Efficient Design Methodology for a Complex DRA-SIW Filter-Antenna Subsystem

1 Jodrell Bank Centre for Astrophysics, The University of Manchester, Manchester, UK 2Department of Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan 3Department of Electrical Engineering, CECOS University Peshawar, Peshawar, Pakistan 4Center for Advanced Studies in Telecommunications, COMSATS Institute of Information Technology, Islamabad, Pakistan 5XLIM, UMR 7252, Université de Limoges/CNRS, Limoges, France


Introduction
High miniaturization trends in wireless communication systems are inspiring researchers to produce compact and costeffective components with easy integration in system-inpackage solutions.Conventionally, in RF front-end modules, the filter and the antenna are realized separately and are connected at 50-ohm reference impedance.This conventional technique limits the degree of freedom in the design of both the filter and the antenna and thus limits the performance of the overall filter-antenna subsystem.A mutual-synthesis technique has been presented in [1] for the design of a filterantenna subsystem, in which the antenna and filters are connected to each other at common optimum impedance rather than the conventional 50 ohms.A more generalized methodology was proposed in [2] which was based on the polynomial optimization that can also handle the outof-band constraints.A number of planar and nonplanar filter-antenna designs have been proposed recently.In [3], a nonplanar structure has been proposed in which a horn antenna is covered with a Frequency Selective Surface (FSS) created using Substrate Integrated Waveguide (SIW) cavities to realize a filter-antenna module.In another study, the impedance bandwidth has been improved along with bandwidth and gain enhancement and design miniaturization by having a shared ground plane between the filter and the antenna [4].A D-shaped filtering antenna has been reported in [5] for GSM, WLANs, LTE, and satellite applications providing flat antenna gain in the pass band and high suppression in the stop band.In [6], a filter-antenna module is integrated through meandered slots.
To reduce the overall size instead of using a classical cascaded basic function approach, a multilayer technology is used in [7] for designing a filter-antenna subsystem.A filtering antenna in [8] is obtained by substituting the resonator and one port of a bandpass filter with fan-shaped patch antenna, where the fan-shaped patch acts as a radiator as well as the second resonator of the filter.A dual band filter-antenna subsystem for Wi-Fi application is achieved by integrating a step impedance resonator filter with a dual band hybrid dielectric resonator antenna (DRA) and monopole antenna [9].A cavity backed slot antenna was proposed in [10] with improved performance.Various SIW based filters have been proposed in the literature.A multimode cavity filter in SIW technology was proposed in [11] which provides more flexibility in choosing the desired filtering topology.
In this paper, a miniaturized filter-antenna system is proposed in which the DRA acts as the last filter element.The filter is realized through SIW technology.At first, the antenna will be characterized based on the operating mode and excitation system (taking into account the complex multimode filter), from which a reliable model will be extracted, and then a filter-antenna subsystem will be designed exploiting the proposed methodology and finally the measured results will be discussed.

Antenna Characterization
Dielectric resonator (DR) can radiate and act as an antenna when not bounded by a conductive boundary.The cylindrical DRA has been employed in this work which is integrated with a SIW cavity.The proposed structure is presented in Figure 1.The DR used in this design has dielectric constant,   , of 9.8 and dielectric loss tangent, tan , of 0.002.The radius of DR is 6.35 mm and height is 9 mm.The SIW filter is fabricated using standard printed circuit technology on Rogers 5880 laminate having dielectric constant of 2.2, loss tangent of 0.0009, and thickness of 1.575 mm.The length and width of the substrate integrated cavity are 38 mm × 38 mm.The DRA is excited for HEM 11 mode through a rectangular slot having length Ls of 10 mm and width  of 2.2 mm.All the simulations were carried out using a commercially available full-wave finite element method based electromagnetic simulator (ANSYS HFSS).

Resonant Frequency and Quality
Factor.The resonant frequency and -factor have been calculated from [12]  0 =  × 6.324 where  = /, r is the radius, and  represents the height of DRA.
The resonant frequency and -factor of HEM 11 mode are calculated theoretically for a radius of 6.35 mm and height of 9 mm and were found to be 5.56 GHz and 7.90, respectively.The resonant frequency and -factor can be optimized as desired using different combinations of radiusto-height ratio.The quality factor can be enhanced at the cost of larger radius.The radius-to-height ratio of 0.35 has been used in the simulation.The cylindrical DRA is excited with the desired mode of HEM 11 , and the  and  fields of this mode are plotted in Figures 2(a) and 2(b), respectively.

Excitation Mechanism.
The DRA is excited through a slot as shown in Figure 1 for the desired mode to be installed.The radiation resistance of DRA depends upon the length and width of the slot.Figure 3 establishes the relation between the slot dimensions and radiation resistance.The optimum slot dimensions have been selected from the graph in Figure 3.    and width of the slot (i.e.,  1 and  2 ) are taken and the impedance values are observed.

Circuit Model of DRA. The equivalent circuit of the antenna is shown in
In order to calculate the length of the coupling slot, a graph is presented in Figure 7 which shows different values of  1 and  2 as a function of slot length for different slot widths.
The antenna structure is further examined, and the circuit element values (extrapolated from Figure 7) for different combinations of  and  are summarized in Table 1.

Impedance transformer
Coupling with DRA

Filter-Antenna Subsystem
After characterization of the antenna for the desired frequency and mode, the antenna model is connected with the filter and integrated in the same package.

Codesigning of Filter with Antenna Model.
A dual mode SIW cavity filter is optimized, and a detailed description of the filter is illustrated in Figure 8.The capabilities to achieve different filtering function through the dual mode filters were explored in [11]; a similar filter structure is adopted for the design methodology proposed in this work.The filter is now codesigned with the antenna model.The antenna and the filter have been connected to each other through common reference impedance which is optimized for both the antenna and the filter.This will increase the degree of freedom for the design of both the antenna and the filter.The dependence of the external quality factor   and coupling coefficient k is detailed in Figures 9(a design parameters that can be easily optimized through these graphs.The circuit model of the filter is shown in Figure 10.Hence, where i, j may have the value = , −, 1, 2, +, . The filter designed at 50-ohm impedance at input and output, when connected with the antenna, will only result in optimum match when the antenna is also designed at 50-ohm input impedance which will restrict the antenna performance to this impedance region.Instead of following the conventional technique, the filter has been codesigned with the antenna model extracted in the previous section.The transmission and reflection properties of the filter are illustrated in Figure 11(a).Return loss of the filter-antenna (2) International Journal of Antennas and Propagation  subsystem designed at common optimized reference impedance is shown in Figure 11(b).Following are the element values for the optimized circuit model as shown in Table 2.The input impedance of the filter-antenna subsystem for  = 8 mm and  = 1.8 mm is shown in Figure 12.
The values of  and  are interpolated from the graph of Figure 7.  parameters of the filter are shown in Figure 11 The realized gain of the subsystem designed at 50 ohms is 6.9 dBi whereas the realized gain of the subsystem designed with the proposed methodology is found to be 7.1 dBi.

Results and Discussion on the Combined Filter-Antenna
Subsystem.The optimized filter-antenna structure has been fabricated as shown in Figure 13(a).The physical dimensions of the optimized structure are given in Table 3.The simulated and measured results are compared in Figure 13(b).It can be observed that the return loss for both the simulated and the measured cases is better than 20 dB for a bandwidth of 150 MHz and 10 dB for a bandwidth of 250 MHz.It can be seen that the filter-antenna subsystem is well matched for the desired frequency of operation.The copolarization and cross-polarization radiation patterns of the antenna are shown in Figure 14.Figures 15(a The radiation characteristics of the proposed filterantenna subsystem are analyzed in detail.The measured and simulated radiation patterns (normalized) for  fields are compared in Figure 16(a) and for  fields in Figure 16(b).It can be concluded that the measured results of the fabricated prototype validate the proposed simulated design.
A comparison is given in Table 4.The proposed structure presents high selectivity and improved gain performance by using a high efficiency dielectric resonator antenna.

Conclusion
An efficient design methodology for the design of filterantenna subsystem is proposed.A prototype has been   proposed and fabricated employing DRA and SIW based dual mode cavity filter.The measured results are in accordance with the simulated results.The proposed design methodology simplifies the design process and optimizes the impedance between the filter and the antenna, thereby relaxing the constraints on the design of both the filter and the antenna, which consequently results in a subsystem with improved performance.

Figure 4 .
In order to verify the proposed model, the input impedance plots of equivalent circuit model and EM model of the DRA are compared in Figure 5.The real and imaginary impedance values show good agreement between circuit and EM model.Plot of impedance values for different models of antenna are shown in Figures 6(a) and 6(b).Different values of length

Figure 3 :
Figure 3: Simulated radiation resistance for different lengths and widths of the slot.

Figure 7 :
Figure 7: Plot of  1 and  2 for different values of length with fixed slot widths.Solid lines are for  = 1, dotted lines are for  = 1.5, dashed lines are for  = 2, and solid-dotted lines are for  = 2.5.

Figure 12 :
Figure 12: The input impedance of the antenna with the filter structure.
(b), which are not optimized for 50-ohm output impedance but at optimum common impedance.The coupling matrix of the filter optimized for the subsystem is given by (3), with  = 5.8 GHz and  = 190 MHz.Here, − and + are the nonresonating nodes.Hence, ) and 15(b) show the realized gain and the radiation efficiency of the proposed filter-antenna subsystem.

Figure 13 :Figure 14 :Figure 15 :
Figure 13: Simulated and measured return loss of the filter-antenna subsystem connected at common impedance.

Table 1 :
Different length and width relation with  1 and  2 .

Table 2 :
Element values for the optimized circuit model.

Table 3 :
Dimensions for the filter-antenna subsystem.

Table 4 :
Comparison with other filter-antenna systems.