Novel Application of Circular Frequency Selective Surface for Broadbanding of Monolithic Radome

Transcription

1 Nvel pplicatin f ircular Frequency Selective Surface fr radbanding f Mnlithic Radme Raveendranath U. Nair, T. Madhulata and R.M. Jha mputatinal Electrmagnetic Lab. erpace Electrnic and Sytem iviin Natinal erpace Labratrie (SIR-NL) angalre -567, India. raviunair@nal.re.in, ha@nal.re.in STRT The applicatin f circular aperture frequency elective urface () fr the enhancement f electrmagnetic perfrmance f mnlithic half-wave radme i preented. The perfrmance parameter f the circular aperture type embedded radme panel are cmputed baed n equivalent tranmiin line methd in cnunctin with equivalent circuit methd (EM). The EM analyi hw that the circular embedded mnlithic half-wave radme panel ha uperir perfrmance characteritic a cmpared t the mnlithic wall alne cnfiguratin. Keywrd: Mnlithic radme, Frequency elective urface, Radme perfrmance parameter fr catering extreme aerdynamic tree at high mach peed. ut the mar diadvantage f mnlithic halfwave wall i very narrw bandwidth, which limit it applicatin. Since mdern airbrne radar antenna have multi-band functinal requirement, uch deign may nt be ufficient. Hence in the preent wrk, the deign parameter f circular aperture embedded mnlithic half-wave wall cnfiguratin are ptimied uch that it prvide uperir EM perfrmance a cmpared t mnlithic radme panel alne, ver the X-band.. IRULR PERTURE EMEE MONOLITHI HLF-WVE ROME PNEL ESIGN. INTROUTION In rder t meet the tringent EM perfrmance requirement f mdern airbrne radme, variu radme deign technique (baed n metallic tructure like wire grid, wire mehe, etc.) have been reprted [- 3]. Mt f thee reprted wrk fcued n the enhancement f tranmiin efficiency f the radme panel. Even thugh the tranmiin efficiency f the radme wall cnfiguratin can be imprved with the incluin f metallic tructure, ther perfrmance parameter like breight errr, cr-plar level etc., may degrade dratically. Thi neceitate the develpment f new technique, which can imultaneuly imprve the EM perfrmance parameter f radme wall. In thi regard, find ptential applicatin in the deign f high perfrmance radme due t their inherent frequency elective characteritic. In thi paper, nvel applicatin f circular aperture frequency elective urface fr bradbanding f mnlithic half-wave radme wall cnfiguratin (Fig. ) i preented. Mnlithic half-wave radme wall i generally ued fr the deign f treamlined necne radme a it prvide uperir EM perfrmance at the deign frequency a well a ufficient tructural rigidity ircular aperture ielectric lab Figure Schematic f circular aperture embedded mnlithic half-wave radme The circular array, cniting f peridic array f circular aperture, i embedded in the mid-plane f mnlithic half-wave radme wall (ε r 4.; tanδ e.5). The thickne f the mnlithic half-wave wall fr ptimum pwer tranmiin, i 7.44 mm at the center frequency GH, and at the nrmal incidence. The inductive uceptance f the aperture type circular element i frmulated uing the equivalent circuit mdel (EM). It i incrprated in the equivalent tranmiin line mdel [4] f the circular embedded mnlithic half-wave wall. The ptimied deign parameter f the

2 circular at nrmal incidence (diameter 7. mm and pitch 9.6 mm) and high incidence angle 6 (diameter.7 mm and pitch 37.8 mm) are cmputed. The circular aperture embedded mnlithic radme wall i cnidered a an equivalent tranmiin line with different ectin crrepnding t lab and tructure. The change in the characteritic impedance f the free pace and mnlithic half-wave wall i a mar urce f reflectin f the wave incident n the tructure. In the preent radme wall tructure, the i ymmetrically laded in the mid-plane f the mnlithic lab. cmpared t the free pace, different layer f circular aperture embedded mnlithic wall can be cnidered a lw impedance line cnnected end-t-end. Hence the whle cnfiguratin can be repreented by a ingle matrix btained by the multiplicatin f matrice crrepnding t the individual layer. Since the circular aperture i lcated in the midplane f the mnlithic lab, it i cnidered t be made up f tw identical ectin, with the circular aperture in between them. Let Z be the characteritic impedance f free pace. The characteritic impedance f each half-ectin f mnlithic lab i repreented by Z. In the equivalent tranmiin line methd, the matrix cniting f i, i, i, and i will repreent the i th layer f the radme wall. Let t and d repreent the thickne f each half-ectin f mnlithic lab and circular aperture repectively. Here t (d m - d )/. One half-ectin f the wall i repreented by Φ be the electrical length crrepnding t each layer, which i a functin f the cmplex permittivity ε *, f dielectric layer, the angle f incidence,θ and the thickne f the dielectric layer, d. The ther half-ectin f the wall i repreented by () The interactin f incident EM wave with aperture type i repreented a a wave prpagating alng a tranmiin line, with hunt uceptance repreenting the. The EM characteritic f ingle unit cell f circular aperture are aumed t be ame a that f infinite peridic array. The mnlithic half-wave dielectric lab exhibit capacitive uceptance, while the aperture type circular aperture exhibit inductive uceptance. The inductive uceptance f the aperture type circular apertrure matche with the capacitive uceptance f the entire mnlithic lab t prvide bradband EM perfrmance. Let,, and be the element f the matrix repreenting circular aperture. Then the circular aperture i repreented by Here repreent the hunt uceptance f the circular aperture. The equivalent circuit mdeling f circular aperture wa dicued in [5]. Uing thi mdeling apprach, the inductive uceptance f aperture type fr perpendicular plariatin i given by () (3) 3abλ (4) 3 πd Here a and b are the pacing between the hle. In the preent cae a b. d i the diameter f the hle. λ i the wavelength f the incident EM wave. In rder t eliminate puriu reflectin and phae ditrtin, the medium preent in the aperture ectin f the circular aperture i aumed t be the ame a that f the mnlithic radme wall. The entire mnlithic half-wave wall with circular aperture i repreented by (5)

3 Uing Eqn (5), the,, and parameter f the final matrix are cmputed. The pwer tranmiin cefficient i given by P tr 4 ( ) The pwer reflectin cefficient i given by (6) the deign frequency f GH but it i very high at the end frequencie f X-band. It i intereting t nte that the pwer reflectin f circular aperture embedded tructure i lw (well within %) thrughut the frequency range a cmpared t the mnlithic lab alne cae. P rf (7) 8 6 The phae ditrtin are determined by the inertin phae delay (IP) f the radme wall. Fr the preent radme wall cnfiguratin, tw ymmetric dielectric layer and ne layer are cacaded. Hence the inertin phae delay i given by 4 π λ IP T ( t + ) cθ (8) d Here T i phae angle aciated with the vltage tranmiin cefficient f the entire radme wall. 8 9 Figure a Pwer tranmiin characteritic f and mnlithic lab alne at nrmal incidence 3. EM PERFORMNE PRMETERS The EM perfrmance parameter (pwer tranmiin, pwer reflectin and inertin phae delay) f the mnlithic cnfiguratin with circular aperture are cmputed fr perpendicular plariatin at and 6. cmparative tudy f the EM perfrmance f circular aperture embedded mnlithic half-wave radme with that f mnlithic lab alne i preented in the fllwing ectin. (a) EM perfrmance at nrmal incidence The pwer tranmiin characteritic f the mnlithic lab alne and circular aperture embedded mnlithic lab ver the frequency range 8 GH at nrmal incidence are hwn in Fig. a. Even thugh the tranmiin efficiency f embedded tructure i ame a that f mnlithic lab alne, arund GH, it i uperir t that f mnlithic lab alne, in the remaining prtin f the X-band. The pwer reflectin f embedded tructure i almt negligible arund 9 GH and it hw increae beynd GH (Fig. b). Fr the mnlithic lab alne, the pwer reflectin characteritic i negligible arund Figure b Pwer reflectin characteritic f and mnlithic lab alne at nrmal incidence

4 Figure c Inertin phae delay characteritic f and mnlithic lab alne at nrmal incidence Figure 3b Pwer reflectin characteritic f and mnlithic lab alne at 6 The inertin phae delay (IP) characteritic f bth tructure are hwn in Fig. c. The IP f bth tructure exhibit teady increae frm 8 GH t GH. Since the rate f change f IP f bth tructure ver X- band i imilar, bth tructure will exhibit imilar phae ditrtin and breight errr characteritic. (b) EM perfrmance at 6 The EM perfrmance characteritic f bth circular aperture embedded and lab alne tructure at high incidence angle 6 are hwn in Fig. 3a-3c. cmpared t the nrmal incidence cae, there i a decreae in the pwer tranmiin efficiency f circular aperture embedded radme panel Figure 3a Pwer tranmiin characteritic f and mnlithic lab alne at 6 Figure 3c Inertin phae delay characteritic f and mnlithic lab alne at 6

5 The pwer reflectin f circular aperture embedded wall i lw a cmpared t that f the lab alne cae belw GH. It i berved that the IP f bth circular aperture embedded tructure and mnlithic lab alne i rapidly increaing tward high frequency ide f the X-band pectrum (Fig. 3c). with the nrmal incidence cae, the IP variatin f bth tructure are imilar. 4. ONLUSIONS The EM perfrmance parameter f circular embedded mnlithic half-wave at nrmal incidence and high incidence angle 6 have been analyed. It i berved that the incluin f circular ha imprved the perfrmance parameter ver the elected frequency range. Since the cnventinal mnlithic half-wave tructure ha very narrw bandwidth, the circular aperture type embedded mnlithic half-wave radme i a better chice fr airbrne radme applicatin. 5. REFERENES [] J.. Waltn, Jr., Technique fr airbrne radme deign, FL Reprt, N , pp. 5-8, 966. [] L.. Rbinn, Electrical prpertie f metal laded radme, W Technical Reprt, N. 6-84, 96. [3].. Munk, Frequency Selective Surface: Thery and eign. New Yrk: Wiley, ISN ,. [4] R.H.J. ary, Radme, in The Handbk f ntenna eign..w. Rudge, K. Milne,.. Olver, and P. Knight (Ed.), Lndn, UK: Peter Peregrinu, ISN , 98. [5] T.Y. Othi, tudy f micrwave tranmiin thrugh perfrated flat plate, JPL Technical Reprt 3-56, vl. II, pp. 8-85, 97.