Dual-Mode Patch Filter with Metal Wall Structures

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.


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 , 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][5][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.

Filter Geometry
The proposed dual-mode patch filter with metal wall structures is shown in Figure 1.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 ℎ 1 ) and substrate 2 (height ℎ 2 ), both of which are characteristic of relative dielectric constant   = 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  1 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.the inductive element of  vias   vias is  via /.Therefore, the increase in  decreases the inductive element of the metal wall, and the resonant frequency increases in accordance with the equation  = 1/(2 √ ).When  0 via is 0, multiplying  gap by  0 via is 0. Therefore, the effect of  gap disappears, and the frequency response of the 0 via is equal to the frequency response of the 0 metal wall in Figure 5.

Dual-Mode Patch Filter
Figure 3 shows the simulated frequency response relative to changes in the height ℎ 1 of substrate 1.Other parameters for the filter excluding ℎ 1 are the same as described above.Because the change of ℎ 1 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 ℎ 1 .The inductive element of six vias  six vias is   via /6.When the inductive element of each via increases, the inductive element of the metal wall increases.Therefore, the resonant frequency decreases.As ℎ 1 increases from 0.435 to 0.634 mm, the 3 dB bandwidth increases from 6.1% to 9.1%.
Figure 4 shows the simulated frequency response relative to changes in the height ℎ 2 of substrate 2. Because a change of ℎ 2 influences the impedance condition like ℎ 1 , the width of the two feed lines is adjusted.In the simulated results, the resonant frequency decreases with the decrease in ℎ 2 .Because ℎ 2 denotes the gap between the patch resonator and the metal strips, the reduction of ℎ 2 increases the capacitive element  gap .Because  gap is connected to  six vias in series, the resonant frequency is inversely proportional  to the product of  gap and  six vias .Thus, an increase of  gap decreases the resonant frequency.As ℎ 2 decreases, the split between the first and second modes increases, and the 3 dB bandwidth increases.When ℎ 2 is reduced from 0.36 to 0.28 mm, the 3 dB bandwidth increases from 7.7% to 9.1%.Figure 5 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 International Journal of Antennas and Propagation 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.
Figure 6 shows the simulated frequency response relative to changes in the length of the long strip  1 .As  1 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  1 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.

Simulated and Measured Results
Figure 7 shows the simulated and measured results of the proposed filter.The parameters of the filter are selected as ℎ 1 = 0.635 mm, ℎ 2 = 0.28 mm, and  1 = 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.

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.

Figure 1 :Figure 2 :
Figure 1: Proposed dual-mode patch filter with the metal wall structures.

Figure 2 Figure 3 :
Figure2shows the simulated frequency response relative to changes in the number of vias.The other parameters are set as follows: ℎ 1 = 0.635 mm, ℎ 2 = 0.28 mm, and  1 = 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  via .Because  vias of the metal wall are practically connected,

Figure 4 :
Figure 4: Simulated frequency response relative to changes in the height ℎ 2 .

Figure 5 :
Figure 5: Simulated frequency response relative to changes in the number of the metal walls.

Figure 6 :
Figure 6: Simulated frequency response relative to changes in the number of the metal walls.

Figure 7 :
Figure 7: Simulated and measured results of the proposed filter.

Figure 8 :
Figure 8: Fabricated prototype of the proposed filter: (a) top view, (b) top of substrate 1, (c) bottom view.