Intrnational Confrnc on Mchatronics Enginring and Information Tchnology (ICMEIT 2016) Low Cross-Polarization Slab Wavguid Filtr for Narrow-Wall Slottd Wavguid Array Antnna with High Gain Horn Guoan Xionga, Jin Panb, Yuchng Jiaoc, Zhaonan Jind and Boyuan Ma School of Elctronic Enginring, Univrsity of Elctronic Scinc and Tchnology of China, Chngdu 611731, China agustav_xiong@foxmail.com, bpanjin@ustc.du.cn, c610178809@qq.com, d403572765@qq.com, 331953406@qq.com Kywords: slottd wavguid antnna, high gain, cross-polarization, filtr. Abstract. A low cross-polarization slab wavguid filtr for narrow-wall slottd wavguid array antnna with high gain horn is prsntd. In narrow-wall slottd wavguid array, th horizontal polarization (HP) is th co-polarization, whil th vrtical polarization (VP) is th cross-polarization. Slottd wavguid array is widly usd in radar systm, and th high lvl cross polarization rflcts th ffct of radar antnna. Physical formulas wr prsntd to giv a concis xplanation how this structur rducs cross-polarization. Th gain of only wavguid slottd array antnna is not high nough for radar systm. On this basis, a 40-lmnt low cross-polarization narrow-wall slottd wavguid array antnna with high gain horn in 9.305GHz was dsignd and simulatd. Th antnna has th charactristics of SLL=-30dB, cross-polarization lowr than -40dB and gain=32db in main lob. 1. Introduction Bcaus of light wight, low sid lob and convninc to control sid lob, slottd wavguid antnna has bn widly usd in radar systm. Narrow walls or wid walls of wavguid ar fabricatd in ordr to intrrupt th currnt distribution. Conduction currnts can bypass slots, whil displacmnt currnts can travrs slots. For narrow-wall slottd wavguid array antnna, th slant slots with diffrnt angls and dpths must intrrupt th surfac currnts to radiat TE10 mod wav. Th slant slots mainly radiat horizontal polarization wav, whil vrtical polarization wav is also radiatd from thm. Cross-polarization in such antnna is not nglctabl, which caus additional lobs in azimuth pattrn. Cross-polarization radial componnt is undsird for narrow-wall slottd wavguid array antnna. Many mthods hav bn prsntd to dcras cross-polarization. Crating untitld slots in th wavguid can chang th filds insid wavguid, which can lowr th cross-polarization [1]. [2] prsnts a slot array wavguid antnna using ridg on its narrow wall and six radiation lmnts having a frquncy of 9 GHz. Th antnna has th charactristics SLL=-20dB and cross- polarization quivalnt to -60dB. [3] dsignd an dg-slottd wavguid antnna array with vry low crosspolarization using a pair of shapd iriss that flank th slot instad of th aformntiond wirs or strips in th masurd rsult for 16-lmnt slot shows cross-polarization lvl lss than -40dB. Anothr way to rduc this problm is polarization filtr. Basd on rcnt rports, it sms that most of th works rportd on low cross-polarization narrow-wall slottd wavguid array antnna hav paid attntion to ways to dsign it. But th analysis mthod ar not rportd systmatically. In this papr, th narrow wall slottd wavguid array antnna with low cross-polarization slab wavguid filtr is dsignd and simulatd by HFSS. Th dsignd array antnna has charactristics cross-polarization lowr than -40dB, SLL=-30dB and gain=32db in main lob. 2016. Th authors - Publishd by Atlantis Prss 266
2. Antnna Dsign 2.1 Polarization Filtr Dsign. Fig. 1 Narrow-wall slottd wavguid array antnna with rctangular wavguid polarization filtr and its E fild distribution Tim-harmonic currnt in smi-unboundd rctangular wavguid systm Bcaus trapzoidal slab wavguid can b rgardd as a kind of spcial gradual rctangular wavguid. Trapzoidal slab wavguid filtr can b analyzd by rctangular wavguid mood. Th slab wavguid polarization filtr is shown in Fig.1. Tim-harmonic currnt in smi-unboundd slab wavguid systm can b analyzd by in smi-unboundd rctangular wavguid systm in Fig.1. By mans of Babints principl [4], many of th problms of slot antnnas including slottd wavguid antnna can b rducd to situations solving complmntary linar antnnas for which solutions hav alrady bn obtaind. For narrow wall slottd wavguid array antnna, E fild is shown in Fig. 1. In Fig. 1, E fild can b dcomposd into E H and E V. By th mans of Babints principl, E H can b xcitd by I H in horizontal lctric dipol antnna, and E V can b xcitd by I V in vrtical lctric dipol antnna. In a wavguid systm, whn th oprating frquncy is lss than th cutoff frquncy of TE01 wav (f<fc), TE01 wav mustnt propagatd. Thr is th sam E fild distribution btwn TE01 wav in a rctangular wavguid systm and vrtical lctric dipol antnna. For th purpos of rducing E V (cross-polarization), th condition as bllow must b fulfilld to nsur th falloff of TE01 mood, b 2 (1) whr: b-lngth of narrow wall of wavguid filtr; -lngth of lctromagntic in a fr spac To nsur th propagation of TE 10 mood (co-polarization), th condition as bllow must b fulfilld a 2 (2) whr: a-lngth of broad wall of wavguid filtr; -lngth of lctromagntic in a fr spac In ordr to nsur th cross-polarization fild in this kind of systm, slots can b rgardd as th xcitation sourc for abov smi-unboundd rctangular wavguid systm in Fig.1. E fild xcitd by y dirction currnt J(r) in (x,y,z) can b prsntd by lctric dyadic Grn function of th first kind as follows Er () j G (, rr) Jr () dv (3) V 1 By dyadic Grn functions in lctromagntic thory, w obtain G ( rr, ) k G ( rr, ) ( rr) (4) 2 1 1 y y 2 2 0 n n x n x m y m y sh z z 1 0 ab m n mn a a b b sh z z G(, rr ) sin sin cos cos (5) 2 2 2 1 G(, ) 1 G(, ) G(, ) G1 (, rr) rr x y [ G(, ) ] 2 y y rr rr rr 2 2 z y (6) k x y k y z y 267
2 m 2 n 2 2 1 n 0 whr: mn ( ) ( ) k ; n0 a b 0 ls 2.2 Horn Dsign. A horn antnna may b rgardd as a flard-out wavguid [5]. Th function of th horn is to produc a uniform phas with a largr aprtur than that of th wavguid and hnc gratr dirctivity. Assuming that th rctangular wavguid is nrgizd with a TE 10 mod wav lctric fild, th horn in Fig. 2 flard out in a plan prpndicular to E. This is th plan of th magntic fild H. Hnc, this typ of horn is calld a sctoral horn flard in th H plan or an H-plan sctoral horn. Optimum horn dimnsions can b got in (7)(8). L L (7) cos 2 cos 2 L 1 cos 2 (8) Fig. 2 H-plan sctoral horn Dsign paramtrs of H-plan sctoral horn 2.3 Array antnna Dsign. Fig. 3 shows th structur of 40-lmnt slottd wavguid array antnna with slab wavguid cross-polarization filtr. BJ100 wavguid (a=22.86mm, b=10.16mm, t=1.27mm) is chosn to b slottd. Th filtr is mad up of FR4 layr with coppr of 0.018μm on it. Tabl 1 shows th structur paramtrs. By Taylor synthsis mthod, th amplitud of ach slot in 40 lmnts of SLL=30dB can b gt. Th quivalnt normalizd conductivity distribution is also calculatd by [6] as Fig. 4. Th quivalnt normalizd conductivity distribution is th maximum at an angl among diffrnt dpths. Using (9), th rlationships among angl, quivalnt normalizd conductivity g and slot dpth can b simulatd by HFSS. Th corrspondnc among angl, quivalnt normalizd conductivity g and slot dpth can b shown in Fig. 4. Using Fig. 4 and 4, th angl and dpth of ach slot can b sur. 2 1 2 d S 12 N g ( ) 1 2 (9) 1 S11 whr: α-wavguid loss pr unit lngth; N-numbr of slots Tabl 1 Structur Paramtrs d dx db dc w L 24mm 12.5mm 11mm 15.5mm 1.5mm 350 14.2 Fig. 3 Slottd wavguid array antnna with slab filtr and horn 268
Fig. 5 shows th simulatd radiation pattrn in horizontal plan in 40-lmnt slottd wavguid antnna with cross-polarization filtr in 9.305 GHz. Th simulatd maximum sid lob lvl is -30dB with cross-polarization lvl of -40dB blow th main lob in th band. Fig. 4 Equivalnt normalizd conductivity distribution Corrspondnc among angl, quivalnt normalizd conductivity g and slot dpth 3. Summary Fig. 5 Simulatd radiation pattrn in horizontal plan in 9.305GHz In this papr th principl of dcrasing cross-polarization by slab cross-polarization filtr and high gain by horn antnna is rportd. On this basis, 40-lmnt narrow wall slottd wavguid array antnna with high gain horn in 9.305GHz with this filtr is dsignd and simulatd by HFSS. Th lvl of cross-polarization lobs blow th main lob is lowr than -40dB with th maximum sid lob lvl of -30dB and 32dB gain in main lob, allowing this antnna adapt to radar systm. Rfrncs [1]. W. Wang, J. Jin, J. G. Lu, t al. "Wavguid slottd array antnna with broadband, dual-polarization and low cross-polarization for X-band SAR applications." IEEE Intrnational Radar Confrnc. Arlington, Virginia, USA, 9-12, May, 2005, p. 653-656. [2]. A. R. Mallahzadh, S. M. A. Nzhad. "Cross-Polarization and Siz Rduction of Slottd Wavguid Array Antnna by Angld Ridgs." Europan Confrnc on Antnnas and Propagation. Pragu, Czch Rpublic, 26-30, March, 2012, p.1-5. 269
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