1999.D. Sievenpiper, E. Yablonovitch, High-Impedance Electromagnetic CrystalGround Planes, Workshop on Electromagnetic Crystal Structures, Design,Synthesis, and Applications (WECS), Laguna Beach, California, Jan. 6-8, 1999.V. Radisic, Y. Qian, R. Coccioli, T. Itoh, A novel 2-D photonic band-gapstructure for microstrip lines, IEEE Microwave and Guided Wave Letters, vol. 8,no.2, Feb. 1998.R. Coccioli, W.R. Deal,T. Itoh, Radiation characteristics of a patch antenna on athin PBG substrate,IEEE A p - S International Symposium 98 , Atlanta, GeorgiaJune 21-26, 1998.F.R. Yang, Y. Qian, R. Coccioli, T. Itoh, A Novel Low Loss Slow-WaveMicrostrip Structure, EEE Microwave and Guided Wave Letters, voL 8, no. 11,K.P. Ma, F.R. Yang, Y. Qian, T. Itoh, NonleakyConductor-Backed CPW usinga novel 2D-PBG latticeAsia-Pacijic Microwave Conference, Yokohama, Japan,December 8-11. 1998.

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