Abstract—A dual-band A-sandwich radome is proposed for air-

Abstract—A dual-band A-sandwich radome is proposed for air-
borne applications. Each layer of the radome wall structure is di-
mensioned in a thickness of 1/12 wavelength corresponding to a
selectable design frequency. The D ray-tracing method is ap-
plied to numerical analysis of antenna-radome interactions. Re-
sults demonstrate the radome feasible for dual-band applications
in the ranges of 1.53–1.60 and 6.49–6.75 times of the design fre-
quency, respectively. The design method is proved convenient for
designing dual-band radomes with selectable passbands, such as a
radome for both X- and Ka-band applications.
Index Terms—A-sandwich, dual-band, radome.
I. I
NTRODUCTION
A
IRBORNE radomes are mounted on aircraft or missiles to protect fragile antennas inside from severe environmental conditions [1], [2]. They should provide sufficient mechanical performances, such as rigidity and thermal resistance, whereas they must not significantly affect antenna radiation patterns.
The existence of a radome always causes worse copolarization transmission, greater cross polarization, undesirable sidelobe level, boresight error (BSE), etc, thereby degrading electromagnetic (EM) performances of the antenna-radome system Therefore, radome design that associates with nosecone radome configuration, radome wall structure (thickness and dielectric properties of each layer, as well as the EM characteristics of the antenna is needed for the aim of improved EM properties.
As for multifunctional antenna systems, the antenna may work in dual bands, resulting in the need for dual-band radome designs. Several dual-band radome structures have been reported in literature [3]–[5]. Mackenzie and Stressing disclosed an X- and W-band radome wall structure in an A-sandwich construction, which was composed of two thin skin layers and one thick core layer [3]. In their design, the high mid-band frequency should be larger than at least 10 times of the low one.
Pei et al. proposed a dual-band C-sandwich structure, which was composed of three thin skin layers and two thick core
Manuscript received March 10, 2015; revised April 09, 2015; accepted May, 2015. Date of publication June 01, 2015; date of current version February, 2016. This work was supported by the National Natural Science Foundation of China under Grants No. 91216301, No. 11227801, and No. the National Basic Research Program of China (973 Program) under Grants
2010CB832701 and CB the Foundation of the Author of National
Excellent Doctoral Dissertation of China under Grant No. 201029; and the Beijing NOVA Program under Grant No. Z151100000315041.
The authors are with the State Key Lab for Turbulence and Complex
Systems, College of Engineering, Peking University, Beijing 100871, China
(e-mail: peiym@pku.edu.cn).
Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LAWP.2015.2438552
Fig. 1. Schematic diagram of the tangent-ogive radome and its wall structure
(A-sandwich structure with a coating paint).
layers [4]. The dual-band C-sandwich structure presented better transmission capability with broader bandwidths compared to the dual-band A-sandwich wall structure. However, C-sand- wich nosecone radome always exhibits higher BSE leading to limited EM ability. Lee et al. used frequency selective surfaces embedded in dielectric media to form a dual-band radome,
which had abroad passband in 2–18 GHz and a narrow waveband at 95 GHz, but wide separation between passbands was also presented In this letter, we propose a dual-band A-sandwich radome,
of which band separation between the higher and lower pass- bands becomes smaller (the higher mid-band frequency can be approximately 4.23 times of the lower one. The radome for airborne applications is assumed to be in atypical tangent-ogive configuration and houses monopulse antennas working at diverse wavebands. Each layer of the A-sandwich radome wall structure, which consists of two dense skin layers and a foam core layer, is dimensioned in 1/12 wavelength corresponding to a selectable design frequency. Numerical results by the 3-D
ray-tracing method prove the dual-band ability of the radome in the frequency ranges of and
With such design method, an X- and Ka-band radome with se- lectable operating frequencies can be easily designed.
II. DESIGN FOR THE
D
UAL
-B
AND
R
ADOME
The nosecone radome considered here is atypical tangent- ogive configuration with a height m and abase diameter mas illustrated in Fig. 1. Assuming a
Cartesian coordinate system placed at the radome base center with the -axis pointing to the radome nose and the -axis in the elevation plane, the tangent-ogive profile can be described as © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

ZHOU et al.: DUAL-BAND A-SANDWICH RADOME DESIGN FOR AIRBORNE APPLICATIONS
219
Fig. 2. Effect of the incidence of angle on the power transmission (TE wave only) of the A-sandwich structure.
The monopulse antenna is assumed to be a slotted waveguide planar antenna array located at a distance of m away from the radome base center. The antenna is fixed to a gimbal system without rotational offsets, which enable the antenna scanning in the elevation and azimuth planes. The diameter of the antenna is considered as 0.4 and 0.2 m for X- and Ka-band,
respectively. The polarization direction of the antenna is treated to be vertical (along -axis direction, and the electric field of antenna aperture to be a Taylor circular-aperture distribution with dB maximum sidelobes and, which leads the electric field in the wavenumber space to have zeros The wall structure of the tangent-ogive radome is considered as an A-sandwich structure, which exhibits higher strength-to- weight ratio, and thereby is widely used in airborne radomes.
In this study, the skin layers of the A-sandwich structure are made of glass composite with complex relative permittivity of, while the foam core layer is made of honeycomb structure with complex relative permittivity of, [7]. Each layer of the A-sandwich structure is dimensioned in 1/12 wavelength corresponding to a design frequency. The thickness of each skin layer and the core layer can be derived as and, respectively, where denotes light speed in vacuum.
In our previous work [9], we have demonstrated such flat radome structure had similar transmission ability around and by using the theory of small reflections, and further calculation by the boundary value method [1] indicated its dual-band ability in the frequency ranges of and. However, the incident angle was considered as zero in the EM analysis of the flat structure. In the antenna-radome system, the incident angle varies with the antenna scan angle and also every single ray in the D ray-tracing method described in
Section III. Therefore, it is necessary to conduct EM analysis for the flat dual-band structure with various incident angles before the design for the dual-band antenna-radome system.
The boundary value solution for multilayer dielectric wall described in [1, Ch. 5] can be used for the EM analysis of the dual-band structure with various incident angles. Because the incident angle between the antenna axis (Fig. 1) and the radome surface normal lies nearly in the range, the effect of the incidence of angle in this range on the power transmission (TE
wave only) of the A-sandwich is studied (Fig. 2). It can be seen that the overlapped frequency ranges of various incident angles within which the power transmission maintains above 1.5 dB
are and. We can expect such wall structure design leads to possibility in the dual-band ability of a tangent-ogive radome within the ranges.
III. DRAY -TRACINGMETHODIn order to conduct EM analysis for antenna-radome interactions, the D ray-tracing method is applied [1]. The method belongs to the geometric optics (GO) method, which can be easily implemented on a personal computer and is more time-saving compared to the physical optics (PO) methods. GO treats EM
wave propagation as light-like behavior and provides reasonably good solutions for antennas as small as about five wavelengths in dimension. In this study, the D ray-tracing method satisfies the accuracy requirement since the antenna diameter is times larger than the wavelength corresponding to the operating frequency. The procedure of the D ray-tracing method
(transmitting mode) can be described as follows) Transform the location of each rectangular slot of the planar array at on the antenna surface in terms of the antenna coordinate system into radome coordinates after all gimbal angular rotations. Let be the elevation angle and the azimuth angle, then the transformation can be derived as) Determine the intercept point of each ray with the tan- gent-ogive prole. Each ray is assumed parallel to the antenna axis and has an electric field intensity, which is determined by the Taylor circular-aperture distribution of each rectangular slot at the th point in the -direction and th point in the -direction) Determine the surface normal at the intercept point of each ray and the angle of incidence. Then, decompose the local plane wave into perpendicular and parallel components.
Afterwards, the copolarization (or cross-polarization)
transmission coefficient at each intercept point can be determined using the boundary value solution. The transmission coefficient includes the effects of relative permittivity, electric loss tangent, and layer thickness of each layer of the wall structure at the local intercept point) The antenna aperture distribution is projected through the tangent-ogive radome wall to form an equivalent aperture outside of the radome in the far field. The normalized far-
field pattern of the array antenna finally can be given by
(3)

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 15, where and are the point (rectangular slot) numbers of the antenna array in the - and -directions, respectively. Both the sum and difference channel signals of the monopulse antenna can be calculated by substituting aperture distribution of the sum or difference pattern into, respectively. The copolarization (or cross-polariza- tion) transmission of a radome is defined as the square of the ratio of maximum value of the antenna electric radiation pattern by substituting the copolarization (or cross-polarization) coefficient into (3) with the radome to that without the radome, namely. As for
BSE, it is defined as the bending of the angle of arrival of a received signal in the presence of the radome, and can be evaluated by searching the elevation and azimuth angles of. Eventually, the EM performances
(copolarization or cross-polarization power transmission,
BSE, etc) of the antenna-radome system can be obtained numerically.
IV. RESULTS AND
D
ISCUSSION
The D ray-tracing method as described in Section III is used to calculate the EM performances of the antenna-radome system. It is also assumed that the outer surface of the tan- gent-ogive radome is coated with atypical coating paint (complex relative permittivity:
) of mm thick [6]. In order to reduce perturbation of the existence of the coating paint on EM properties of the nosecone radome, the outer skin layer thickness is modified to guarantee the electric thickness of the outer two layers (coating and outer skin layers) equivalent to 1/12 wavelength corresponding to the design frequency as illustrated in Fig. 1. The outer skin layer thickness can be modified as
,
where denotes the wavelength in vacuum corresponding to the design frequency.
Here, we present the copolarization power transmission,
BSE, and cross-polarization power transmission in the elevation plane ( -plane) with respect to the elevation scan angle for the design frequency of
GHz of the wall structure in Fig. 3. In this case, the core layer thickness is 3.97 mm according to the dual-band wall structure design. We can see that the copolarization power transmission efficiency maintains above 1.5 dB and BSE below 4.0 mrad in the two frequency bands (9.2–9.7 and 39.0–40.0 GHz) as shown in Fig. a) and
(b). It is seen that the copolarization power transmission is lower at higher frequencies because electric loss tangent of the composite material has a more significant effect on the transmission loss at higher frequencies. We can also see that
BSE reaches a maximum value near the radome nose because the incident angle of the rays varies rapidly as the antenna scans near the nose. The cross-polarization transmission is also presented in Fig. c. It shows that the cross-polarization power transmission is nearly below 25 dB in the 9.2–9.7 and GHz ranges. The cross-polarization transmission tends to be minor at high frequencies as the antenna diameter is smaller in dimension. The nosecone radome exhibits dual bands over and for the design frequency. Such ranges fall in, though are not as wide as,
the overlapped bandwidths of various incident angles of the flat
Fig. 3. EM performances of the tangent-ogive radome with the wall structure designed in a design frequency of 6.0 GHz (a) copolarization transmission;
(b) BSE; (c) cross-polarization transmission.
structure (and) as presented in
Fig. 2. It is concluded that dual-band design fora flat structure contributes to the design of a dual-band nosecone radome,
though the bandwidth will be narrower for the nosecone radome. With this design method for the wall structure, it is convenient to design a dual-band nosecone radome, such as an
X- and Ka-band radome as seen from Fig. The EM performances of the nosecone radome with various core layer thickness ( ) distributions are also presented in Fig. 4. Three cases with constant thickness distributions are considered:
mm,
mm, and mm.
One case of which the core layer thickness varies according to antenna scan angle is also considered, and when is in the, and

ZHOU et al.: DUAL-BAND A-SANDWICH RADOME DESIGN FOR AIRBORNE APPLICATIONS
221
Fig. 4. EM performances of the tangent-ogive radome with various core layer thickness distributions (a) transmission (b) BSE.
ranges, respectively. We can see that has more effects on the EM characteristics at lower frequency, especially near the nose of the radome. As near the nose increases, it is seen that power transmission is elevated, while BSE will become worse.
If is below 3.7 mm, the power transmission will be below dB. When is higher than 4.3 mm, BSE will exceed mrad. As for the case with varying thickness, power transmission is slightly improved while BSE becomes worse when compared to the previous design (mm. Thus, we can conclude that it is better to design the dual-band structure with constant thickness distribution because it is easier to fabricate and also has good EM properties compared to thickness-varying designs.
It should be noted that the complex relative permittivity of the radome materials should not vary significantly over the two wavebands. Otherwise, the nosecone radome may not satisfy the
EM requirements simultaneously over two passbands. For wide waveband separation (higher value for the design frequency
),
it is better to use radome material with stable permittivity over a wide frequency range, such as silicon nitride.
V. C
ONCLUSION
A dual-band A-sandwich nosecone radome, of which the higher mid-band frequency can be approximately 4.23 times of the lower one, is proposed for airborne applications. The nosecone radome is designed as atypical tangent-ogive config- uration and the wall structure as an A-sandwich wall structure with a coating paint on the outer skin layer. Each layer of the A-sandwich radome wall structure is dimensioned in wavelength corresponding to a selectable design frequency. Numerical results prove the dual-band ability of the tan- gent-ogive radome in the ranges of and. With this design method for the wall structure,
it is convenient to design a dual-band nosecone radome with desirable wavebands, such as an X- and Ka-band radome with selectable passbands.
R
EFERENCES
[1] DJ. Kozakoff, Analysis of Radome Enclosed Antennas. Norwood,
MA, USA Artech House, 1997.
[2] VB. Yurchenko, A. Altintas, and AI. Nosich, Numerical optimization of a cylindrical reflector-in-radome antenna system IEEE Trans.
Antennas Propag., vol. 47, no. 4, pp. 668–673, Apr. 1999.
[3] SB. Mackenzie and D. W. Stressing, “W-band and X-band radome wall US. Patent 60 285 65, Feb. 22, 2000.
[4] Y. Pei, A. Zeng, L. Zhou, R. Zhang, and K. Xu, Electromagnetic optimal design for dual-band radome wall with alternating layers of staggered composite and kagome lattice structure Prog. Electromagn.
Res., vol. 122, pp. 437–452, 2012.
[5] D. Lee, S. Chakravarty, and R. Mittra, Design of dual-band radomes for high off-normal incidence using frequency selective surfaces embedded in dielectric media Electron. Lett., vol. 36, no. 18, pp, Aug. 2000.
[6] RU. Nair and RM. Jha, Electromagnetic performance analysis of a novel monolithic radome for airborne applications IEEE Trans. An-
tennas Propag., vol. 57, no. 11, pp. 3664–3668, Nov. 2009.
[7] RU. Nair and RM. Jha, Novel A-sandwich radome design for airborne applications Electron. Lett., vol. 43, no. 15, pp. 787–789, Jul TA. Milligan, Modern Antenna Design. Hoboken, NJ, USA Wiley L. Zhou, Y. Pei, R. Zhang, and D. Fang, Method for design of dual-band flat radome wall structure AIAA J., vol. 51, no. 12, pp, View publication stats
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