Evanescent-Mode Waveguide Filter with Transmission Zeroes Created by Shorted

Waveguide Shunted in Coupling Region

Liu He(&), Xingjian Zhong, Zhendong Fan, Quan Zhang,

and Wei Zhang

College of Communications Engineering, The Army Engineering University

of PLA, Nanjing, China

15651766397@163.com

Abstract. This paper presents a direct method to generate a transmission zero in evanescent-mode waveguide filter by adding a shorted waveguide shunted in the coupling region between two resonators. First, a traditional evanescent-mode waveguide filter with series coupling topology is designed. To introduce a transmission zero to enhance the stopband performance, a shorted waveguide is shunted in the coupling region between two adjacent resonators. Adjusting length of the shunted waveguide can easily adjust the position of the transmission zero. To maintain the bandpass performance of the whole filter, the coupling region needs to be tuned to produce suitable coupling strength as the same as the original one. A fourth-order example is designed and measured, measured results show good accordance with the simulated ones, validating the proposed method in this paper.

Keywords: Transmission zero Evanescent mode Shorted waveguide Fourth order

1 Introduction

In recent years, the performance of filters becomes more demanding than ever before, which requires the filters should poss small size, high selectivity, low insertion loss, and so on. In the late 1950s, Jaynes [1] and Edson [2] proposed resonators in evanescentmode waveguide to design filters, which are called evanescent-mode waveguide filters now. Compared with general coupled resonant filters, the spurious response of evanescent-mode waveguide bandpass filters could be far away from the center frequency. Thus, these filters are widely used in satellite communication systems.

There are two common kinds of evanescent-mode waveguide filter, evanescentmode E-plane finned waveguide bandpass filter [3] and evanescent-mode waveguide bandpass filter with non-touching-E-plane fins [4]. In this paper, by means of introducing capacitive posts in E-plane [5, 6], a new structure is proposed as shown in Fig. 1, the equivalent circuit model of the filter is shown in Fig. 2.

© Springer Nature Singapore Pte Ltd. 2020

Q. Liang et al. (eds.), Communications, Signal Processing, and Systems, Lecture Notes in Electrical Engineering 517, https://doi.org/10.1007/978-981-13-6508-9_48

400 L. He et al.

Fig. 1. Evanescent-mode waveguide filter

Fig. 2. Equivalent circuit model

2 Filter Design

Basic structure of evanescent-mode waveguide bandpass filter is shown in Fig. 1. Four capacitive posts are placed at suitable intervals along the evanescent waveguide to realize fourth order within the passband. The input and output of the filter are realized by coaxial feedings. Cut-off wavelength of the waveguide can be computed as

The fundamental mode propagating in the waveguide is TE10 mode whose cut-off wavelength is 2a (a is the width of the waveguide’s broadside), a can be computed as kc/2. The size of the capacitive posts and the intervals is determined by the coupling coefficient matrix. A 6 6 normalized coupling matrix corresponding to a fourth-order Chebyshev filter [7] is shown in the following:

Coupling coefficient Ki,i+1 can be computed by the following equations:

where FBW is fractional bandwidth of the filter and mi,i+1 is the corresponding elements in the normalized coupling matrix. Adjust the intervals between posts until Ki,i+1 in (3) equals or approaches closely to Ki,i+1 in (4). Then, a shorted waveguide is shunted in the coupling region between the second and the third resonators as shown in Fig. 3.

Fig. 3. Simulated model proposed in this letter by introducing a shunted waveguide

Length of the shorted waveguide is about 1/2 kg, kg is waveguide length of the transmission zero’s frequency and can be computed as

2

1 k

kc

Due to the introduced shorted waveguide, the coupling between the second and the third resonators will change. Then tune the distance between the two resonators until obtaining the same bandpass performance.

An example is given to show the transmission zero created by the shorted waveguide as shown in Figs. 4 and 5. Width and height of the short waveguide’s crosssection are 14 and 5 mm, length of the waveguide is set to 17.75 mm.

Fig. 4. A shorted waveguide shunted at the center of an evanescent-mode waveguide