Abstract A design procedure for filter-antenna modules in substrate integrated waveguide technology is presented. The filter antenna module is designed as a second order filter, with the output resonator loaded with the radiating element. The design procedure is developed in three stages, which are explained in detail through the design of two filter-antenna modules with different electrical specifications. The integrated filter-antenna response of the designed modules is verified through electromagnetic simulations and experimental results. One of the designed modules is fabricated and measured. Measurement results yield central frequency of 1.94 GHz, -10 dB-fractional bandwidth of 1.55 % and directive radiation pattern with maximum gain of 4.87 dBi and front-to-back ratio of 25.60 dB. These experimental results present good agreement with electromagnetic simulations.

O. A. Nova (&) J. C. Bohorquez´ N. M. Pen˜a Department of Electrical and Electronic Engineering, Universidad de los Andes, Carrera 1 No. 18A-12, Bogota´, Colombia

e-mail: oa.nova254@uniandes.edu.co

J. C. Boho´rquez

e-mail: jubohorq@uniandes.edu.co

N. M. Pen˜a

e-mail: npena@uniandes.edu.co

G. E. Bridges L. Shafai C. Shafai

Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada

e-mail: bridges@ee.umanitoba.ca

L. Shafai

e-mail: shafai@ee.umanitoba.ca

C. Shafai

e-mail: cshafai@ee.umanitoba.ca

Keywords Bandpass filter Coupled-resonator circuit Filter-antenna Slot antenna Substrate integrated waveguide

Abbreviations

1 Introduction

The 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 the communication systems that fit the imposed restrictions.

The SIW technique makes possible to put together volumic structures with planar ones within a circuit fabricated by means of conventional planar processes such as printed circuit board (PCB) or low-temperature co-fired ceramic (LTCC) [2]. This technique consists of fabricating waveguide-like structures in a planar form by implementing the lateral walls as arrays of metalized holes.

Using the SIW technology, different contributions have been reported for filters and antennas. The SIW filter’s topology explored in [3] consists of the coupling of side-by-side horizontally oriented SIW cavities by means of evanescent waveguide sections. Antennas were integrated as open-end waveguide radiators or waveguide slot antennas [4]. Recently, in [5] a SIW cavity-backed antenna has been presented, in

which the properties of the TE101 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, integrating both the filtering and the radiation functions into just one module has been proposed. Several approaches have been reported regarding the integration of a filter with an antenna. In [6] a filterantenna is obtained for a central frequency (f0) of 10 GHz, by integrating the filtering function in an electromagnetic horn antenna by means of the introduction of some metallic posts inside the antenna. The reported return loss is greater than 11 dB and the -10 dB fractional bandwidth is 6.0 %. In [7], the integration filter-antenna is obtained by covering a horn antenna’s aperture with frequency selective surfaces implemented with SIW cavities (SIWC-FSS). This solution presents f0 = 9.8 GHz, return loss greater than 13 dB and a -10 dB fractional bandwidth of 11.2 %.

Some planar approaches have been proposed to achieve the filter-antenna integration, as described in [8]. The idea consists of the integration of a slot antenna as the third resonator of a second order bandpass filter conformed by the combination of two resonators implemented in thin film microstrip lines (TFMS) and coupled by admittance inverters in coplanar waveguide (CPW). The obtained fil- ter-antenna response has a return loss greater than 12 dB with a -10 dB fractional bandwidth of 14.3 %.

In this paper, a design procedure for filter-antenna modules in SIW technology is presented. The procedure is structured by using a coupled-resonator circuit approach. Therefore, the design is developed based on three parameters: the external quality factors of input and output and the coupling coefficient between resonators, as presented in [9]. The value of the physical dimensions that control the abovementioned parameters is determined after a characterization procedure. Details of the design procedure, in all of its three stages, are presented in this paper, by developing, step by step, the design process of two filter-antenna modules, with different electrical specifications. One of these proof- of-concept modules is fabricated and measured while the other is validated by means of electromagnetic simulations.

2 Structure of the filter-antenna module

The filter-antenna module to be designed is of second order, so that it is made up of two resonant cavities. The configuration used to couple the two cavities is by vertically stacking them, one on the top of the other (Fig. 1). The coupling is done by means of two slots located at the inter-cavity metallization.

The basic constitutive parts of the filter-antenna module are presented below:

Fig. 1 Configuration of the SIW filter-antenna module

2.1 Input cavity

This cavity is the first resonator of the module. It contains the access line on the upper metallization. This line was designed according to [10], in order to excite the mode TE101, and combines a short-circuited 50 X CPW-line terminating in a perpendicular slot line. A current maximum is established at the termination of the CPW, independently of the frequency, as required for an optimum magnetic coupling of the energy at the input of the cavity, by means of the perpendicular slot. A 50 X microstrip line is added to the CPW in order to weld a standard SMA (Sub-Miniature version A) connector at the input of it.

2.2 Inter-cavity coupling

The coupling between the two cavities is of mixed electric and magnetic nature, and is realized by means of two slots located at the inter-cavity metallization. The electrical nature of the coupling is due to the location of the slots close to the middle of the cavity (in the y direction, Fig. 1), where the maximum of electric field is obtained. The magnetic nature is attributable to the establishment of a maximum current at the point where the two slots meet each other. In this way, the radiating element of the module (meandered slots located at the output cavity) is appropriately fed, by matching the common point of the meandered slots (point where the two slots meet each other) with the maximum current point. Besides, the length of the coupling slots must be as large as possible, in order to establish two points of maximum current at the other two ends of the meandered slots. With this configuration, the electric and magnetic field distribution on the radiating element is the same as that obtained for the cavity-backed slot antenna designed in [5].