4.1.1 A CLL-based filtenna design

The elaborate geometry of the filtenna is shown in Figure 14. As depicted in Figure 14(a) and (e), the compact electrically small antenna (ESA) with NFRP was chosen as the radiating element. The NFRP element is proposed to etch upon one side of the semi-circle board, while the monopole microstrip is located on the other side, with the design principle corresponding to the reported NFRP ESAs [23–25]. The composite structure of this radiator element and filtering element, which is based on CLL resonators, is well shown in Figure 14(a)–(d). The enlarged filter is shown in Figure 14(b). This filter structure is a typical band-pass design [21, 26, 27], and is set to be symmetric about the S–S<sup>0</sup> line. One end is connected to the printed monopole and the other to the SMA.

The third resonator is an additional CLL element, shown in blue in Figure 12(a). Its gap position coincides with the driven CLL element, and it has an arm included to facilitate its coupling to the NFRP element. This collocated arrangement of the two CLLs provides a means to control the mutual coupling, further expanding the bandwidth without increasing the total overall dimensions of the filtenna. Three slots were etched in the ground strip directly beneath the two CLL elements to achieve a smoother realized gain curve. The length of the additional CLL element is set nearly equal to the driven CLL's size to make their resonance frequencies close to

Enhanced bandwidth filtenna with slots in its ground strip. (a) Top and (b) back views of the HFSS simulation

The simulated and measured |S11| and realized gain values of the second filtenna

with the ground strip slots are given in Figure 13. The simulated (measured) realized gain values indicate that the simulated peak realized gain value is improved from 1.659 to 1.75 dBi. The corresponding measured value is 1.376 dBi, revealing more losses than expected in fabrication. For the simulated |S11| values exhibited in Figure 13, the impedance bandwidth ranges from 2.264 to 2.46 GHz (about 8.3% fractional bandwidth, i.e., a 32.2% improvement) and was from 2.261 to 2.447 GHz (7.9% fractional bandwidth, i.e., a 26% improvement) in the measurement. Similarly, the simulated ka 0.94 and measured ka 0.938 values verify that the filtenna is electrically small. Furthermore, the simulated radiation efficiency across the entire operational bandwidth is higher than 82.87%. Again, very good agreement between the simulated and measured performance characteristics was

one another.

Figure 12.

model. (c) Front and back views of the fabricated prototype.

Electromagnetic Materials and Devices

obtained.

262

#### Figure 14.

Prototype of miniaturized filtenna with a NFRP structure. (a) 3D graphic of the NFRP filtenna. (b) Filtering structure. (c) Side views of the ESA and filtenna. (d) Fabricated module of the filtenna in various side views. (e) 3D graphic of the ESA with a NFRP structure.

Within the operational band, the realized gain (along +z axis) and radiation efficiency ranges in 5.73–5.94 dBi and 94–95%, respectively. The expected two overlapping resonances are depicted in the result of the fabricated filtenna. As exhibited in Figure 15(a), the measured (simulated) |S11|min is respectively situated at 1.23 (1.24) and 1.265 (1.272) GHz. Then 10 dB bandwidth is expanded to 49 (50) MHz, ranging in 1.223 (1.23)–1.272 (1.28) GHz, i.e., the proposed filtenna processes a 3.93% (4.2%) FBW. It is comparably flat for the peak realized gain values in this operational band, which ranges from 4.73 (4.25) to 5.43 (5.23) dBi. This fairly flat realized gain curve indicates that an essentially stable response is obtained through the whole operational band. As observed, the measured ones

The simulated (measured) peak realized gain patterns in the E- and H-planes at the lower resonance frequency

The simulated and measured |S11| and peak realized gain values versus the source frequency (a) for the filtenna

design shown in Figure 14(a) and (b) for the CLL-based NFRP ESA alone in Figure 14(e).

Compact, Efficient, and Wideband Near-Field Resonant Parasitic Filtennas

DOI: http://dx.doi.org/10.5772/intechopen.82305

Figure 16 indicates the measured (simulated) E- and H-plane peak realized gain

As shown in Figure 17, by altering the orientation, position, and configuration of

patterns for the proposed filtenna at the lower resonance frequency of 1.230 (1.240) GHz. The measured (simulated) peak gain was 5.12 (5.36) dBi. On the whole, the measured results of our proposed filtenna are in good agreement with

the filter element, certain advantages could be obtained. In contrast with the

shifted a little to the lower band.

of the filtenna shown in Figure 14—1.24 (1.230) GHz.

4.2 Variations of compact filtennas

the simulated ones.

265

Figure 15.

Figure 16.

#### 4.1.2 Simulation and measured results

Figure 15(a) demonstrates the simulated and measured |S11| and peak realized gain values versus the source frequency of the optimized filtenna. As a reference, the simulated reflection coefficient of the optimized NFRP ESA alone (depicted in Figure 14(e)), is shown in Figure 15(b). The Agilent E8361A PNA vector network analyzer (VNA) is exploited to quantify the impedance matching. With regard to the NFRP ESA, a 30.3 MHz 10 dB impedance bandwidth is realized corresponding to the center frequency which is located at 1.26 GHz (corresponding to FBW of 2.4%) and with karad calculated to be 0.81 (while arad represents the smallest radius for the sphere which could entirely cover the radiating structure at the lowest operation frequency fL, then k=2π/λ<sup>L</sup> = 2πfL/c represents the number of relevant waves in free space). It is worth noting that the ground size is R1 = 75 mm, i.e., kaground 1.96. Although the ground size has a certain influence on the gain and front-to-back ratio, its effect on the impedance matching level and bandwidth is deemed slight.

Compact, Efficient, and Wideband Near-Field Resonant Parasitic Filtennas DOI: http://dx.doi.org/10.5772/intechopen.82305

Figure 15.

The simulated and measured |S11| and peak realized gain values versus the source frequency (a) for the filtenna design shown in Figure 14(a) and (b) for the CLL-based NFRP ESA alone in Figure 14(e).

#### Figure 16.

4.1.2 Simulation and measured results

(e) 3D graphic of the ESA with a NFRP structure.

Electromagnetic Materials and Devices

deemed slight.

264

Figure 14.

Figure 15(a) demonstrates the simulated and measured |S11| and peak realized gain values versus the source frequency of the optimized filtenna. As a reference, the simulated reflection coefficient of the optimized NFRP ESA alone (depicted in Figure 14(e)), is shown in Figure 15(b). The Agilent E8361A PNA vector network analyzer (VNA) is exploited to quantify the impedance matching. With regard to the NFRP ESA, a 30.3 MHz 10 dB impedance bandwidth is realized corresponding to the center frequency which is located at 1.26 GHz (corresponding to FBW of 2.4%) and with karad calculated to be 0.81 (while arad represents the smallest radius for the sphere which could entirely cover the radiating structure at the lowest operation frequency fL, then k=2π/λ<sup>L</sup> = 2πfL/c represents the number of relevant waves in free space). It is worth noting that the ground size is R1 = 75 mm, i.e., kaground 1.96. Although the ground size has a certain influence on the gain and front-to-back ratio, its effect on the impedance matching level and bandwidth is

Prototype of miniaturized filtenna with a NFRP structure. (a) 3D graphic of the NFRP filtenna. (b) Filtering structure. (c) Side views of the ESA and filtenna. (d) Fabricated module of the filtenna in various side views.

The simulated (measured) peak realized gain patterns in the E- and H-planes at the lower resonance frequency of the filtenna shown in Figure 14—1.24 (1.230) GHz.

Within the operational band, the realized gain (along +z axis) and radiation efficiency ranges in 5.73–5.94 dBi and 94–95%, respectively. The expected two overlapping resonances are depicted in the result of the fabricated filtenna. As exhibited in Figure 15(a), the measured (simulated) |S11|min is respectively situated at 1.23 (1.24) and 1.265 (1.272) GHz. Then 10 dB bandwidth is expanded to 49 (50) MHz, ranging in 1.223 (1.23)–1.272 (1.28) GHz, i.e., the proposed filtenna processes a 3.93% (4.2%) FBW. It is comparably flat for the peak realized gain values in this operational band, which ranges from 4.73 (4.25) to 5.43 (5.23) dBi. This fairly flat realized gain curve indicates that an essentially stable response is obtained through the whole operational band. As observed, the measured ones shifted a little to the lower band.

Figure 16 indicates the measured (simulated) E- and H-plane peak realized gain patterns for the proposed filtenna at the lower resonance frequency of 1.230 (1.240) GHz. The measured (simulated) peak gain was 5.12 (5.36) dBi. On the whole, the measured results of our proposed filtenna are in good agreement with the simulated ones.

#### 4.2 Variations of compact filtennas

As shown in Figure 17, by altering the orientation, position, and configuration of the filter element, certain advantages could be obtained. In contrast with the

filtenna depicted in Figure 14, the configurations of the NFRP element, the printed monopole unit and the CLL resonator part shown in Figure 17(a)–(c) were all left the same. The results of the corresponding simulation are presented in Figure 18.

Figure 18 exhibits any of the three proposed filtennas that could introduce two adjacent resonance frequencies and thus reveals an expected, notably enhanced operation bandwidth. As is depicted, there is nearly no fluctuation for the values for peak realized gain traced with the +z-axis over the whole operation band. Furthermore, the improved suppression along the edges of the band remains unchanged as well. Table 1 summarizes the performance properties of the various filtenna designs. In addition, Table 1 reveals that it is electrically small (i.e., ka < 1) for all of the new simulated geometries which are composed of the radiating and filtering

elements, and also the fractional bandwidth remains two times broader than the

ka Radiator & filter

Compact, Efficient, and Wideband Near-Field Resonant Parasitic Filtennas

Figure 17(a) 6.07 0.79 4.85–5.83 76–86 Figure 17(b) 6.32 0.89 5.04–6.04 78–88 Figure 17(c) 6.13 0.90 4.92–5.89 77–86 ESA alone 2.4 0.81 5.73–5.94 94–95

Realized gain (dBi)

Radiation efficiency (%)

A wider impedance bandwidth could be obtained by adding more stages to the filter structure. As shown in Figure 19, this filtenna is evolved from the design in Figure 14. It is composed of the NFRP ESA and a two-stage filtering structure. The filter structure consists of two rectangular CLLs etched on the substrate with a gapto-gap orientation. This arrangement produces a known electrical coupling between

The details of the design parameters of the filtenna shown in Figure 19 are listed in Table 2. Referring to the inset figure, the microstrip transmission line is placed on the right side of the upper CLL. It has a 50 Ω characteristic impedance and is connected directly to the center conductor of the coaxial feedline. A straight coupling line, which lies between the two CLLs along the y-axis, is utilized to further tune the coupling levels between the two CLLs. The impedance matching and

The NFRP filtenna with two filter stages. (a) The geometry of the two filter stages and (b) fabricated prototype

electrically small CLL-based NFRP antenna alone.

Summary of the performance characteristics of the proposed one-stage filtennas.

FBW10dB (%)

DOI: http://dx.doi.org/10.5772/intechopen.82305

4.3 Bandwidth enhancement of the filtennas

the two elements.

Figure 19.

267

of the filtenna.

Reported filtennas

Table 1.

#### Figure 17.

Exploring variations of the filtenna design shown in Figure 17. Change in the filter (a) position, (b) orientation, and (c) structure.

#### Figure 18.

The simulated |S11| and peak realized gain values as functions of the source frequency for the three cases shown in Figure 17.


Compact, Efficient, and Wideband Near-Field Resonant Parasitic Filtennas DOI: http://dx.doi.org/10.5772/intechopen.82305

Table 1.

filtenna depicted in Figure 14, the configurations of the NFRP element, the printed monopole unit and the CLL resonator part shown in Figure 17(a)–(c) were all left the same. The results of the corresponding simulation are presented in Figure 18. Figure 18 exhibits any of the three proposed filtennas that could introduce two adjacent resonance frequencies and thus reveals an expected, notably enhanced operation bandwidth. As is depicted, there is nearly no fluctuation for the values for peak realized gain traced with the +z-axis over the whole operation band. Furthermore, the improved suppression along the edges of the band remains unchanged as well. Table 1 summarizes the performance properties of the various filtenna designs. In addition, Table 1 reveals that it is electrically small (i.e., ka < 1) for all of the new simulated geometries which are composed of the radiating and filtering

Exploring variations of the filtenna design shown in Figure 17. Change in the filter (a) position, (b)

The simulated |S11| and peak realized gain values as functions of the source frequency for the three cases shown

Figure 17.

Figure 18.

266

in Figure 17.

orientation, and (c) structure.

Electromagnetic Materials and Devices

Summary of the performance characteristics of the proposed one-stage filtennas.

elements, and also the fractional bandwidth remains two times broader than the electrically small CLL-based NFRP antenna alone.

#### 4.3 Bandwidth enhancement of the filtennas

A wider impedance bandwidth could be obtained by adding more stages to the filter structure. As shown in Figure 19, this filtenna is evolved from the design in Figure 14. It is composed of the NFRP ESA and a two-stage filtering structure. The filter structure consists of two rectangular CLLs etched on the substrate with a gapto-gap orientation. This arrangement produces a known electrical coupling between the two elements.

The details of the design parameters of the filtenna shown in Figure 19 are listed in Table 2. Referring to the inset figure, the microstrip transmission line is placed on the right side of the upper CLL. It has a 50 Ω characteristic impedance and is connected directly to the center conductor of the coaxial feedline. A straight coupling line, which lies between the two CLLs along the y-axis, is utilized to further tune the coupling levels between the two CLLs. The impedance matching and

#### Figure 19.

The NFRP filtenna with two filter stages. (a) The geometry of the two filter stages and (b) fabricated prototype of the filtenna.


Table 2.

Dimensions of the NFRP filtenna with two filter stages shown in Figures 4–6 (all dimensions are in millimeters).

Figure 20.

The simulated and measured |S11| and realized gain values as functions of the source frequency for the two-stage NFRP filtenna shown in Figure 19.

Author details

269

Ming-Chun Tang\*, Yang Wang and Ting Shi

Chongqing University, Chongqing, China

provided the original work is properly cited.

School of Microelectronics and Communication Engineering,

Compact, Efficient, and Wideband Near-Field Resonant Parasitic Filtennas

DOI: http://dx.doi.org/10.5772/intechopen.82305

\*Address all correspondence to: tangmingchun@cqu.edu.cn

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

far-field radiation performance characteristics of this two-stage NFRP filtenna were also studied experimentally. The simulated (measured) results shown in Figure 20 demonstrate that the addition of the second CLL resonator introduces another resonance and produces a 55 (50) MHz impedance bandwidth, from 1.321 (1.29) to 1.376 (1.34) GHz, i.e., a 4.0% (3.8%) fractional bandwidth. The measured operational frequency range exhibits only a slight red shift from the simulated one. A flat realized gain response and excellent band-edge selectivity are again witnessed. The measured and simulated realized gain curves demonstrate that the two-stage NFRP filtenna also exhibits an essentially uniform and stable radiation performance over its entire operational bandwidth.

Compact, Efficient, and Wideband Near-Field Resonant Parasitic Filtennas DOI: http://dx.doi.org/10.5772/intechopen.82305
