Nose-ConeConformalSubstrate-IntegratedWaveguideSlotArray …downloads.hindawi.com/journals/ijap/2019/6262574.pdf · 2020-01-27 · Nose-coneparameters Values Cylinderparameters Values

Research ArticleNose-ConeConformal Substrate-IntegratedWaveguide SlotArrayAntenna for X-Band Radar Applications

Hisham Khalil M Mansoor Ahmed and Umair Rafique

Department of Electrical Engineering Capital University of Science and Technology Islamabad Pakistan

Correspondence should be addressed to Umair Rafique umairrafique1987gmailcom

Received 4 October 2019 Accepted 2 December 2019 Published 23 December 2019

Academic Editor Chien-Jen Wang

Copyright copy 2019 Hisham Khalil et al is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

is paper presents the design of nose-cone conformal substrate-integrated waveguide (SIW) slot array antenna for modern radarapplications Firstly the wave propagation characteristics have been investigated in doubly curved SIW and it has been observedthat they are non-uniform along the longitudinal direction of nose-cone conformal SIW To ensure the constant wave propagationalong the length of conformal SIW the conventional design of SIW is reformulated for nose-cone conformal SIW and circuitmodel modification has been demonstrated Secondly the procedure for designing a SIW-based array on curved surfaces has beendeveloped In the proposed design rectangular waveguide (RWG) to SIW feeding structure has been used to avoid spuriousradiations Finally 1times 6 element-based nose-cone conformal slotted array has been designed and compared with planar andcylindrical conformal arrays It has been observed from the results that the nose-cone conformal slot array offers low sidelobelevels (SLLs) and high gain For the validation of the proposed design the conformal slotted array has been fabricated andmeasured which exhibited a reasonable agreement between the measured and the simulated data

1 Introduction

In modern aircraft and unmanned aerial vehicles (UAVs)most of the communication systems depend on antennasGenerally planar antenna arrays are utilized in these sys-tems which could potentially increase the payload and thusmake the system inefficient [1] To overcome these problemsnow-a-days conformal antennas are being used on thesurface of aircraft and UAVs [2 3] Also substrate-inte-grated waveguide (SIW) based conformal antennas areconsidered good candidates for airborne applications SIWhas the inherent property of the microstrip line with anadded advantage of a waveguide Moreover it is compact insize and cost-effective and offers low insertion loss and betterintegration capability for microwave circuits compared withconventional waveguides [4ndash9] e closed configuration ofSIW suppresses the spurious radiations from feedingstructure which otherwise could lead to narrow beamwidth[4 10]

In the literature many conformal antenna arrays havebeen presented for multiple applications In [11] a

conformal Vivaldi antenna array with high gain and lowsidelobe level (SLL) is presented for airborne applicationse array elements are synthesized using the Dolph-Che-byshev technique and the presented array operates in thefrequency range of 400ndash800MHz In [12] a conformalrectangular waveguide (RWG) is presented for aircraft ap-plications It covered 360deg azimuth angle for scanningpurpose in the frequency range of 16ndash166 GHz Howeverthe presented array is non-planar which is considered anegative feature from the design perspective In [13] acylindrical conformal SIW-based traveling wave slot array isdesigned and implemented e array consisted of 4times16elements and employed microstrip to SIW type feedingstructure conformed on a cylindrical surface It was notedthat this kind of configuration degrades radiation charac-teristics especially SLL Another cylindrical conformalslotted array with large beam tilt (84deg) is presented in [14]e reported array had minus 13 dB SLL but the design com-promised over the beam scanning A microstrip fed circularpatch antenna array conformed on a cylinder for C-bandapplications is discussed in [15]

HindawiInternational Journal of Antennas and PropagationVolume 2019 Article ID 6262574 11 pageshttpsdoiorg10115520196262574

It is worth mentioning that the designs presented in[11ndash15] are simulation based and their experimental oranalytical validation is not available In [16] an analyticaldesign approach is adopted and equations are presented for acylindrical curved slot array In [17] an experimental studyfor conformal antennas on aircraft surface is presentedwherein suitable material for array fabrication and theirmechanical aspects are reported Pelham et al [18] estimatedthe gain of the conformal array on the wings of an aircraftand compared it with the gain of the conventional planararray In [19] a conformal antenna array factor is simulatedand optimized using a MATLAB program for radar scan-ning applications e authors reported a peak SLL ofminus 20 dB

In the current state of conformal arrays there is a need toinvestigate the integration of antenna arrays on the surfaceof high-speed jets Owing to this in this work a nose-coneconformal SIW slotted array is designed fabricated andcharacterized at X-band Since the conical surface is anessential part of aerodynamic applications such as spacerockets supersonic aircraft and missiles [3] thereforedesign consideration of nose-cone conformal SIW is em-phasized in this work In cylindrical conformal SIW thecross-section of SIW remains the same along longitudinaland circumferential directions therefore the wave propa-gation characteristics have to be uniform in the conformedstructure us a standard slot array design procedure canbe implemented on the cylindrical surface [20ndash24]e samedesign strategy of a slot array cannot be implemented di-rectly on doubly curved surfaces due to the involvement of θand ϕ in the conformed SIW Because of the variation inboth θ and ϕ the uniform wave propagation along thedirection of nose-cone would be the first design challengewhile keeping the SLL at the desired level would be thesecond major consideration In [25] a conical conformalSIW slot array antenna is presented for millimeter-waveapplications A flexible SIW transition from conical-to-cy-lindrical is designed for improved impedance matching epresented configuration provides better return loss in theband of interest but it has no effect on gain compared to itscounterpart ie conical surface Furthermore conical-to-cylindrical SIW transition can increase the size of the an-tenna structure which limits its use in many aerodynamicapplications

In this paper a design of the nose-cone conformal slotarray antenna is presented for X-band radar applicationse problems in the wave propagation characteristics forsuch a medium are identified and to overcome this issue ageneralized solution is proposed It has also been demon-strated that the proposed design offers better performancecompared with the design reported in [25] Moreover theproposed array is cost-effective and lightweight and offersenhanced radar coverage without involving mechanicalsteering Furthermore the conventional planar slot arrayneeds protective radome for aircraft applications that im-plies dielectric losses [3] which are not there in the proposeddesign hence making it a good candidate for suchapplications

2 Characterization of SIW on Curved Surface

is section describes the wave propagation characteristicsof conformal SIW on curved surfaces Two curved surfacesare under consideration cylindrical and conical A planarSIW of width W and length L is conformed onto thecylinder of radius r along the longitudinal direction z asshown in Figure 1(a) e planar cross-sectional face of SIWis converted into a curved surface and due to this changethe propagation constant β and input impedance Zin willchange Since the curvature of a cylindrical surface remainsunchanged therefore β will remain the same

When planar SIW is conformed to the doubly curvedsurface such as nose-cone defined by spherical coordinates(r ϕ θ) as shown in Figure 1(b) to maintain the conformitythe cross-section of SIW will not remain the same along theheight of the cone In other words the cross-sectional face ofSIW is more conforming on the vertex of the cone as shownin Figure 1(b) is continuous structural variation causes achange in β In this situation the design of the slot array onSIW is not feasible as there could be nthβ propagations on thesurface of SIW It means that every element on SIW willresonate at different β and due to this phenomenon thearray cannot be analyzed practically

To represent nose-cone conformal SIW in sphericalcoordinates the base of excitation of SIW is at (r1ϕ1 θ) andshort circuit end is at (r2 ϕ2 θ)as shown in Figure 1(b) ecross-section at base has a radius r1 at ϕ1 and cross-section atvertex has a radius r2 at ϕ2 For wave propagation in conicalSIW the following condition must be validated [25 26]

r1ϕ2 r2ϕ1 (1)

On the other hand the analytical expression for thechange in β can be written as follows [26]

β unp

rn

(2)

where unp is the zeros of Besselrsquos function and rn representsradii of the cone from base to vertex If rn changes β alsochanges accordingly and if rn of a cone decreases or ap-proaches towards zero the corresponding ϕ increasesaccording to equation (1) and consequently β increases asdescribed in [25]

For slot array it is assumed that nth element is lying onany radius rn e mutual coupling between the array ele-ments is difficult to establish and practically the design isinvalid for N number of elements To overcome thisproblem a design technique has been proposed whose ex-planation is provided in Section 3

3 Modified Nose-Cone Conformal SIW

Two design topologies are presented for β regulation alongthe conformed surface one is the cylindrical equivalent andother is the conical equivalent modification Here an as-sumption is made that at any location on the cone (Ln) asshown in Figure 2(a) one can consider that specific portionas a cylinder having radius according to the conical radius at

2 International Journal of Antennas and Propagation

that specific location defined by n For this purpose it isrequired to find the radius at the corresponding length LCwhich can be calculated as follows

rn minus r0 Ln times cos(α) (3)

where

Ln LCoSIW minus Ld (4)

As long as Ld sweeps from 0 to LC Ln traverses along thegeneratrix of nose-cone In equation (4) LCoSIW correspondsto the length of conformal SIW which can be calculatedthrough triangle ΔABC as shown in Figure 2(a) In the figurethe lines AC LCoSIW BC LC and AB rn minus r0 By usingPythagoras theorem LCoSIW can be represented as follows

LCoSIW

rn minus r0( 11138572

+ L2C

1113969

(5)

In equation (3) the variable α represents cone angle Anumber of HFSS simulations of conformal SIW have been

carried out for βn curves against the tilted cylindrical con-formed SIW and the results are shown in Figure 3(a) One canobserve from Figure 3(a) that the slope of β curves by in-creasing the value of LC as evident from Figure 2(b) is re-ducing Examination of Figure 3(a) also shows that all the βn

curves are passing through a region labeled as area of interestIt is worth mentioning that all the βn curves converge for arange of ϕtsim03∘ minus 05∘ is range of ϕt provides unchangedβ along the conformal SIW thus for an appropriate design avalue of ϕt asymp 043deg has been selected which gaveβ 2305 radm as marked area of interest in Figure 3(a)

In the second case the geometrical structure has beenmodified and now it is truly represented by a cone havingbottom radius r1 and top radius r2 as shown in Figure 2(b)Series of simulations were performed by moving the probefrom higher to the lower radius (r1 to r2) and the variationin β has been observed It has been noted that for ϕt asymp 043degβ asymp 228 radm is the value which is independent of LC usthis value is taken to calculate the length LCoSIW of

r

LCSIW

W

ϕ

(a)

LCoSIW

r1

r2

ϕ1

ϕ2

W(ϕ)

(b)

Figure 1 Conformal substrate-integrated waveguide on (a) cylindrical and (b) conical surfaces

r0

rn

α

LCoSIW

LC

Ld

A B

CD

E

(a)

x ϕ1

LC

θ

y

z

r2

ϕ2

r1

(b)

ϕt

W(ϕ)

(c)

Figure 2 Cross-sectional view of nose-cone conformal SIW (a) Side view (b) Front view (c) Tilted CoSIW

International Journal of Antennas and Propagation 3

conformed SIW which is equal to two guided-wavelength2λg Whereas the tapering of via-holes along the longitudinaldirection increases the width of SIW at the base (near to r1)as shown in Figure 2(b) A new variable W(ϕ) is introducedshown in Figure 2(c) while keeping the width of SIW fixed atthe vertex of the cone and the arrays of conducting via holesare being tilted as shown in Figure 2(c) e simulated dataof geometrical variables such as ϕt LC and β have beenanalyzed in Figure 3(b) and it is noted that the region ofconvergence of βn curves is little bit expanded compared tothe cylindrical case (Figure 3(a)) e vias of conformal SIWare tilted outward as shown in Figure 2(c) therefore thewidth W(ϕ)gt λ02

4 Design of Conformal SIW Slotted Array

is section describes the steps involved in the designing ofnose-cone conformal SIW slot array including feedingstructure

41 Array Feeding To excite the array two feeding tech-niques are considered the first one is microstrip to SIWfeeding and the second one is RWG to SIW feeding efirst technique is relatively easy to fabricate as the flexiblesubstrate can be conformed comfortably to a predefinedsurface [27] However this could cause spurious radiationswhich can possibly affect the overall performance of thesystem [6]

RWG to SIW feeding is a better option if adopted forconformal array design One major advantage of RWG toSIW feed is that it suppresses spurious radiations e halfcosine incident wave will intersect with the lower couplingslot of SIW and can be tuned by off-setting from the centere coupling slot is similar to the radiating slot [22] and itis located on SIW where the magnetic field is maximumis kind of technique was utilized in [28] where the au-thors designed SIW slot array antenna on a cylindricalplatform ey utilized RWG to SIW feed structure toexcite 1 times 6 elements slot array eir simulation results

Prop

agat

ion

cons

tant

(rad

m)

220

225

230

235

240

245

250

255

02 04 06 08 10 12 14 16 18 2000ϕt (deg)

Area of interest

205mm

305mm255mm

355mm

LC = 05mm

105mm55mm

155mm

455mm405mm

505mm

(a)

Prop

agat

ion

cons

tant

(rad

m)

220

225

230

235

240

ϕt (deg)00 02 04 06 08 10

Area of interest

LC = 55mm105mm155mm205mm

255mm305mm355mm

405mm455mm505mm

(b)

Figure 3 Wave propagation characteristics of conformal SIW along distance LC and tilted width W(ϕ) (Figure 2(c)) (a) cylindrical(b) nose-cone

3λg4

Input port Output port

LW

Coupling slot

Figure 4 A scheme in which a rectangular waveguides (RWGs) are used to excite and observe field pattern of substrate-integratedwaveguide (SIW)

4 International Journal of Antennas and Propagation

showed that the presented array had SLL of minus 2172 dB witha gain of 98 dBi

Figure 4 illustrates RWG to SIW feeding structure Fromthe figure it can be observed that two coupling slots areplaced on SIW structure e distance from the center of theslot to conducting vias is 3λg4 as shown in Figure 4 Tworectangular waveguides represented as input and outputports are used to calculate the power transfer characteristicsfrom one port to another

S 11 (

dB)

ndash50

ndash40

ndash30

ndash20

ndash10

0

975 1000 1025 1050950Frequency (GHz)

(a)

S 21 (

dB)

ndash60

ndash50

ndash40

ndash30

ndash20

ndash10

0

975 1000 1025 1050950Frequency (GHz)

996 999 1002993Frequency (GHz)

ndash30ndash25ndash20ndash15ndash10

S 21 (d

B)

(b)

Figure 5 S-parameters of RWG to SIW feeding structure (a) reflection parameter and (b) insertion loss the inset of Figure 5(b) represents azoomed view of region of interest

Feedingwaveguide

h

Feedingslot

Metallicvias

2Lr

Xn

λg2

3λg4

Figure 6 Configuration of hollow nose-cone conformal SIW slot array antenna

Table 1 Design parameters of nose-cone and cylindrical conformal SIW slot array antennas

Nose-cone parameters Values Cylinder parameters Valuesr1 (mm) 5λ0 r (mm) 166λ0r2 (mm) 4λ0 LC (mm) 5λ0LC (mm) 5λ0 LCSIW (mm) 5λg

LCoSIW (mm) 5λg W (mm) λ02

Table 2 Slot parameters for nose-cone conformal antenna withinterelement spacing of λg2

S No Xn (mm) 2Lr (mm)

1 0630 113332 0858 115153 0974 115834 0980 115915 0875 115086 0672 11290

International Journal of Antennas and Propagation 5

e S-parameters of RWG to SIW feeding structure areshown in Figure 5 It can be observed from Figure 5(a) that theproposed feeding technique is operating well for the frequencyof interest which is 10GHzerefore from Figure 5(a) it canbe noted that the proposed feeding structure provides100MHz bandwidth from 993 to 1003GHz e reasonbehind such a narrow bandwidth is that the conducting viasare tightly spaced which tend to cause electric field leakagee dielectric losses could be another reason for the observednarrow bandwidth for the proposed feeding structure [29] Onthe other hand the broadband transition such as reported in[30] cannot be implemented directly in the proposed designbecause it is difficult to conform E-plane iris of RWG on thenose-cone surface

Figure 5(b) shows the insertion loss of the proposedfeeding structure From the figure it is observed that thevariation in insertion transition is 125 dB which couldprimarily be associated with dielectric losses of the medium[29] and coupling losses of both the ports of the proposeddesign

42 Array Design and Simulation e graphical view oflongitudinal slot array on the nose-cone surface is repre-sented in Figure 6 It is pertinent to mention here that ahollow conical surface has been selected for the proposeddesign Moreover substrate chosen for the design of slottedarray is Rogers RTDuroid 5880 having h 0508mm

Table 3 Comparative analysis for planar cylindrical and nose-cone conformal SIW slot arrays (N 6 and d λg2)

Parameters Planar SIW Cylindrical SIW [28] Nose-cone SIW1st SLL (dB) minus 1978 minus 2172 minus 20822nd SLL (dB) minus 1785 minus 1883 minus 2763SIW tilt (deg) 0deg 0deg 043degBeam position (deg) 0deg 0deg 0degHPBW (deg) 2029deg 2124deg 1980degMax gain (dBi) 936 98 1191

Nor

mal

ized

gai

n (d

B)

ndash50

ndash40

ndash30

ndash20

ndash10

0

ndash100 ndash50 0 50 100 150ndash150Theta(deg)

Planar SIWCSIW [28]CoSIW

Figure 8 H-plane radiation characteristics of planar cylindrical and nose-cone conformal slot arrays

Shor

t

Zin

Y1

G0

Y2

G0

Y3

G0

Y4

G0

Y5

G0

Y6

G0

βl βl2

Figure 7 Equivalent circuit model of slotted array

6 International Journal of Antennas and Propagation

εr 22 and tan δ 00009 e center frequency of thedesigned array is 10GHz e array is projected on theconical surface of lower radius r1 and upper radius r2 asshown in Figure 2(b) while the width W of the nose-coneconformal SIW is selected according to the process definedin Section 3 e diameter of the metal vias is 05mm andthe separation between them is 09mm e rest of thedesign parameters are listed in Table 1 is table providesvariables for two different designs one deals with the

proposed nose-cone design listed in column 1 whereas forcomparison purposes cylindrical design parameters are alsoprovided in the table

e road map for the designed array is the same as that ofElliotrsquos design technique [20 24] To get optimum radiationcharacteristics with low SLL the slot offsets and their corre-sponding lengths listed in Table 2 are obtained by using themethod of least square (MLS) [4 10]e error function whichis used for pattern synthesis is given in equation (6) [4]

Nor

mal

ized

gai

n (d

B)

ndash50

ndash40

ndash30

ndash20

ndash10

0

ndash100 ndash50 0 50 100 150ndash150Theta (deg)

With carrier (400mm)With larger curvature (r2 gt 10λ0)

Figure 9 Effect of large curvature and carrier surface on antennarsquos radiation characteristics

ndash100 ndash50 0 50 100 150ndash150Theta (deg)

Nor

mal

ized

gai

n (d

B)

ndash50

ndash40

ndash30

ndash20

ndash10

0

LC = 150mm200mm250mm

300mm350mm

(a)

ndash100 ndash50 0 50 100 150ndash150Theta (deg)

Nor

mal

ized

gai

n (d

B)

ndash60

ndash50

ndash40

ndash30

ndash20

ndash10

0

r1 = 90mm r2 = 60mmr1 = 120mm r2 = 90mmr1 = 180mm r2 = 150mm

(b)

Figure 10 Effect of nose-cone parameters (a) LC and (b) r1 and r2 on antennarsquos performance

International Journal of Antennas and Propagation 7

εsyn Wupper

1113944

M

m1S θm( 1113857 minus h

upperm

11138681113868111386811138681113868111386811138681113868 + W

lower1113944

M

m1S θm( 1113857 minus h

lowerm

11138681113868111386811138681113868

11138681113868111386811138681113868

(6)

where Wupper and Wlower are the weights while hupperm and

hlowerm represent upper and lower limits of SLL respectively

S(θm) denotes array factor and M represents maximumnumber of iterations

Once the conformal SIW slot array is designed then itsequivalent circuit model can be implemented by the classicalslot array theory [23] as shown in Figure 7Wherein the interelement spacing is λg2 Y represents admittance of the

elements G0 is the normalized conductance of slots Zin isthe input impedance of the array and the array is shortcircuited at 3λg4

e proposed nose-cone conformal SIW slot array an-tenna is validated by comparing its radiation characteristicsshown in Figure 8 with planar and cylindrical conformal[28] SIW slot arrays From the figure it can be observed thatthe radiation characteristics are almost the same for all thethree cases but nose-cone conformal array provides lowSLLs which could be considered as a major achievement ofnose-cone conformal design From Figure 8 it can also beobserved that the arrays have the main beam position in the

(a) (b)

Figure 11 Prototype of the fabricated nose-cone conformal slot array antenna (a) Slotted array conformed on conical platform(b) Rectangular waveguide with SMA connector

Retu

rn lo

ss (d

B)

975 1000 1025 1050950Frequency (GHz)

ndash30

ndash25

ndash20

ndash15

ndash10

ndash5

0

SimulatedMeasured

Figure 12 Return loss characteristics of the proposed nose-coneconformal slot array

ndash100 ndash50 0 50 100 150ndash150Theta (deg)

SimulatedMeasured

Nor

mal

ized

gai

n (d

B)

ndash50

ndash40

ndash30

ndash20

ndash10

0

Figure 13 H-plane radiation characteristics of the proposed nose-cone conformal slot array

8 International Journal of Antennas and Propagation

broadside direction (θ 0∘) Different electrical parametersof all the three distinct surfaces are listed in Table 3 wherethe comparative analysis for the presented cases is given

In nose-cone conformal case the 1st SLL given in Ta-ble 3 is equal to minus 2082 dB while the 2nd SLL is equal tominus 2763 dB Such low values of SLL could possibly be asso-ciated with the conformity of the radiating array with thesupporting surface e maximum gain of the nose-coneconformal slot array antenna at 10GHz is 1191 dBi whilethe half power beam width (HPBW) is 1980degemaximumgain values for planar and cylindrical conformal arrays are

936 dBi and 98 dBi respectively as listed in Table 3 Innutshell it can be claimed that nose-cone conformal SIWarray antenna provides better gain and low SLLs

Figure 9 shows the effect of large curvature and carriersurface on antennarsquos performance It has been observed fromFigure 9 that if the curvature of the nose-cone is increased aslarge as 10λ0 then the effect of conformity tends to beminimum and the surface will behave as a planar surface [16]Accordingly the radiation pattern due to the large curvature ofthe nose-cone conformal antenna is approximately equal toplanar antenna as shown in Figure 9 On the other hand whennose-cone conformal antenna is embedded with carrier sur-face shown in inset of Figure 9 an increase in SLLs andbeamwidth has been observed is is due to the fact that thecarrier surface provides extra reflections at the edges of nose-cone conformal antenna

Figure 10(a) shows the simulated radiation patterns fordifferent values of LC It has been noted from the figure thatfor LC 150ndash250mm the SLLs are reducing For valuesgt250mm the SLLs increases with the spread in the mainbeam up to some extent as shown in Figure 10(a)

e effect of r1 and r2 on antennarsquos performance is il-lustrated in Figure 10(b) From the figure it has been ob-served that SLLs tend to reduce with the increase in r1 and r2value while the gain variation is about 05 dB

43 Fabrication and Measurements e prototype of theproposed nose-cone conformal SIW slot array antenna isshown in Figure 11 Four major steps are involved in thefabrication of the proposed antenna array First fabricationof SIW is performed which involves the etching of slotelements drilling and metallization of via holes Second acone is fabricated in accordance with the design parameterslisted in Table 1 by using a metallic sheet of thickness 14AWG as shown in Figure 11(a) ird RWG (WR-90)having one end compatible to the conical surface is attachedto the inner surface of the cone through a machined windowas shown in Figure 11(b) while an SMA connector isinserted at a distance of λg4 from shorted end of a wave-guide e last step is to wrap SIW slot array on the conicalsurface to excite the array by feeding slot

e return loss characteristics of the fabricated antennaare measured using the vector network analyzer (VNA) andcompared with the simulated results From Figure 12 areasonable match is observed between simulated andmeasured return loss An acceptable match between simu-lated and measured results validate the proposed nose-coneSIW design Some discrepancies are observed between thesimulated and measured return losses which could be as-sociated with the fabrication tolerances especially themounting of RWG on conical platform

In Figures 13 and 14 a comparison between simulated andmeasured H-plane and E-plane radiation patterns is shown forthe proposed nose-cone conformal SIW slot array ForH-planeshown in Figure 13 the simulated results offer SLLs ofminus 2082dB and minus 2763dB respectively while themeasured SLLsare minus 2082dB and minus 16dB respectively An obvious reason ofthe observed discrepancy could be that the feeding part of the

ndash100 ndash50 0 50 100 150ndash150Phi (deg)

SimulatedMeasured

Nor

mal

ized

gai

n (d

B)

ndash20

ndash15

ndash10

ndash5

0

Figure 14 E-plane radiation characteristics of the proposed nose-cone conformal slot array

ndash15 ndash10 ndash5 0 5 10 15 20ndash20Theta (deg)

Nor

mal

ized

pow

er p

atte

rn (d

B)

ndash5

ndash4

ndash3

ndash2

ndash1

0

980GHz990GHz100GHz

101GHz102GHz

Figure 15 Main beam positioning vs frequency

International Journal of Antennas and Propagation 9

conformed array is located at the base of hollow nose-cone asshown in Figure 11(b) and a mismatch between RWG andSIW because of its tapered nature caused an extra reflectionand hence increased the SLL which ultimately degrades radi-ation characteristics of the proposed antenna On the otherhand the designed antenna consists of only one rowof radiatingslots therefore its radiation characteristics are deteriorated inthe E-plane by the fixed screw blockage and the lateral size of anSIW slot array antenna as shown in Figure 14 Furthermore theproposed antenna provides linear polarization and radiationefficiency of 70

By using the proposed nose-cone conformal SIW arrayantenna the main beam can be steered by changing thefrequency to increase the coverage e patterns thusachieved indicating the position of main beam as a functionof frequency are shown in Figure 15

A comparative analysis between proposed and previouslypresented conformal arrays is given in Table 4 By examiningthe values of return loss and the gain as shown in the table itcan be noted that the proposed array has better performancethan the design presented in [25] It is worth mentioning herethat the proposed nose-cone conformal array offers high gaineven with less number of elements

5 Conclusion

is paper presents the design of X-band nose-cone con-formal substrate-integrated waveguide longitudinal slotarray antenna e wave propagation on nose-cone has beeninvestigated and a rectification process has been proposedthrough EM simulations e simulated radiation charac-teristics of the proposed array are compared with planar andcylindrical conformal arrays and it is observed that theproposed nose-cone conformal array offers relatively highgain with low SLLs Furthermore the nose-cone conformalarray is validated through fabrication and a good agreementis observed between the measured and the simulated datae presented study demonstrate that the proposed antennaarray can be utilized in modern radars and on the conicalsurface of aircrafts and missiles

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that there are no conflicts of interestregarding the publication of this paper

References

[1] S W Schneider C Bozada R Dettmer and J TenbargeldquoEnabling technologies for future structurally integratedconformal aperturesrdquo in Proceedings of the IEEE Antennasand Propagation Society International Symposium Held inConjunction with USNCURSI National Radio ScienceMeeting vol 2 pp 330ndash333 Boston MA USA July 2001

[2] D Wingert and B Howard ldquoPotential impact of smartelectromagnetic antennas on aircraft performance and de-signrdquo in Proceedings of the NATO Workshop on SmartElectromagnetic Antenna Structures pp 1ndash10 Brussels Bel-gium November 1996

[3] R J Mailloux Conformal Array Antenna eory and Design(Reviews and Abstracts) Wiley-Interscience PublicationHoboken NJ USA 2007

[4] S E Hosseininejad and N Komjani ldquoOptimum design oftraveling-wave SIW slot array antennasrdquo IEEE Transactionson Antennas and Propagation vol 61 no 4 pp 1971ndash19752012

[5] D Deslandes ldquoDesign equations for tapered microstrip-to-substrate integrated waveguide transitionsrdquo in Proceedings ofthe 2010 IEEE MTT-S International Microwave Symposiumpp 704ndash707 Anaheim CA USA May 2010

[6] M I Nawaz Z Huiling and M Kashif ldquoSubstrate integratedwaveguide (SIW) to microstrip transition at X-bandrdquo inProceedings of the 2014 International Conference on CircuitsSystems and Control pp 61ndash63 Yueyang China June 2014

[7] M Bozzi F Xu D Deslandes and K Wu ldquoModeling anddesign considerations for substrate integrated waveguidecircuits and componentsrdquo in Proceedings of the 8th Inter-national Conference on Telecommunications in Modern Sat-ellite Cable and Broadcasting Services pp 7ndash16 Nis SerbiaSeptember 2007

[8] D Deslandes and K Wu ldquoIntegrated microstrip and rect-angular waveguide in planar formrdquo IEEE Microwave andWireless Components Letters vol 11 no 2 pp 68ndash70 2001

[9] J Wu Y J Cheng and Y Fan ldquoA wideband high-gain high-efficiency hybrid integrated plate array antenna for V-bandinter-satellite linksrdquo IEEE Transactions on Antennas andPropagation vol 63 no 4 pp 1225ndash1233 2015

[10] S E Hosseininejad N Komjani and A Mohammadi ldquoAc-curate design of planar slotted SIW array antennasrdquo IEEEAntennas and Wireless Propagation Letters vol 14 pp 261ndash264 2015

[11] J Dong Y Wang F Meng and W Feng ldquoA research onairborne conformal array with high gain and low SLLrdquo inProceedings of the 2014 International Conference on Com-putational Intelligence and Communication Networkspp 334ndash338 Bhopal India November 2014

[12] A Traille J Ratner G D Hopkins and V Tripp ldquoDevel-opment of a novel faceted conformal slotted-waveguidesubarray for sensor applications with full 360deg azimuthtracking capabilitiesrdquo in Proceedings of the 2007 IEEE

Table 4 Comparative study of proposed and previously presented antenna arrays

Parameters [25] ProposedArray size 1times 8 1times 6Cone type Solid cone Hollow nose-coneWaveguide type Wire-cut waveguide Suspended waveguideDesign procedure β and Zin optimization on cylindrical surface β optimization on conical surfaceReturn loss gtminus 10 dB ltminus 10 dBGain 99 dBi 1191 dBi

10 International Journal of Antennas and Propagation

Antennas and Propagation Society International Symposiumpp 3828ndash3831 Honolulu HI USA June 2007

[13] O Bayraktar and O A Civi ldquoCircumferential traveling waveslot array on cylindrical substrate integrated waveguide(CSIW)rdquo IEEE Transactions on Antennas and Propagationvol 62 no 7 pp 3557ndash3566 2014

[14] Y Y Liu M Guo and S S Zhong ldquoConformal slottedwaveguide array antennardquo in Proceedings of the 2012 IEEEInternational Workshop on Antenna Technology (iWAT)pp 56ndash59 Tucson AZ USA March 2012

[15] P Chopra M Bhandari and S Saxena ldquoConformal antennausing circular microstrip patches in C bandrdquo in Proceedings ofthe 3rd International Conference on Signal Processing andIntegrated Networks (SPIN) pp 759ndash762 Noida IndiaFebruary 2016

[16] H Yang Z Jin G Montisci et al ldquoDesign equations forcylindrically conformal arrays of longitudinal slotsrdquo IEEETransactions on Antennas and Propagation vol 64 no 1pp 80ndash88 2016

[17] P Knott C Loker and S Algermissen ldquoAntenna elementdesign for a conformal antenna array demonstratorrdquo inProceedings of the 2011 Aerospace Conference pp 1ndash5 Big SkyMT USA March 2011

[18] T Pelham G Hilton E Mellios and R Lewis ldquoPredictingconformal aperture gain from 3-D aperture and platformmodelsrdquo IEEE Antennas and Wireless Propagation Lettersvol 16 pp 700ndash703 2017

[19] A K Aboul-Seoud A D S Hafez A M Hamed andM Abd-El-Latif ldquoA conformal conical phased array antennafor modern radarsrdquo in Proceedings of the 2014 IEEE AerospaceConference pp 1ndash7 Big Sky MT USA March 2014

[20] R Elliott and L Kurtz ldquoe design of small slot arraysrdquo IEEETransactions on Antennas and Propagation vol 26 no 2pp 214ndash219 1978

[21] R Elliott and W OrsquoLoughlin ldquoe design of slot arrays in-cluding internal mutual couplingrdquo IEEE Transactions onAntennas and Propagation vol 34 no 9 pp 1149ndash1154 1986

[22] M Orefice and R S Elliott ldquoDesign of waveguide-fed seriesslot arraysrdquo IEE Proceedings H Microwaves Optics and An-tennas vol 129 no 4 pp 165ndash169 1982

[23] R S Elliot Antenna eory and Design John Wiley amp SonsHoboken NJ USA 2006

[24] R Elliott ldquoAn improved design procedure for small arrays ofshunt slotsrdquo IEEE Transactions on Antennas and Propagationvol 31 no 1 pp 48ndash53 1983

[25] Y F Wu and Y J Cheng ldquoConical conformal shaped-beamsubstrate-integrated waveguide slot array antenna withconical-to-cylindrical transitionrdquo IEEE Transactions on An-tennas and Propagation vol 65 no 8 pp 4048ndash4056 2017

[26] R Harrington Time-Harmonic Electromagnetic Fields IEEEPress Series on ElectromagneticWaveeory Wiley HobokenNJ USA 2001

[27] L Li X Chen R Khazaka and K Wu ldquoA transition fromsubstrate integrated waveguide (SIW) to rectangular wave-guiderdquo in Proceedings of the 2009 Asia Pacific MicrowaveConference pp 2605ndash2608 Singapore December 2009

[28] H Khalil M M Ahmed U Rafique Saeed-ur-RehmanM Latif and W Nazar ldquoDesign of X-band cylindrical con-formal substrate integrated waveguide slot antenna arrayrdquo inProceedings of the 2018 21st Saudi Computer Society NationalComputer Conference (NCC) pp 1ndash4 Riyadh Saudi ArabiaApril 2018

[29] A Rhbanou S Bri and M Sabbane ldquoAnalysis of substrateintegrated waveguide (SIW) resonator and design of

miniaturized SIW bandpass filterrdquo International Journal ofElectronics and Telecommunications vol 63 no 3 pp 255ndash260 2017

[30] R Vincenti Gatti R Rossi andM Dionigi ldquoBroadband right-angle rectangular waveguide to substrate integrated wave-guide transition with distributed impedance matching net-workrdquo Applied Sciences vol 9 pp 1ndash16 2019

International Journal of Antennas and Propagation 11