диафрагмированные волноводные фильтры / 6e447123-4f14-4b43-ab52-48697d449a75

Abstract—A design procedure for filter-antenna modules based on substrate integrated waveguide cavities is presented in this letter. The filter-antenna module is modeled as an asynchronously tuned coupled-resonator circuit in which the last resonator also contains the radiating element. The design of the filter-antenna module is based on the classical process applied to obtain filters through the use of coupled resonators. The designed filter-antenna module is manufactured and measured, obtaining the following: a central frequency of 1.94 GHz, a 3-dB fractional bandwidth (FBW) of 5.57%, a gain of 4.87 dBi, a front-to-back ratio (FTBR) of 25.60 dB, and a co-to-cross-polarization ratio of 22.86 dB in the direction of maximum radiation. The integration of the filter and the antenna into just one module leads to a reduction of size and weight in the RF front-end, while the implementation by means of the substrate integrated waveguide technique makes the integration with planar circuits easier.

Index Terms—Bandpass filters, coupled-resonator circuits, filter-antenna modules, slot antennas, substrate integrated waveguide (SIW).

I. INTRODUCTION

T HE TREND of communications systems has lead designers to require light weight, robustness, and easy integration with planar circuits [1]. Thus, technologies such as substrate integrated waveguide (SIW) emerge as an option to develop communication systems that fit these imposed

restrictions.

Using the SIW technology, different contributions have been reported for filters and antennas. The configuration of the SIW filter explored in [2] consists of side-by-side coupling of horizontally oriented SIW cavities by means of evanescent waveguide sections. Antennas have been integrated as open-end waveguide radiators or waveguide slot antennas [3]. Recently, in [4], an SIW cavity-backed antenna was presented, in which

Manuscript received December 15, 2010; accepted January 11, 2011. Date of publication January 20, 2011; date of current version March 14, 2011. This work was supported by the Department of Electrical and Electronic Engineering, Los Andes University, Bogotá, Colombia, and the Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, MB, Canada.

O. A. Nova, J. C. Bohórquez, and N. M. Peñais are with the Department of Electrical and Electronic Engineering, Los Andes University, Bogotá, Colombia (e-mail: oa.nova254@uniandes.edu.co; jubohorq@uniandes.edu.co; npena@uniandes.edu.co).

G. E. Bridges, L. Shafai, and C. Shafai are with the Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada (e-mail: bridges@ee.umanitoba.ca; shafai@ee.umanitoba.ca; cshafai@ee.umanitoba.ca).

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.2011.2107724

the properties of the mode are exploited to obtain the smallest possible cavity, lower power loss, and an antenna layout that fits within the surface cavity area.

In order to increase the compactness of the RF front-end of a communication system, the integration of the filtering and the radiation functions into just one module has been proposed. Several approaches have been reported regarding the integration filter-antenna. In [5], a filter-antenna is obtained by adding the filtering function to an electromagnetic horn antenna by means of the introduction of metallic posts inside the antenna. In [6], the integration filter-antenna is obtained by covering the horn antenna’s aperture with frequency selective surfaces implemented with SIW cavities (SIWC-FSS). In [7], a planar approach is proposed to incorporate a slot antenna as the third resonator of a second-order thin-film microstrip-lines coupled-resonator filter.

In this letter, the design procedure of a second-order filter-an- tenna module based on the coupling of two SIW cavities is presented. The module is modeled as a coupled-resonator circuit, which enables us to design it by means of a structured procedure used for this kind of circuit. The design procedure is systematically developed, highlighting the settings required to apply it to the filter-antenna module. In this way, a completely integrated filter-antenna module is achieved using SIW cavities. This letter is a contribution to the obtaining of a structured synthesis process of filter-antenna modules and their integration with planar circuits.

II. STRUCTURE OF THE FILTER-ANTENNA MODULE

The structure of the filter-antenna module is presented in Fig. 1. The configuration used to couple the two SIW cavities consists of placing them in a vertical orientation with a top-to-bottom arrangement. The SIW cavities are designed with a standard procedure for rectangular waveguide cavities [8] for the resonance mode .

The input cavity constitutes the first resonator of the module and contains the access line in the upper metallization, which is designed according to the procedure described in [9] to excite the mode of the cavity. The output cavity constitutes the second resonator of the module and contains the radiating element in the lower metallization. The radiating element is obtained as described in [4]. This leads to a cavity-backed slot antenna in which the radiating elements are two meandered slots, each of length . The coupling between the two cavities is achieved by means of two slots placed at the shared or intercavity metallization.

Fig. 1. Structure of the filter-antenna module.

Fig. 2. Coupled-resonator model of the filter-antenna module.

III. DESIGN OF THE FILTER-ANTENNA MODULE

The proposed filter-antenna module is modeled as a coupledresonator circuit, shown in Fig. 2.

With this model, the design procedure consists of setting the values of three design parameters: the input and output external quality factors ( and , respectively) and the coupling coefficient . The values of these parameters depend on the desired characteristic of response. The relationship of the design parameters with the physical dimensions of the structure should be characterized.

A. Determination of the Theoretical Design Parameters

The determination of these values is performed by using a general synthesis method for Chebyshev filtering functions [10]. This method can be applied to an asynchronously tuned cou- pled-resonator circuit, as is required for the proposed filter-an- tenna module, given that each of its resonators has a different self-resonance frequency. The filtering specifications used as input parameters for the synthesis method are the following:

B. Characterization of the Design Parameters

Most of the physical dimensions of the upper and lower metallization (Fig. 3) have been established from previous works [4], [9]. However, it is noticed that and are the dimensions that mainly control the values of and , respectively. Therefore, these two design parameters are characterized in terms of these two dimensions. Since the coupling of the intercavity metallization [Fig. 3(b)] is designed in this letter, the coupling coefficient is characterized in terms of the dimensions , , and by keeping the two

Fig. 3. Physical dimensions of the filter-antenna module. (a) Upper metallization. (b) Intercavity metallization. (c) Lower metallization.

coupling slots symmetric along the y-axis of the metallization. Following this procedure, it can be noticed that all the three dimensions affect the value of . However, it was decided to set and with the values that best enable the establishment of the field distribution required for the antenna’s radiation, and characterize only in terms of .

The method used for the characterization of as a function of is based on

(1)

where represents the difference between the frequencies for which the phase of is 90 higher and 90 lower than the phase of in resonance [11]. The parameter is that obtained for the input resonator when it is loaded only by the external circuit, that is to say, the parameter obtained for the input resonator after shorting the intercavity metallization and eliminating the output resonator. Therefore, this characterization method does not take into account the intercavity coupling nor the antenna radiation. The relationship between and is obtained through simulations of the referred parameter, in magnitude and phase [Fig. 5(a)]. The variables of (1) are determined for each simulated value of as follows: is the frequency where the minimum magnitude occurs, and can be readily calculated from the phase response as indicated above, taking the phase of in resonance as the phase corresponding to the frequency .

The characterization of is based on

(2) where and are the self-resonance frequencies of each resonator, while and are the eigenfrequencies of the whole resonant structure [11]. Through electromagnetic simulation of each resonator, the self-resonance frequencies are found to be GHz and GHz. The eigenfrequencies are determined from simulations of the two coupled cavities while changing the position of the intercavity slots [ in Fig. 3(b)]. The eigenfrequencies correspond to the two reflection zeros observed in the band of interest of the parameter (Fig. 4). The curve versus is presented in Fig. 5(b).

The characterization of is based on

(3)

Fig. 5. Characterization curves for the three design parameters with the theoretical values represented by a dashed line. (a) Input external quality factor . (b) Coupling coefficient . (c) Output external quality factor .

where and are respectively the average magnetic and electric energies stored in the output resonator, and is the power radiated from the module. The variables , , and

are calculated for different positions of the meandered slots [ in Fig. 3(c)] by means of the HFSS Fields Calculator, with the two cavities coupled by the intercavity slots. The curve versus is presented in Fig. 5(c).

The values of the physical dimensions , , and are determined from the crossing point between the characterization curve and its respective theoretical value in Fig. 5. These values are presented in Table I along with the values of the other dimensions of the module.

IV. EXPERIMENTAL RESULTS

The designed SIW filter-antenna module was fabricated by means of a standard printed circuit board (PCB) process on two

The measured and simulated parameters of the filter-an- tenna module are shown in Fig. 6, along with the theoretical response obtained from the Chebyshev model. The return loss difference is due to a coupling problem originated by technological defaults, mainly due to a misalignment between the two cavities. The experimental central frequency of 1.94 GHz and the 3-dB fractional bandwidth of 5.57% show good agree-

The radiation patterns were measured in a compact range anechoic chamber. Fig. 7 shows the measured and simulated copolarization and cross-polarization radiation patterns at GHz both in the E-plane ( plane) and in H-plane ( plane). The gain measured in the direction of maximum

The experimental co-pol patterns present good agreement with those obtained from simulations in Ansoft HFSS. In the

Fig. 7. Measured and simulated radiation patterns of the filter-antenna module at GHz. (a) E-plane ( plane). (b) H-plane ( plane).

E-plane, the cross polarization of the compact range in the anechoic chamber is about 30 dB at GHz, which is why the measured cross polarization cannot be as low as that predicted by the simulation. In the H-plane, the cross-pol nulls are well predicted, but the peak level of the measurement is about 10 dB higher than the simulated one. It could have been due to a measurement system misalignment. Oscillations in the cross-pol patterns are due to reflections from the antenna cable.

V. CONCLUSION

In this letter, the design procedure of a filter-antenna module based on SIW cavities is presented. The design procedure is systematically developed and can be reproduced in order to obtain different filter-antenna modules from the desired electrical

specifications. Although the determination of the theoretical design parameters is made in this letter for a Chebyshev characteristic, this can be done for different types of responses. The rest of the design procedure is independent of the type of response. The design principles presented here are readily applied to obtain an th-order filter-antenna module. The electrical response of the designed modules presents integrated filtering and radiating functions without damaging any of them. The proposed module exhibits real filter-antenna integration, given that the cavity containing the radiating element is one of the filter’s resonators. This integration can be seen as a contribution to greater compactness of the RF front-end.

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