Contents I. Introduction
There are several RF band-pass filters based on microstrip technology [1]; and the majority of these filters used the principle of the parallel-coupled microstrip line filters (PCMFs) that was originally proposed in [2]. PCMF first order provides a versatile design for several applications, offering the capability to adjust operation bandwidth with impedance changes in their lines. However, one of the disadvantages lies in its limited rejection at the second harmonic . The unrejected harmonics can cause interference to other systems, thus it is highly desirable in microstrip filters to increase the rejection at such frequency. The techniques presented to reduce or reject the natural response to the second harmonic in PCMF are several. The work presented in [3] is a third order PCMF that employed wiggly-lines instead of the classical straight lines, reaching a high rejection at second harmonic. Works reported in [4] and [5] have geometric designs placed periodically into each of the couplers. Metamaterials as for example Defected Ground Structure (DGS) are used to overcome this problem [6]–[7]. Other techniques use complex geometric forms as those proposed by [8] and [9]. These solutions are effective, however, is required electromagnetic (EM) simulations to design them, due to the large number of variables involved in obtaining the desire frequency response, and the lack of analytical solution.
