Raveendranath U.Nair,Preethi D.S,and R.M.Jha

1 Introduction

Several techniques for broadbanding of radomes based on metallic wire grids/meshes,resonant/semi-resonant inclusions,anisotropic materials are reported in the open literature[Walton(1966);Bodnar and Basset(1975);Cary(1983);Kozakoff(2010)].Due to superior filter(band-pass/band-stop)characteristics,both conventional and metamaterial based frequency selective surfaces are widely used in the design of radomes[Coryet al.(2007);Basiryet al.(2011);Costa and Monorchio(2012),Narayanet al.(2012);Nairet al.(2013)].Further,materials/structures with controllable dielectric properties have been used in the design of radome wall configurations to meet the high-end electromagnetic(EM)performance requirements of modern radar antenna systems[Chenet al.(2010);Peiet al.(2012)].The practical realization of radome wall configurations with control-lable dielectric characteristics is often a challenging problem[Nairet al.(2012)].In the present work,inhomogeneous planar layer(IPL)radome wall configuration with graded dielectric parameters is proposed,which can be easily fabricated by cascading different dielectric layers(Fig.1).Here the dielectric parameters are increased from the outer wall to the middle layer in a graded manner(or step-wise variation).The EM performance analysis indicates that the graded dielectric IPL radome wall configuration has superior EM performance parameters as compared to that of conventional monolithic half-wave radome of identical thickness.

Figure 1:(a)Schematic of graded dielectric IPL radome and(b)Graded variation of dielectric parameters across the radome wall.

2 Graded Dielectric IPL Radome:EM Design Aspects

The radome wall consists of five cascaded dielectric layers with different dielectric properties(Fig.1a).The total thickness of the entire radome wall is 7.44 m-m,which is the optimized thickness of conventional monolithic half-wave radome(design frequency of 10 GHz).Both Layer 1 and Layer 5 are made of quartz honeycomb(dielectric constant,εr=1.04;and loss tangent,tan δe=0.0008).Layer 2 and Layer 4 are made of polyurethane foam(εr=1.15 and tan δe=0.002).Layer 3(central layer)is made of glass epoxy having the highest dielectric parameters(εrm=4.0 and tan δem=0.015).Thus graded(or step-like)variation of dielectric parameters is incorporated in the radome wall(Fig.1b).

Table 1:Optimized design parameters of graded IPL radome(Polarization:perpendicular).

Figure 2:Equivalent transmission line model of graded dielectric IPL radome.

The graded dielectric IPL radome is modeled as an equivalent transmission line with different sections corresponding to respective dielectric layers(Fig.2).The radome performance parameters are computed using equivalent transmission line model[Nairet al.(2012)].The voltage-current transmission matrix of the entire graded dielectric IPL radome wall is obtained as,

HereZ1,Z2,Z3,Z4,andZ5are the intrinsic impedances of the layers.Z0is the freespace impedance.The electrical length φ of each layer is a function of the thickness,complex permittivity,angle of incidence,and wavelength of incident wave.

The power transmission is expressed as

Similarly,the power reflection is given by

The insertion phase delay(IPD)of the graded dielectric IPL radome is given by

Here∠T1,...,∠T5are the phase angles associated with the voltage transmission coefficients of the dielectric layers with thicknessesd1,...,d5respectively.θ1,...,θ5are the corresponding incidence angles at the interfaces of the dielectric layers.

3 EM Performance Analysis

The EM performance envelopes corresponding to power transmission,power reflection,and IPD characteristics of graded dielectric IPL radome,and monolithic half-wave radome of identical thickness over the entire range of incidence angles from 0?to 80?and X-band are shown in Figures 3 through 5.Since the EM performance degradation for perpendicular polarization represents the worst-case scenario,the EM performance parameters are analyzed only for perpendicular polarization in the present work.

Figure 3:Power Transmission characteristics of graded IPL radome over a wide range of incidence angles and X-band(Polarization:perpendicular).

Figure 4:Power reflection characteristics of graded IPL radome over a wide range of incidence angles and X-band(Polarization:perpendicular).

Figure 5:Insertion phase delay characteristics of graded IPL radome over a wide range of incidence angles and X-band(Polarization:perpendicular).

It is observed that the graded IPL radome shows excellent EM performance characteristics upto 60?incidence angle.However,there is a drastic degradation of performance parameters beyond 60?.A comparative study of EM performance parameters(power transmission,power reflection,and IPD)of graded dielectric IPL radome,and monolithic half-wave radome has been carried out at normal incidence,30?,45?,and at high incidence angle 60?in the figures below.

Figure 6:Power transmission characteristics of graded IPL radome and monolithic half-wave radome at normal incidence.

Figure 7:Power transmission characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 30?(Polarization:perpendicular).

Figure 8:Power transmission characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 45?(Polarization:perpendicular).

Figure 9:Power transmission characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 60?(Polarization:perpendicular).

The power transmission efficiency of graded dielectric IPL radome is superior to that of conventional monolithic radome at both the normal incidence and high incidence angles representing different classes of radome applications(Figures 6-9).It is pointed out that the transmission efficiency of graded dielectric IPL radome is above 90%in the entire X-band.Such high transmission efficiency,over wide band and a range of incidence angles,is required for modern radome applications.

Low power reflection characteristic is achieved for the graded IPL radome design at both normal incidence,and high incidence angles over the entire X-band frequency regime(Figures 10-13).

Figure 10:Power reflection characteristics of graded IPL radome and monolithic half-wave radome at normal incidence.

Figure 11:Power reflection characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 30?(Polarization:perpendicular).

Figure 12:Power reflection characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 45?(Polarization:perpendicular).

Figure 13:Power reflection characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 60?(Polarization:perpendicular).

Such low power reflection characteristics are desirable for radome applications as it facilitates the reduction of sidelobe level(SLL)degradations of antenna radiation pattern,minimization of pattern ripples,and elimination of fl ash lobes in the scan volume.The variation of IPD for graded dielectric IPL radome over the X-band frequency range is less than that of monolithic radome(Figures 14-17).Such IPD characteristics(low magnitude and small variation across the entire X-band)indicate low phase distortions,which in turn reduces the bore sight error(BSE)for streamlined nosecone radomes for airborne applications.

Figure 14:Insertion phase delay characteristics of graded IPL radome and monolithic half-wave radome at normal incidence.

Figure 15:Insertion phase delay characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 30?(Polarization:perpendicular).

Figure 16:Insertion phase delay characteristics of graded IPL radome and monolithic half-wave radome at incidence angle 45?(Polarization:perpendicular).

Figure 17:Insertion phase delay characteristics of graded IPL radome and monolithic half-wave radome at 60?(Polarization:perpendicular).

4 Conclusion

The present work shows that the graded IPL radome has superior EM performance characteristics at both the normal and high incidence angles,as compared to the conventional monolithic radome.This establishes that the graded IPL radome wall configuration has promising application in the design of hemispherical,cylindrical radomes and streamlined nosecone radomes.Since this structure offers superior power transmission and low IPD characteristics,it has also potential application in the design of walls for structurally integrated radiating systems for aircraft and coverings for embedded antennas in aircraft wing and fuselage.

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