диафрагмированные волноводные фильтры / 1017de8e-36fe-4461-8104-0e4c63f38a79

Abstract — This paper presents electromagnetic simulation of a dielectric-filled rectangular waveguide antenna filter structure. A microstrip rectangular patch antenna represents the radiating element of this structure and it is endwall coupled with a waveguide that incorporates E-plane filter via a slot in the ground plane. Microstrip antenna, dielectric filling of the waveguide and direct connection between the antenna and the filter ensure compactness as the primal goal. The example antenna filter is designed and simulated at the centre frequency of 11.85 GHz.

Keywords — antenna filters, dielectric-filled waveguides, patch antennas, waveguide filters.

I. INTRODUCTION

Planar antennas are optimal with respect to weight, antenna depth (low profile size) and overall occupied volume, cost, availability of printed circuit technology and easiness of fabrication. However, they suffer from limited electromagnetic properties - high losses, mainly due to the feeding network, high quality factor, poor polarization purity, spurious feed radiation and narrow bandwidth. Extra losses and manufacturing complexity are introduced when components are realised individually and connected via external circuits. Integration of subsystems in single elements is advantageous both for reduced losses as well as for size reduction and simple fabrication. Furthermore, suitable slot feeding of a patch antenna apart from this can be used for impedance matching and increasing antenna bandwidth, where symmetrical feeding, which maximizes coupling on the one hand, also suppresses crosspolarization and spurious feed radiation. Finally, the filter segment of the structure was chosen so as to be easily incorporated within the waveguide feeding. It also manifests good compromise between low insertion loss and size since the waveguide filters are preferable for their low insertion losses and their unwanted bulkiness was suppressed by dielectric filling which lossy character does not mutilate insertion loss considerably. This paper

therefore proposes mainly compact, but also increased

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Financial support was provided by the EUErasmus Mundus Action 2 project EUROWEB.

U. Jankovic and V. Petrovic are with School of Electrical Engineering, University of Belgrade, Serbia (e-mail: ju113096m@student.etf.rs)

N. Mohottige and D. Budimir are with Wireless Communications Research Group, University of Westminster, London, W1W 6UW, UK (e-mail: d.budimir@wmin.ac.uk)

Z. Golubicic is with TTI Technologías de Telecomunicaciones y de la Informacíon Norte, PCTCAN c. Albert Einstewin 14 39011, Santander, Spain.

978-1-4673-2984-2/12/$31.00 ©2012 IEEE

bandwidth and low cost antenna filters. Such combinations of microstrip antennas and waveguide feed are especially needed at millimeter-wave frequencies [1], [2].

II. PROPOSED CONFIGURATION

The layout of the proposed structure is shown in Fig. 1. Description of its configuration begins with the waveguide filter building block, which configuration is described in [3]. It consists of a rectangular waveguide section of 1 mm thick brass conductor, filled with dielectric of relative permittivity of 3.38, having 10 mm x 2.25 mm inner cross section in which interior are distributed six 0.1 mm thick copper septa along E-plane that separate five resonators. It is a 5th order Chebyshev bandpass filter with midband frequency of 11.85 GHz.

Fig. 1. Configuration of proposed antenna filter structure.

Patch antenna as the second building block, is designed on a Rogers/Duroid 5800 substrate with a thickness of 1.6 mm, the metallization thickness of 0.017 mm and relative permittivity of 2.2. The dimensions of the designed prototype have later been corrected in optimization process of simulation model that not only provides better calculation of the very patch element, but also accounts for influence of surrounding feeding elements. In order to make guided, faster optimization, it was useful to know that antenna input impedance can be controlled by the patch width - wider patch leads to lower impedance, because the radiation from the radiating edge increases, hence efficiency and directivity increase too as well as it leads to better broadband characteristic and downshifts the resonant frequency, while patch length could be used to correct matching of resonant frequency of patch with center simulation frequency of 11.85 GHz.

III. RESULTS

The structure shown in Fig. 2 has been simulated in

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commercial full wave EM simulator, namely Microwave Studio tool of CST Studio Suite software package [4]. The simulation model accurately presented characteristics of the real structure in the sense of shapes and materials except neglecting losses in dielectric materials. As the excitation source was used a waveguide port, the most accurate one, which represents a special kind of boundary condition of the calculation domain, simulating an infinitely long waveguide connected to the structure.

Fig. 2. Layout of the proposed prototype. All dimensions are given in millimeters.

Simulation parameters have been set so as to fulfill settings appropriate for both waveguide filters and planar antenna structures, which are described in CST Microwave Studio templates intended for these applications. As a side effect of centrally positioned slot had emerged very useful possibility for using two symmetry planes, reducing the time required for the simulation for about four times. Frequency domain solver of general purpose method with tetrahedral mesh of second order elements was used for calculating S-parameters and radiation pattern. The antenna filter dimensions that were analyzed in parametric studies were set as variables and optimized in order to get optimal characteristics of the antenna filter. Slot width was bounded from the lower side to be not less than 0.1 mm because of harder control of characteristics, meaning possible difficulties in fabrication where relative errors can be huge, hence changing results significantly, and vast influence of effects like edge effect that give extreme field intensity values, which makes simulation results uncertainties greater too. The simulated S-parameters of the designed filter and patch antenna are shown in Figs. 3 and 4 respectively. The S11 parameter of the antenna filter shows very good characteristic with quiet low reflection in the passband. Using VSWR<2 criterion, relative bandwidth is about 8%, which is larger than usual patch antenna passband of 2%-5% [5], [6]. Filter, on the other hand, provides much steeper transition to the stopbands than solely patch antenna, and this transition can be optimized for wanted passband criterion.

Fig. 3. Simulated S-parameters of the designed filter and patch antenna.

Fig. 4. Simulated S11-parameter of the designed antenna filter.

Fig. 5 shows the radiation diagrams of the proposed antenna filter. Radiation efficiency is about -0.32 dB, and total efficiency is about -0.36 dB and directivity is about of 9 dB at the centre frequency of 11.85 GHz. Here should be noted that dielectric materials have been modeled as lossless and that at higher frequencies losses in dielectric start to overpower losses in conductors, which are dominant at lower frequencies.

Fig. 5. Simulated radiation diagrams: (a) H-plane and

(b) E-plane.

IV. CONCLUSION

A dielectric-filled rectangular waveguide antenna filter

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structure is presented. The antenna filter that was designed and simulated at centre frequency of 11.85 GHz consisted of only low-profile microstrip antenna directly integrated with the 54 mm x 12 mm x 4.25 mm size box of the dielectric-filled waveguide filter. Moreover, using optimized slot coupling of the antenna and the filter instead of adding external circuits has helped keeping the antenna filter structure sustaining sufficiently low loss and easier to fabricate as well as reaching above average relative bandwidth compared to patch antennas of about 8% for VSWR<2 criterion and with low spurious radiation.

REFERENCES

[1]M. Kanda, D. C. Chang and D. H. Greenlee, “The Characteristics of Iris-Fed Millimeter-Wave Rectangular Microstrip Patch Antennas”,

IEEE Trans. Electromagnetic Compatibility, Vol. 27, No. 4, pp. 212220, November 1985.

[2]D. M. Pozar, “Aperture Coupled Waveguide Feeds for Microstrip Antennas and Microstrip Couplers”, IEEE Int. Symp. Antennas and Propagation, Baltimore, MD, July 1996.

[3]N. Mohottige, Z. Golubicic and D. Budimir, “Compact Dielectricfilled Waveguide Filters and Diplexers”, IEEE Int. Symp. Antennas and Propagation and CNC/USNC/URSI National Radio Science Meeting (APS2012), Chicago, Illinois, USA, July 8-14, 2012.

[4]Microwave Studio, CST, http://www.cst.com/.

[5]C. A. Balanis, Anthenna Theory: Analysis and Design, John Wiley & Sons, Publishers, Inc., New York, 2005.

[6]G. Kumar, K.P. Ray, Broadband Microstrip Antennas, Artech House, Norwood, 2003.

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