Microstrip Bandpass Filter with Slotted Hexagonal Resonators and Capacitive Loading

A compact microstrip elliptic-function bandpass filter (BPF) is introduced in this article. The techniques, etching slot and adding open stubs with capacitive loading, are applied to enhance the self-inductance and self-capacitance of the hexagonal open-loop resonators, which increases the electrical length of the resonator. At the same time, the capacitive coupling between two hexagonal resonators is enhanced. Size reduction and improved insertion-loss performances are obtained. Compared to the conventional design, the center frequency and insertion loss of the proposed filters are reduced by 28 percent and 3.1 dB, respectively. The measurements show that the proposed filter, called Type II, has a fractional bandwidth of 23 percent at the center frequency of 1.84 GHz and its insertion loss in the passband is less than 1.5 dB. The BPF occupies only 12 × 21.2 mm.

From: Vol. 52 | No. 11 | November 2009 | Page 88

by Wenming Li, University of Electronic Science and Technology of China, Chengdu, China; Haiwen Liu, Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, China

Modern mobile systems impose strict requirements for bandpass filters, such as small size, high selectivity, wide upper stopband, low insertion loss and cost. A planar microstrip elliptic-function BPF is a good choice for this purpose. BPF advances on the base of circular, square and triangular patch/ring resonators have been reported widely. For example, Hong proposed an elliptic-function BPF using meander square resonators for size reduction,¹ while Zhang presented another elliptic-function BPF with capacitive loading for compactness and sharp rejection performances.²

Recently, there has been a growing interest in planar hexagonal resonators/filters because of their small size and high electric coupling. For instance, a compact BPF using hexagonal resonators was presented by Zhu.³ By adopting a skew-symmetric feed structure, an additional transmission zero occurs within the upper stopband of this filter, which generates a wide bandstop performance. In 2002, Chang provided a high-selectivity quasi-elliptic BPF with five hexagonal resonators,⁴ which proved that some frequency responses can be achieved by a fewer number of hexagonal resonators than with square resonators. Also, Mao described a dual-mode BPF using hexagonal resonators in 2006.⁵ The filter is easy to fabricate and possesses enhanced power-handling capability. However, the circuit sizes are large and occupy at least 0.83λg × 0.71λg, where g is the guided wavelength at the center frequency of the passband. To make the circuit size compact and enhance the coupling between resonators, Ni reported a hexagonal filter with a source-load coupling structure.⁶ The filter requires a size amounting to 0.32λg × 0.21λg.

In this article, a novel and small elliptic-function hexagonal BPF is proposed. The design techniques, etching slot and adding open stubs are analyzed and verified by simulations and measurements. Also, an asymmetrical 50 Ω feed structure is used. Compared to the reported hexagonal filters in the literature, the proposed filter exhibits size reduction and excellent transmission performances.

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