диафрагмированные волноводные фильтры / 340dbd81-3d1c-4260-83cd-e864367b7c3f

I. INTRODUCTION

R adome is an important component of aircraft. It not

only requires its shape to meet certain aerodynamic layout, but also meet certain structure and strength requirements. For the antenna system, the radome is transparent within the antenna frequency band. Ordinary, filtering performance of radome depends on the structural parameters and transfer coefficient varies with relatively flat frequency response.

Frequency Selective Surface (FSS), an infinite twodimensional period array structure by the same element, can play the role of electromagnetic wave frequency selection and polarization selection [1]. The combination of FSS and media is an important means to improve the quality factor. FSS radome not only can protect antenna from the impact of the physical environment, but also to filter out of band waves. For the characteristics of FSS radome, it has been attracted the attention of many international scholars. Pelton and B. A. Munk has been research a specific streamlined metallic radome [2]. The slot geometry was a triangular grid structure for suitable for maintaining the required surface periodicity on radome shapes and its super resonant frequency stability. R. Pous and D. M. Pozar designed a new type of FSS based on the aperture coupled microstrip patch for a narrow bandpass response using for building radomes with low out-of- band radar cross section (RCS) [3]. Later, the Coupled Resonator Spatial Filter (CRSF) FSS used for filters [4-7] and radome design [8]. Such FSS radome and a filter-antenna consisting of monopole antenna were designed by H. Zhou et al. A CRSF FSS was used to design a conical radome. The FSS array showed stable performances.

In this paper, a planar CRSF FSS was designed used commercial software High Frequency Structure Simulator (HFSS) version 13.0. A new composite structure was firstly

used in W-band. Firstly, a new composite CRSF structure

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used to design a FSS array was designed to transmit W-band with the resonator frequency 94GHz. The CRSF consists of a hexagon patch and circular resonant aperture. The planar FSS array shows stable characteristic of different incident angles and polarizations. Secondly, a system consisting of a horn antenna and a FSS radome was designed and investigate. Then, the radar cross-section (RCS) of system was analyzed.

II.DESIGN PROCEDURE

A.CRSF-FSS Design and Results

The coupled-resonator spatial filter was firstly designed in [3] by R. Pous et al. The size of patch determines the resonant frequency. The aperture determines the coupling degree and a second order band pass response. This surface as a narrow bandpass resonance is useful for building radomes with low out-of-band RCS. In this paper, we chose hexagon patch elements and a circular coupling aperture to consist a band pass filter in W-band.

The geometry of unit cell is shown in Fig.1. It is composed of three metallic surfaces. The top and bottom surfaces are hexagon metallic patch elements. The middle surface is a circular-aperture square. The dielectric is Rogers 5880, whose relative dielectric constant and loss tangent are 2.2, 0.0009 respectively. The thickness of dielectric substrate is 0.127mm. Optimizes dimensions of the unit cell are: a=1.4mm, l=0.57mm, r=0.25mm.

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Fig.1 Geometry of CRSF unit cell.

We used HFSS 13.0 to calculate the transmission and reflection coefficient. Some results are shown in Fig.2 and Fig.3. In HFSS 13.0, we designed an infinite planar FSS array

through the establishment of two pairs of master and slave boundaries. For normal incidence, from Fig.2 (a), two pole transmission responses are 90.9GHz and 93.2GHz, respectively. Fig.2 (b) shows that the transmission response has a rather flat without any ripples from 88.4GHz to 95.5GHz, and the insertion loss is lower than -0.1dB. The 3dB bandwidth is about 19.5GHz from 82.6GHz to 102.1GHz and the relative bandwidth is about 21.1%. Fig.3 shows the transmission response with different incident angle and polarization wave. When the TE and TM incident wave angle from normal to 40 degree, the FSS shows relative stable performance with slightly change in the pass band with insertion loss reduction to -0.68dB. The results show that the CRSF FSS is a good candidate for low insertion loss, stable characteristics.

Fig.2 Frequency response (a) Reflection (b) Transmission

In Fig.4 (b), we can see that the return loss of the antenna and radome system is lower than -18.8 dB from 92GHz to 94GHz in the transmission band. The simulated reflection coefficient of horn without radome is about -26dB.

In Fig.5, we compared the antenna and antenna radiation patterns. In the pass band, the patterns are nearly the same in E plane and H plane. The radome has played a role of stop band. The compared results are shown in Table.II.

Fig.3 Different angle response (a) TE (b) TM

B. Antenna and FSS Radome Design

The system consisting of a W-band antenna and finite planar FSS radome is shown in Fig.4. The geometrical parameters of the horn are listed in Table 1. The radome has 912 elements.

Fig.4 System model and return loss (a) Horn with radome system (b) Simulated input reflection coefficient of antenna and radome system

TABLE I

GEOMERTRY OF THE HORN

(e) 120GHz-E-plane (f) 120GHz-E-plane Fig.5 Radiation patterns of the antenna and FSS radome at different

frequency

TABLE II

COMPARED RESULTS

C. Monostatic RCS Results

In HFSS 13.0, the three models, horn, FSS radome and horn with FSS radome in Fig.6, were designed to calculate the RCS. The element of FSS radome is composed of two substrates with the same thickness shown in Fig.1.

Fig.6 Models of RCS calculation (a) Horn (b) FSS radome (c) Horn with FSS radome.

It is well known that the RCS of any antenna can be significantly reduced by placing a suitable shaped bandpass radome in front of it [1]. Ben A. Munk et al. studied that when the radome was exposed to an incident field, most of the incident energy would be reflected leading to a very weak signal in the backscattering direction and thus to a low RCS because of the shape of radome. When the radome is opaque, the incident signal will primarily be reflected in the specular direction while the backscattered signal will be low. However, when the radome is transparent, no significant reduction of the antenna RCS will take place.

We calculated the RCS of three models using plane wave with different incident angles for excitations. The monostatic RCS results are shown in Fig.7. The RCS of horn with radome shown in Fig.7 (a) is lower than RCS of horn when the incident angles from 10 to 70 degrees, and in Fig.8 (b) the angle is from 10 to 50 degrees. The design of FSS radome shows good performance advantage in reducing RCS at 70GHz and 120GHz for stop-band. The results provides the basis for our later studies of conformal shape radome.

(a)

(b)

Fig.7 Monotatic RCS results in out of band. (a)70GHz (b)120GHz.

VII. CONCLUSION

In this paper, a system consisting of a finite planar FSS radome and a horn was designed. A new CRSF element was firstly used in W-band showing stable performance with the polarization state and angle of incident wave. The transmission response of the infinite FSS array has a rather flat without any ripples from 88.4GHz to 95.5GHz, and the insertion loss is lower than -0.1dB. The 3dB bandwidth is about 19.5GHz from 82.6GHz to 102.1GHz and the relative bandwidth is about 21.1%. The pattern of the system coincides with the antenna pattern in pass-band. In stop-band the radome reflects most of the energy. The design of FSS radome shows good performance advantage in reducing RCS for its planar shape at stop-band. The results provide the basis for our later studies of conformal shape radome.

ACKNOWLEDGEMENT

The authors wish to acknowledge the assistance and support of the ICMTCE 2013 Steering Committee.

REFERENCES

[1]B. A. Munk, Frequency Selective Surface Theory and Design, New York: J. Wiley & Sons, 2000.

[2]E. L. Pelton and B. A. Munk, “A streamlined Metallic Radome,”

IEEE Trans. Antennas Propag., pp. 799-803, November 1974.

[3]R. Pous and D. M. Pozar, “A frequency-selective surface using coupled microstrip patches,” IEEE Trans. Antennas Propag., vol. 39, no.12, pp. 1763-1769, 1991.

[4]A. A. Tamijiani, K. Sarabandi, and G. M. Rebeiz, “Antenna-filter- antenna arrays as a class of bandpass frequency-selective surfaces,”

IEEE Trans. Microwaves Theory Tech., vol. 52, no.8, pp.1781-1789, 2004.

[5]M. Al-Joumayly and N. Behdad, “A new technique for design of lowprofile, second-order, band-pass frequency selective surfaces,” IEEE Trans. Antennas Propag., vol.57, no.2, pp. 452-459, 2009.

[6]N. Behdad, M. A. Joumayly, and M. Salehi, “A low-profile third-order bandpass frequency selective surface,” IEEE Trans. Antennas Propag., vol. 57, no.2, pp. 460-466, 2009.

[7]Y. Q. Li, Z. B. Pei, et al. “Design and verification of a good passband performance of second-order frequency selective surface,”Journal of Air Force engineering University (Natural Science Edition), vol.12, no.4, pp73-77, Aug. 2011.

[8]H. Zhou, S. B. Qu, J. F. Wang, et al. “Filter-antenna consisting of conical FSS radome and monopole antenna,” IEEE Trans. Antennas Propag., vol. 60, no.6, pp.3040-3045, June 2012.