**3. UWB BPF with three-stub-loaded slot-line multiple mode resonator (MMR)**

#### **3.1. Three-stub-loaded slot-line UWB BPF**

where *c* is the speed of light in vacuum, εeff is the effective dielectric constant, and *a* and *g* are

A UWB BPF with notch band is designed based on the abovementioned method. To further increase the attenuation of the notch band in the UWB band, two microstrip open-loop resonators are loaded to the slot-line resonator. By proper setting the position of the two openloop resonators, a narrow notch band can be achieved in the UWB passband. The layout of the proposed notched UWB BPF is shown in **Figure 3(a)**. **Figure 3(b)** illustrates a full-wave

**Figure 4.** Measured frequency responses of proposed band-notched UWB BPF using open-loop resonator-loaded

**Figure 3.** (a) Layout of the proposed band-notched UWB BPF using open-loop resonator-loaded stepped-impedance

slot-line resonator and (b) simulated result of the band-notched UWB BPF with varying *a*.

the side length and the gap width of the microstrip open-loop resonator, respectively.

**2.3. Experimental results and discussions**

46 UWB Technology and its Applications

stepped-impedance slot-line resonator.

**Figure 5** shows the configurations of the proposed UWB BPF with three-stub-loaded slot-line MMR. Three-stub-loaded slot-line MMR is fed by microstrip feed line. The MMR and the feed lines are folded and orthogonal coupling is applied.

The slot-line MMR consists of a stepped-impedance resonator and three loading stubs, with one located at the middle of the resonator. Compared with traditional SIR and stub-loaded resonator (SLR), the proposed one has more degrees of freedom to control its resonant frequencies. Once the original parameters of the slot-line resonator are determined, EM solver is invoked to tune the structure to achieve an optimized characteristic. **Figure 6** depicts the simulated transmission characteristics of the resonator with and without additional stub. The solid line and dashed line indicate the transmission characteristic of the resonator with and without additional stub, respectively. Additional stub increases the electrical length of the stub, and an additional resonant mode is shown in the UWB frequency range.

**Figure 5.** Configurations of the proposed UWB filter with three-stub-loaded slot-line MMR. (a) Top view and (b) bottom view.

**Figure 6.** Characteristics of the slot-line MMR with and without the additional stub.

Next, the influence of two parameters of the resonator on the resonant mode of the resonator is studied. **Figure 7** shows the variation of resonant-mode frequencies against *L*<sup>8</sup> , length of the stub, and *r*, radius of the additional stub. As can be seen from **Figure 7**, four resonant modes (i.e., *f* 1 , *f* 2 , *f* 3 , and *f* 4 ) are created in the studied frequency range, which are applied to generate the UWB transmission characteristic. **Figure 7(a)** shows the variation of resonant mode against the length of the stub. As *L*<sup>8</sup> increases from 2.8 to 4.8 mm, *f* 3 drops from 10.5 to 8 GHz, and *f* 4 drops from 11.5 to 10 GHz, which are located in the UWB range, while *f* 1 and *f* 2 keep almost unchanged. **Figure 7(b)** depicts the variation of resonant-mode frequencies against the radius of the additional stub. As *r* increases from 0.4 to 1.0 mm, *f* 1 , *f* 2 , and *f* 3 remain stationary, while *f* 4 drops from 12.5 to 10.5 GHz. These resonant frequencies are basically related to the stepped-impedance resonator, and some ones also can be separately controlled by tuning the loaded stubs, which shows great convenience in relocating the required resonant modes of the resonator.

EM solver is invoked to analyze the relationship between the parameters and the transmission characteristic of the bandstop filter. **Figure 9(a)** shows the transmission characteristics of the resonator versus *L*, the length of the loading stub. When *L* increases from 4.5 to 6.0 mm, the lower resonant mode keeps unchanged, and the higher mode decreases from 3.06 to

**Figure 8.** Dual-mode microstrip resonator-loaded slot-line. (a) Structure and (b) its equivalent circuit model.

and (b) *r.*

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As we all know that the bandwidth of the bandstop filter is determined by the separation of two poles and the band position of the bandstop filter can be adjusted by the length of the stub *L*, a transmission pole is also created, which is located near the lower transmission zero, sharping the transition band. **Figure 9(b)** illustrates the effect of varying S on the two modes of the dual-mode resonator. When *S* increases from 3.0 to 4.5 mm, the bandwidth of the filter increases from 60 to 480 MHz, and the central frequency decreases from 4.6 to 4.2 GHz. Obviously, modifying *S* and *L* can change the position and the bandwidth of the stopband.

Based on the proposed structure, a UWB BPF with notch band from 5.15 to 5.35 GHz is designed and fabricated. The feed lines and the MMR in slot-line are folded and orthogonally coupled to acquire the desired strong coupling. A dual-mode loaded-stub open-loop

2.84 GHz.

**3.3. Filter implementation and results**

**Figure 7.** Resonant-mode frequencies against (a) *L*<sup>8</sup>

#### **3.2. Band-notched UWB BPF**

Based on the UWB filter mentioned above, a notch band is produced and a band-notched UWB BPF is designed. **Figure 8(a)** illustrates the circuit model of a dual-mode resonatorloaded slot-line, where the white section indicates the slot-line, the gray part is the ground, and the black one is a microstrip dual-mode resonator. Equivalent circuit of dual-mode resonator-loaded slot-line is provided in **Figure 8(b)**. Two resonant modes of the stub-loaded resonator are coupled to the slot-line, providing a bypass for the adjacent signal of its resonance. The degree of separation of two modes determines the bandwidth of the notch band, and the coupling between resonator and slot-line influences the amplitude of the attenuation.

Slot-Line UWB Bandpass Filters and Band-Notched UWB Filters http://dx.doi.org/10.5772/intechopen.80004 49

**Figure 7.** Resonant-mode frequencies against (a) *L*<sup>8</sup> and (b) *r.*

Next, the influence of two parameters of the resonator on the resonant mode of the resonator

stub, and *r*, radius of the additional stub. As can be seen from **Figure 7**, four resonant modes

ate the UWB transmission characteristic. **Figure 7(a)** shows the variation of resonant mode

almost unchanged. **Figure 7(b)** depicts the variation of resonant-mode frequencies against the

stepped-impedance resonator, and some ones also can be separately controlled by tuning the loaded stubs, which shows great convenience in relocating the required resonant modes of

Based on the UWB filter mentioned above, a notch band is produced and a band-notched UWB BPF is designed. **Figure 8(a)** illustrates the circuit model of a dual-mode resonatorloaded slot-line, where the white section indicates the slot-line, the gray part is the ground, and the black one is a microstrip dual-mode resonator. Equivalent circuit of dual-mode resonator-loaded slot-line is provided in **Figure 8(b)**. Two resonant modes of the stub-loaded resonator are coupled to the slot-line, providing a bypass for the adjacent signal of its resonance. The degree of separation of two modes determines the bandwidth of the notch band, and the coupling between resonator and slot-line influences the amplitude of the attenuation.

drops from 11.5 to 10 GHz, which are located in the UWB range, while *f*

increases from 2.8 to 4.8 mm, *f*

drops from 12.5 to 10.5 GHz. These resonant frequencies are basically related to the

) are created in the studied frequency range, which are applied to gener-

3

1 , *f* 2 , and *f* 3 , length of the

drops from 10.5 to 8 GHz,

1 and *f* 2 keep

remain stationary,

is studied. **Figure 7** shows the variation of resonant-mode frequencies against *L*<sup>8</sup>

radius of the additional stub. As *r* increases from 0.4 to 1.0 mm, *f*

**Figure 6.** Characteristics of the slot-line MMR with and without the additional stub.

(i.e., *f* 1 , *f* 2 , *f* 3 , and *f* 4

and *f* 4

while *f* 4

the resonator.

against the length of the stub. As *L*<sup>8</sup>

48 UWB Technology and its Applications

**3.2. Band-notched UWB BPF**

**Figure 8.** Dual-mode microstrip resonator-loaded slot-line. (a) Structure and (b) its equivalent circuit model.

EM solver is invoked to analyze the relationship between the parameters and the transmission characteristic of the bandstop filter. **Figure 9(a)** shows the transmission characteristics of the resonator versus *L*, the length of the loading stub. When *L* increases from 4.5 to 6.0 mm, the lower resonant mode keeps unchanged, and the higher mode decreases from 3.06 to 2.84 GHz.

As we all know that the bandwidth of the bandstop filter is determined by the separation of two poles and the band position of the bandstop filter can be adjusted by the length of the stub *L*, a transmission pole is also created, which is located near the lower transmission zero, sharping the transition band. **Figure 9(b)** illustrates the effect of varying S on the two modes of the dual-mode resonator. When *S* increases from 3.0 to 4.5 mm, the bandwidth of the filter increases from 60 to 480 MHz, and the central frequency decreases from 4.6 to 4.2 GHz. Obviously, modifying *S* and *L* can change the position and the bandwidth of the stopband.

#### **3.3. Filter implementation and results**

Based on the proposed structure, a UWB BPF with notch band from 5.15 to 5.35 GHz is designed and fabricated. The feed lines and the MMR in slot-line are folded and orthogonally coupled to acquire the desired strong coupling. A dual-mode loaded-stub open-loop

**Figure 9.** The transmission characteristic of the simplified resonator with varied parameters. (a) L and (b) S.

**4. Triangular loop-loaded band-notched UWB filter**

**Figure 11.** Comparison between measured and EM-simulated results of the band-notched UWB BPF.

and **(c)** gives equivalent circuits of the slot-line MMR.

Two stubs are symmetrically loaded to slot-line, which forms a dual-stub-loaded slot-line MMR, as shown in **Figure 12(a)**. Because the proposed slot-line MMR is a symmetrical structure, even-odd mode theory can be applied to analyze its resonant characteristics. **Figure 12(b)**

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Under odd-mode excitation, the symmetrical plane can be seemed as short-circuited, and its

*ZB* tan *θ<sup>B</sup>* tan *θ<sup>C</sup>* + *ZB* tan *θ<sup>B</sup>* tan *θ<sup>A</sup>* + *ZA* tan *θ<sup>A</sup>* tan *θ<sup>C</sup>* = 0 (2)

where *Z*A and *Z*B and *θ*A, *θ*B, and *θ*C are the characteristic impedance and electrical length of

Under even-mode excitation, the symmetrical plane can be seemed as open-circuited, and its

*ZA* tan *θ<sup>A</sup>* + *ZB* tan *θ<sup>B</sup>* = *ZB* tan *θ<sup>A</sup>* tan *θ<sup>B</sup>* tan *θ<sup>C</sup>* (3)

Resonant modes of the resonator can be controlled and allocated according to the require-

To have a clear knowledge of slot-line resonator, resonant characteristics of the dual-stubloaded slot-line resonator are performed by invoking the 3D EM simulator. Resonant modes

**4.1. Dual-stub-loaded slot-line MMR**

resonant condition can be derived as

the dual-stub-loaded MMR, respectively.

resonant condition can be summarized as

ments by changing the parameters of the resonator.

**Figure 10.** Configurations of the proposed band-notched UWB BPF using dual-mode microstrip resonator-loaded threestub-loaded slot-line MMR. (a) Top view and (b) bottom view.

resonator is loaded to the slot-line, and a notch band for WLAN is produced, as shown in **Figure 10**. The dual-mode resonator is folded in order to improve the slow-wave effect for miniaturization. A substrate of 28 mm × 18 mm with a relative dielectric constant of ε<sup>r</sup> = 3.5 and a thickness of h = 0.8 mm is used in the design. Finally obtained parameters of the filter shown in **Figure 1** are *W*<sup>0</sup> = 1.80 mm, *L*<sup>1</sup> = 3.80 mm, *L*<sup>2</sup> = 4.70 mm, *W*<sup>1</sup> = 1.50 mm, *L*<sup>3</sup> = 8.00 mm, *W*<sup>2</sup> = 0.30 mm, *L*<sup>4</sup> = 17.40 mm, *W*<sup>3</sup> = 0.60 mm, *L*<sup>5</sup> = 3.70 mm, *W*<sup>4</sup> = 0.50 mm, *L*<sup>6</sup> = 3.80 mm, *W*<sup>5</sup> = 0.30 mm, *L*<sup>7</sup> = 2.50 mm, *W*<sup>6</sup> = 0.30 mm, *L*<sup>8</sup> = 3.50 mm, and *r* = 0.55 mm. Measurements are performed by using vector network analyzer AV3926.

A comparison between the simulated and measured results is shown in **Figure 11**, where the solid lines and the dashed lines indicate the EM-simulated results and measured results, respectively. Simulated results show that the 3-dB bandwidth of the filter covers from 3.1 to 5.15 GHz and from 5.35 to 10.6 GHz, while the measured ones show that the 3-dB bandwidth covers from 3.2 to 5.15GHz and from 5.35 to 10.6GHz. Simulated and measured insertion losses of each passband are about 2 dB; return losses are −12 dB/−18 dB and − 18 dB/−10 dB. Return loss in the notch band is greater than 15 dB. Except for the deviation that may be caused by the fabrication, the simulated results agree well with the measured results.

**Figure 11.** Comparison between measured and EM-simulated results of the band-notched UWB BPF.
