Wideband slot antennas for radar applications - University of ...

WIDEBAND SLOT ANTENNAS FOR RADAR APPLICATIONSAbdelnasser A. Eldek, Atef Z. Elsherbeni, Charles E. Smith and Kai-Fong LeeCenter of Applied Electromagnetic Systems Research (CAESR)Department of Electrical Engineering, The University of Mississippi, University, MS 38677Abstract -In this paper a printed dipole antenna and threeslot antennas are designed to operate at 10 GHz for use in radarsystems. A parametric study of each antenna and comparisonbetween their radiation properties including return loss,bandwidth, directivity, efficiency and radiation patterns for 6-element linear array are introduced. Microstrip slot antennasshow wider bandwidth, less coupling and smaller antenna suecompared with the traditional printed dipole.I. INTRODUCTIONIn present-day radar systems, the need for antennas ofsmall size and high efficiency has generated much attentionin the study of compact microstrip antennas. These antennasexhibit low profile and lightweight properties as well as lowcross polarization radiation in some designs. However,microstrip antennas inherently have narrow bandwidths (BW)and in general are half-wavelength structures operating at thefundamental resonant mode TMoI or TMID [l]. The coplanarpatch antennas (CPAs) introduced in [2] and [3] have 3.4%and 8.8% BW. Researchers have made efforts to overcomethe problem of narrow BW and various configurations havebeen presented to extend the BW. Adding a short on theupper slot of the CPA and varying its length achieved 30 to40% BW [4] at higher frequencies for radar applications.In this paper, printed dipole, coplanar patch-slot (CPA),slot dipole and bow-tie slot antennas have been designed forradar applications with emphasis on size reduction, and.improved BW, coupling and efficiency. Characteristics ofarrays of 6 elements of these antennas are compared with theprinted dipole design, and their S-parameters and radiationproperties are introduced. The related analysis is performedusing the commercial computer software package,Momentum of Agilent Technologies, Advanced DesignSystem (ADS), which is based on the method of moment(MOM) technique for layered media. Momentum solvesmixed potential integral equations (MPIE) using full waveGreen's functions. Validation is performed by comparingsample ADS results with simulations based on the finitedifference time domain (FDTD) technique.width and G is the gap width. In addition to these parameters,h is the height of the substrate, and gr is thc dielectricconstant.The retum loss of the printed dipole based on ADSMomentum is confirmed by comparing the numerical resultsfrom a FDTD simulation. This comparison reveals goodagreement, as shown in Fig. 2. The presented printed dipolehas (K tl, 22, G, /,and h) = (12.4, 0.5, 0.3, 0.4, 0.3 and 1.57mm) and (gr = 2.2).The parametric study starts with the feed line depth L/. Byincreasing Lfi it is noticed that the resonant frequencydecreases, then increases back towards the original frequencyat certain length. It is known that the input impedance for atransmission line is given byzi, = z,Z, + jZ, tan p/Z, + jZ, tan p/ 'At I = .2g/2, Z, = ZL, and from that numerical experiment.2g of this antenna can be defined, and then the effectivepermittivity, can be calculated fromThe Ag of this printed dipole is 23 mm and E,~ is about1.7, which is 77 % of &,. LineCalc, a program within the ADSsoftware package, calculates creJof the two strip feed line tobe 1.78 which shows that the printed dipole antenna structuredecreases the feed line grCfiWA. Printed Dipole Antenna11. ANTENNA ANALYSISThe geometly of printed dipole and its parameters areshown in Fig. 1 where Wrepresents the dipole width, L,is thefeed line length, fl is the dipole height, 12 is the feed lineFig. I. Printed dipole antenna parameters0-7803-7920-9/03/$17.00 02003 IEEE 79 2003 IEEE Radar Conference

SI1 IdB)Fig. 2. ADS Momentum and FDTD results for the printed dipoleantenna.Fig. 4.f(GHnADS Momentum and FDTD results for the slot dipoleantenna.Additional parametric study for this antenna shows thatincreasing W, h and E, reduces the resonant frequency, andthat tl and the feed line parameters control the return losslevel. Further study shows that the dominant factor in thedesign of printed dipoles is W, which traditionally assumed tohe 4 /2. This antenna has more than 12 % BW and 90 %efficiency.8. Slot Dipole AntennaThe slot dipole geometry and its parameters are shown inFig. 3 where W represents the dipole width, SI is the slotheight, L,, is the length of the coplanar waveguide (CPW)feed line, and S2 and G are the width and gap width of theCPW. In addition to these parameters, h is the height of thesubstrate, and E, is the dielectric constant.The slot dipole presented in this paper has for thefollowing parameters, (W, SI, L,,, S2, G and h), the values(19.3, 1.5, 1.5, 0.25, 1, and 1.57 mm) and E~ =2.2. Figure 4shows a comparison between ADS Momentum and FDTDresults for the presented slot dipole. This comparison revealsgood agreement and confirms our design procedure usingMomentum.rIWwc: ,--,-Fig. 3. Slot dipole antenna geometry and parameters.Lcpw behaves like L/ in the printed dipole, and .2, and ~,*fare calculated by the same procedure. The calculated 4 of theslot dipole is found to be 23.5 mm and 1.63 (74% of.$, respectively. The ~,g of the CPW feed line based onLineCalc calculations is 1.576, which shows that the slotdipole antenna structure increases the feed line E,@,By observing the influence of various parameters on theantenna performance, it is found that increasing W, SI, h andE, decreases the resonant frequency, and SI and the feed lineparameters control the return loss level. Further study showsthat the total slot length, calculated at the centerline of theslot, is ahout LE and W is ahout 0.82 4. This antenna canprovide more that 21 % BW and 80 %efficiency.C. Coplanar Patch-Slot AntennaThe CPA antenna consists of a rectangular patchsurrounded by a non-uniform width slot. Antenna geometryand parameters are shown in Fig. 5, where W represents thepatch width, L is the patch length, SI, S2 and S3 are thewidths of the upper, left-right, and lower slots, respectively,S4, Si,and Lcpw are the gap width, feed line width and thelength of the CPW, h is the height of the substrate and isthe dielectric constant. Figure 6 shows a comparison betweenADS and FDTD results for the presented CPA that has (K L.SI , S2, S3. 5’4, S5, L,, and h) = (12.4, 2, 0.5,0.25,0.5,0.25,0.75, 1.5and1.57mm)and~,=2.2.In this antenna, Lcpw behaves like L/ in the printed dipoleantenna design, and ,IK and &fare calculated, using the sameprocedure used for the dipole, to be 23.5mm and 1.54 (70 %E~), respectively. LineCalc calculation of Emfof the CPW feedline is 1.58, which shows that the CPA antenna decreases thefeed line80 2003 IEEE Radar Conference

By observing the influence of various parameters on theantenna performance it is found that increasing W, L, h, E,, SIand S2 and decreasing S3 reduce the resonant frequency. TheCPW feed line parameters control the retum loss level.Although the effect of all these parameters is clear onh, it isnot clear which one parameter can primarily increase the BWof the antenna. In CPA design, the dominant factors are W, Land the total slot length (Ltotal), calculated at the centerlineof the slot, whereLtotal= 2(w+L+Lcpw+s2+s3)+sI-s4-s5. (3)By studying the given design at various center frequencies,it is clear that W is about 0.5& and the Ltoral is about 1.5&At the same time L is about O.l& In general Ltotal controlsthe resonant frequency while patch dimensions and slotwidths control the level of return loss and the resulting BW.This antenna can give more that 17 % BW and 80 %efficiency.D. Bow-Tie Slot AntennaThe geometry of the bow-tie slot antenna and itsparameters are shown in Fig. 7 where W, represents thewidth, L, is the feed line length, L,, L2, L,, L4 and W, definethe bow shape, and SI and S, define the feed line parameters.In addition to these parameters, h is the height of thesubstrate, and E, is the dielectric constant. The presented bowtiedesign has the parameters ( WI, W,, L,, L2, L3, L4, L, s,,SI, and h) being set equal to (22.9, 8.7, 3.5, 20.75, 19.45,7.35, 18.5, 0.25, 3, and 1.57 nun) and &,=2.2. Figure 8 showsa comparison between ADS Momentum and FDTD resultsfor the presented bow-tie slot antenna. Although a stair casegeometry is used in FDTD to define the bow-tie geometry,and only one cell is used in the feed line slot due to memoryrestrictions, the comparison reveals acceptable agreement,which confirms our design procedure using Momentum.W,Fig. 5. Coplanar patch antenna geometly and parameters.Fig. 7. Bow-tie slot antenna geometly and parameters.SI1 (dB)0, Return Loss (dB)0, I--- FDTD jI-ADS If (GHz)Fig. 6. Comparison between the ADS Momenhlm and FDTDresults for the presented CPA.Fig. 8. Comparison between the ADS Mamentnm and FDTDresults for the presented Bow-tie slot antenna.812003 IEEE Radar Conference

....9 10f (GHz)11 12(d)-20 Is2 1-20S51-701-1 -.- CPA '.... .......--2030.. ... . * I d ? % . ... 1 .........-- ; I ---_ Icc ..... .+ .*.-.-.-.-1-.-. -+--40.. .*. ... .........., -.-. I--.>,. .,... I, *....* --I-.... ....8-20,S61I- , -.Slot dipoleI .CPABow-tie .......9 f (GHzl 10 11 12Fig. 11. Rehlm loss and coupling between elements of h-element array module for printed dipole, slot dipole, CPA andbow-tie slot antenna with distance between elements equals to 20.8, 3, 8.5, and 4 mm, respectively. (a) SII, (b)S2l,(c)S31 (d)S41,(e)S51 and(f)S61.83 2003 IEEE Radar Conference

ttSlot dipoletCPABow-tieFig. 12. Radiation pattern in x-y plane..-Printed dipole Slot dipoleBow-tie(a) x-z planewPrinted diwle$6Slot diooleII.3"S"CPA Bow-tie *IFig. 14. Total 3D radiation panem.IV. CONCLUSIONSIn this paper a printed dipole antenna and three microstripslot antennas operating at IO GHz in the X-band (8-12 GHz),are presented. Parametric studies for each antenna showingthe effect of each geometric parameter and antennas'dimensions in terms of h8 are presented. Slot antennasachieve better BW that reaches 40% over the entire band. Inaddition to smaller and less than -24 dB coupling betweenelements is obtained for the 6-element arrays. The efficiencyof the slot antennas is near 80% but less than printed dipole.The cross-polarization level is less than -27 dB in the x-zplane and 4 0 dB in the y-z plane. All antennas show goodradiation pattern stability over the entire band of operation.REFERENCES[I] K-L Wong, "Compact and Broadband MicrostripAntennas", John Wiley andsons, New York, NY, 2002.[2] K. Li, C. H. Cheng, T. Matsuni and M. Izutsu, "Coplanarpatch antennas: principal, simulation and experiment,"Proc. Antennas Propagat. Soc. Int. Symp., Boston, MA,July 2001, vol. 3, pp. 402-405.[3] K. F. Tong, K. Li, T. Matsuni and M. Izutsu, "Widebandcoplanar waveguide fed coplanar patch antennq" Proc.Antennas Propagat. Soc. Int. sy"., Boston, MA, July2001, vol. 3, pp. 406-409.[4] A. Z. Elsherbeni, Abdelnasser A. Eldek, B. N. Baker, C.E. Smith and K-F Lee, "Wideband coplanar patch-slotantennas for radar applications," Proc. AntennasPropagat. Soc. Inl. Symp., Houston, TX, June 2002, vol.2, pp. 436-439.(b) y-z planeFig. 13. Radiation panem in x-z and y-z planes84 2003 IEEE Radar Conference