362 A. HUELTES, J. VERDU, C. COLLADO, J. MATEU, E. ROCAS, J. L. VALENZUELA, FILTENNA INTEGRATION ACHIEVING IDEAL … Filtenna Integration Achieving Ideal Chebyshev Return Losses Alberto HUELTES, Jordi VERDU , Carlos COLLADO , Jordi MATEU 1 , Eduard ROCAS , Jose Luis VALENZUELA Dept. of Signal Theory and Communications, Universitat Politecnica de Catalunya, PMT, 08660 Castelldefels, Spain alberto.hueltes@tsc.upc.edu, jordi.verdu@tsc.upc.edu Abstract. This paper demonstrates that it is possible to find an ideal filter response (Chebyshev, Butterworth,..) considering the antenna as the last resonator of a filter under certain circumstances related with the antenna per- formance and the bandwidth of the filtenna device. If these circumstances are not accomplished, we can achieve ex- cellent performance as well, by means of an iterative proc- ess the goal of which is defined by either a filter mask or a classical filter function itself. The methodology is based on the conventional coupling matrix technique for filter design and has been validated by fabricating a microstrip prototype using hairpin resonators and a rectangular patch antenna. Keywords Filtenna, filter, antenna, return losses, Chebyshev, optimal bandwidth. 1. Introduction One of the major topics in telecommunications and electronics had been for a long time how to match the im- pedance of a certain load to any circuit through a matching network. In particular, a widely studied example consists on matching the impedance of two of the most important parts of a RF/MW front-end: the antenna and the filter. The most straightforward way to do this is by introducing a matching network between both devices [1]. Neverthe- less, in terms of compactness and overall subsystem per- formance, matching the load and the circuit, being the circuit itself the matching network, has advantages. This topic has been studied by several authors; in 1964 Matthaei [2] already detailed how to adapt a RLC load by means of synthesizing a filter matching network preceding the load. More recently, many authors have faced the problem of combining both the filter and the antenna by use of differ- ent techniques and technologies. In [3] a mutual-synthesis approach, that simultaneously optimizes the filtering and radiation functions to obtain an optimal matching, is pre- sented. Similarly to [2], but specifically for UWB commu- nications, [4] introduces a filtenna device which is de- signed by use of a filter as a matching network to match the antenna. References [5] and [6] propose two different tech- niques to synthesize the filtenna network making use of the coupling matrix and considering the antenna as the last resonating element of the filter. In [7] that last considera- tion is also assumed but they go further by proposing an automatic method to carry out a mutual-synthesis based on [3]. In addition, several other authors have proposed solutions that combine the antenna with active devices to provide the desired overall circuit response. Other authors have improved the miniaturization of radio front-ends by use of active integrated antenna (AIA) theories [8], [9]. Besides the techniques and concepts introduced on some of the previous references, this work also uses the antenna bandwidth as one of the parameter to account for the filter design. To this respect, when the filter bandwidth is used as synthesis parameter, this work demonstrates the existence of a unique filter bandwidth that offers an ideal filter response for any given shunt RLC antenna. This concept is fully detailed in Section 2. In addition to that and for those cases where the filter bandwidth and the given antenna is not selected by the designer, an optimiza- tion procedure has to be followed to achieve the best pos- sible response. Sections 3 & 4 detail on the optimization procedure for the design of the filtenna when a certain patch antenna is given. Fabrication and measurements of the resulting design are reported in Section 5. 2. Theoretical Background 2.1 Antenna Integration as the Last Filter Stage The filter-antenna subsystem can be analyzed as the case where the antenna acts as the last stage of a filter network given its intrinsic resonant behavior. The inverse process that we follow here is transforming the last stage of a conventional filter in our antenna in order to keep the load matched and achieve, at the same time, the desired overall performance response. The following analysis is valid as long as our antenna can be accurately character- ized as a shunt RLC circuit with values R ant , L ant , C ant .