# Dual-Mode Patch Filter with Metal Wall Structures

Authors: D. Kang; Y. Sung
Journal: International Journal of Antennas and Propagation
 (2013)

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## Abstract

A dual-mode patch filter with metal wall structures is presented. The proposed structure consists of  substrate 1 with metal wall structures and  substrate 2 with a patch resonator. Because the symmetry of the structure can be perturbed by both long and short strips of the metal wall structures, the dual mode is achieved. The inductive element is introduced to the patch resonator through vias of the metal wall structures. The capacitive element is introduced through a gap between the patch resonator and the metal strips. The measured 3 dB fractional bandwidth for the passband is 10.4%, and the measured minimum insertion loss is 1.3 dB.


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## Body

## 1. Introduction

Technological advancements in wireless communication systems are leading to the creation of highly efficient bandpass filters with high selectivity and low insertion loss [1]. Microstrip bandpass filters using dual-mode resonators have received a great deal of attention in this regard. The microstrip bandpass filter has a compact size, narrow band, high Q, and low radiation loss and is easy to design [2]. Dual-mode resonators are attractive because each resonator can be used as a doubly tuned resonant circuit, and hence, the number of resonators required for a filter of a given degree is reduced by half, resulting in a smaller filter structure [3]. These advantages have led to studies on microstrip dual-mode bandpass filters [4–6]. These filters are the filter with cross-slotted patch resonator [4], the filter with open-loop resonator [5], and the filter with ring resonator [6], respectively.Based on the circular polarization patch antenna with a substrate integrated irregular ground [7], a dual-mode patch filter using metal wall structure is designed in this letter. The filter consists of the metal wall structures located on substrate 1 and the patch resonator located on substrate 2. The metal strips of the metal wall structures consist of long strips and short strips. The dual mode is operated by the difference between the length of the long strips and the length of the short strips. The first and second modes are controlled by the long and short strips, respectively. The influence of the capacitive and inductive elements that are generated by the metal wall structures is analyzed.


## 2. Filter Geometry

The proposed dual-mode patch filter with metal wall structures is shown in Figure1. The cross-sectional view is the view looking at a central dotted line of top view. As seen in this cross-sectional view, the proposed structure is realized on substrate 1 (height h1) and substrate 2 (height h2), both of which are characteristic of relative dielectric constant εr=9. The patch resonator is located on the upper side of substrate 2. In the top view, the width of the two feed lines is selected to be 1 mm which corresponds to a characteristic impedance of 50 Ω. Eight metal strips consisting of four long strips and four short strips are located on the upper side of substrate 1. The length of the long strip denotes l1 and that of the short strip is 4.6 mm. Because the length of the long strip is different from that of the short strip, the symmetry of the structure is perturbed, which makes dual-mode operation possible. The strips are connected to the ground plane which is on the lower side of substrate 1, through vias. The metal wall structure is a group of two adjacent metal walls. Each metal wall consists of a metal strip and six vias. The diameter of the vias is equal to the width of the metal strips, that is, 0.5 mm. Also, the distance between two adjacent metal strips is 0.5 mm. The structure has the inductive element of the vias of the metal wall structures and the capacitive element between the metal strips and the patch resonator.Figure 1
Proposed dual-mode patch filter with the metal wall structures.


## 3. Dual-Mode Patch Filter

Figure2 shows the simulated frequency response relative to changes in the number of vias. The other parameters are set as follows: h1=0.635 mm, h2=0.28 mm, and l1=8.4 mm. Simulation is carried out using IE3D. As the number of vias that constitute the metal walls increases from 2 to 6, the resonant frequency increases. To analyze the operation of the filter, the inductive element of each via denotes  Lvia. Because n vias of the metal wall are practically connected, the inductive element of nvias Lnvias is Lvia/n. Therefore, the increase in n decreases the inductive element of the metal wall, and the resonant frequency increases in accordance with the equation  f=1/(2πLC). When L0via is 0, multiplying Cgap by L0via is 0. Therefore, the effect of Cgap disappears, and the frequency response of the 0 via is equal to the frequency response of the 0 metal wall in Figure 5.Figure 2
Simulated frequency response relative to changes in the number of the vias.Figure3 shows the simulated frequency response relative to changes in the height h1 of substrate 1. Other parameters for the filter excluding h1 are the same as described above. Because the change of h1 influences the impedance condition, the width of the two feed lines is adjusted to be characteristic of a 50 Ω impedance. In the simulated results, the resonant frequency decreases as the height of vias is increased by the increase in  h1. The inductive element of six vias Lsix  vias is Lvia/6. When the inductive element of each via increases, the inductive element of the metal wall increases. Therefore, the resonant frequency decreases. As h1 increases from 0.435 to 0.634 mm, the 3 dB bandwidth increases from 6.1% to 9.1%.Figure 3
Simulated frequency response relative to changes in the heighth1.Figure4 shows the simulated frequency response relative to changes in the height  h2 of substrate 2. Because a change of h2 influences the impedance condition like  h1, the width of the two feed lines is adjusted. In the simulated results, the resonant frequency decreases with the decrease in  h2. Because h2 denotes the gap between the patch resonator and the metal strips, the reduction of h2 increases the capacitive element Cgap. Because Cgap is connected to Lsix  vias in series, the resonant frequency is inversely proportional to the product of Cgap and Lsix  vias. Thus, an increase of Cgap decreases the resonant frequency. As h2 decreases, the split between the first and second modes increases, and the 3 dB bandwidth increases. When h2 is reduced from 0.36 to 0.28 mm, the 3 dB bandwidth increases from 7.7% to 9.1%.Figure 4
Simulated frequency response relative to changes in the heighth2.Figure 5
Simulated frequency response relative to changes in the number of the metal walls.Figure5 shows the simulated frequency response relative to changes in the number of metal walls. As the number of the metal walls that constitute the metal wall structure increases, the resonant frequency decreases in a state that the matching characteristic is improved. Also, the 3 dB bandwidth increases. When the number of metal walls increases from 0 to 2, the 3 dB bandwidth increases from 0% to 9.1%. Because the 0 metal wall represents the filter without the metal wall structures, the effect of this size reduction on the filter can be known by comparing the 0 metal wall with 2 metal walls. Therefore, the size of the filter is decreased by 12.5% through the metal wall structures.Figure6 shows the simulated frequency response relative to changes in the length of the long strip l1. As  l1 increases, the first mode decreases in a state that the movement of the second mode slightly occurs. Therefore, the split between the first and second modes increases, and the 3 dB bandwidth increases. As l1 increases from 8.4 to 9.4 mm, the 3 dB bandwidth increases from 9.1% to 11.5%. Through these findings, the first and second modes can be controlled by the long and short strips, respectively.Figure 6
Simulated frequency response relative to changes in the number of the metal walls.


## 4. Simulated and Measured Results

Figure7 shows the simulated and measured results of the proposed filter. The parameters of the filter are selected as h1=0.635 mm, h2=0.28 mm, and l1=8.4 mm. The impedance of the two feed lines is selected to be 1 mm. The prototype of the filter is based on the abovementioned parameters and is fabricated as shown in Figure 8. In Figure 7, the 3 dB bandwidth of the measured passband is 10.4%, and its measured minimum insertion loss is 1.3 dB. The simulated result is similar to the measured result, but a deviation between the simulated and measured results is observed. This deviation results from errors that occur during the fabrication of the filter. An air gap between the two substrates is generated when substrate 1 is being attached to substrate 2. This air gap reduces the capacitive element, which increases the resonant frequency. Because the air gap is not considered in the simulation, the deviation occurs.Figure 7
Simulated and measured results of the proposed filter.Fabricated prototype of the proposed filter: (a) top view, (b) top of substrate 1, (c) bottom view.
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(b)
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## 5. Conclusions

In this work, a dual-mode patch filter with metal wall structures is proposed. The symmetry of the structure can be perturbed by both long and short strips of the metal wall structures, which makes dual-mode operation possible. The first and second modes are controlled by the long and short strips, respectively. The effect of the capacitive and inductive elements formed by the metal wall structures is proved. This filter is suitable for multilayers circuit applications at microwave frequencies.



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*Source: 576898-2013-08-28.xml*