1. Introduction

A filtenna is a co-designed antenna which integrates a radiating element and filter to be a single device. Due to its self-contained filtering characteristic, filtenna possesses several main properties compared with other general antennas while receiving a signal. First of all, the interconnection losses could be decreased, which emerge while a common receiving antenna is assembled to a filter in the fabrication process. In addition, it restrains unwanted signals which occur out of the operational band. Finally, from the aspect of practice, it promoted a RF front-end system with more compact and lower cost features. Consequently, more attention has been seized to propose all kinds of filtennas into engineering practice.

In this chapter, three main sorts of filtennas are introduced to demonstrate their design methods and performance characteristics. For the first sort, band-pass or band-stop filtennas focus on introducing band-notch filters into ultra-wideband (UWB)/wideband antennas using a variety of high-Q band-stop structures or embedding band-pass filter structures into various types of basic antennas [1–4]. Two printed planar ultrawideband (UWB) antennas are designed and fabricated. To further improve its high frequency characteristics, a multimode-resonator filter

consisting of a single-wing element is combined with the slot-modified UWB antenna. These filtennas would be depicted in Section 2 in detail. For the second sort, multi-resonator-cascaded filtennas are obtained by altering the coupledresonators in the last stages of the filters to act as the radiating elements [5, 6]. In Section 3, two planar efficient wideband electrically small monopole filtennas are proposed. The first one is directly evolved from a common planar capacitively loaded loop (CLL)-based filter. The second filtenna consists of a driven element augmented with a CLL structure and with slots etched onto its ground plane. Both the filtennas are electrically small. For the third sort, near-field resonant parasitic (NFRP), bandwidth-enhanced filtennas are accomplished through organically combining radiator and filtering structures. In Section 4, a filtenna possessing compact geometry with bandwidth enhancement is developed by a novel design method. It expanded an impedance bandwidth which is over three times improvement compared to its component near-field resonant parasitic (NFRP) monopole antenna alone. Then, a set of compact filtennas with the NFRP element is simulated, fabricated, and analyzed to validate the filtennas' reliability.

## 2. Planar ultrawideband filtennas

The degradation of the radiation pattern at higher frequency of the UWB range reveals a serious drawback for the planar design. For the purpose of decreasing this defect, some design methods have been published, such as adding electromagnetic band gaps (EBGs) [7], varying the radiating patches [8], reconstructing the ground planes [9], and turning to a trident-shaped strip integrated with a tapered impedance transformer connected to the feedline [10].

Alternatively, through assembling an asymmetrical single-wing filter into a feedline section of a modified arc-slot UWB antenna, the broadside gain of the antenna in the upper portion of the UWB band is increased. For example, the simulated broadside gains at 10 GHz are increased from 3.89 to 4.16 dBi for the single-wing antenna. Moreover, integrating a filter element into the antenna strengthens the sharp cutoff performance at both edges of the frequency range for the UWB. Additionally, the developed co-design method makes the size compact for the whole system constituted by the filter and antenna effectively. Eventually, the experiment results in good agreement with simulations that could validate the proposed strategy.

of the UWB band, e.g., a 6 dB increase in the realized gain near 10 GHz. The reason is that the arc-shaped slot produces a parasitic element to resonate at TM10 mode around 10 GHz to remedy the radiation performance. However, as shown in Figure 3, the arc-shaped slot modification could ensure the broadside gain

Comparisons between the reference antenna and the antenna with the arc-shaped slot. (a) |S11| values and

The printed planar UWB antenna with an arc-shaped slot and its reference design. (a) Reference antenna,

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

(b) top view, (c) side view, and (d) the fabricated prototype.

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

improvement in the upper portion of the UWB band. But there is no corresponding improvement in the H-plane, i.e., it does not exhibit an omni-directional radiation

Numerous stub-loaded multimode-resonator-based UWB bandpass filters have been reported in recent years [12, 13]. We found one compact filter design that has several attractive features, including simple designs, compact sizes, low losses, flat group delays, enhanced out-of-band rejection, and easy integration with other

First, based on these advantages, a circular stub-loaded single-wing filter was designed, fabricated, and measured. In detail, its layout and the equivalent circuit network, together with the fabricated prototype and S-parameters, are shown in Figure 4. The filter is composed of a single-wing resonator and a pair of interdigitalcoupled lines. The resonator creates and adjusts several sequential modes within the UWB passband [13]. The interdigital-coupled lines are equivalent to two pairs of single transmission lines attached in their middle to a J-inverter susceptance. The simulated (measured) results demonstrate that the single-wing filter provides 3 dB passband bandwidth from 2.806 (2.824) to 10.892 GHz (10.760 GHz), which covers the entire UWB band. Moreover, the +10 dB return loss bandwidth is from 3.025

pattern.

253

Figure 2.

(b) broadside realized gain.

Figure 1.

2.2 UWB multimode resonant filter

microwave components in the UWB frequency range.

#### 2.1 UWB antenna with an arc-shaped slot

The geometries of a traditional patch UWB monopole antenna as a reference together with its arc-slot modified case are illustrated in Figure 1. The reference antenna is evolved from the reported printed planar UWB monopole antenna designs [11]. Its radiating patch is elliptical in shape, and its ground plane is designed with a rectangular slot at its upper edge for impedance matching. As its modified case, an arc-shaped slot is engineered to be symmetric within the radiating patch and to be close to the throat of the microstrip-feed strip. Both the reference antenna and the antenna with an arc-shaped slot have their comparison on Sparameters, and broadside realized gains are shown in Figure 2. It is shown that the reference antenna has a very wide 10 dB impedance bandwidth from 2.855 up to 14.0 GHz in the simulation. In contrast, the simulated (measured) bandwidth of the antenna with the arc-slot is shown to be from 2.615 (2.775 GHz) up to 14.0 GHz. Moreover, by etching the arc-shaped slot, the antenna with the arc-shaped slot achieves improved broadside realized gains, particularly at the high frequency side

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

Figure 1.

consisting of a single-wing element is combined with the slot-modified UWB antenna. These filtennas would be depicted in Section 2 in detail. For the second sort, multi-resonator-cascaded filtennas are obtained by altering the coupledresonators in the last stages of the filters to act as the radiating elements [5, 6]. In Section 3, two planar efficient wideband electrically small monopole filtennas are proposed. The first one is directly evolved from a common planar capacitively loaded loop (CLL)-based filter. The second filtenna consists of a driven element augmented with a CLL structure and with slots etched onto its ground plane. Both the filtennas are electrically small. For the third sort, near-field resonant parasitic (NFRP), bandwidth-enhanced filtennas are accomplished through organically combining radiator and filtering structures. In Section 4, a filtenna possessing compact geometry with bandwidth enhancement is developed by a novel design method. It expanded an impedance bandwidth which is over three times improvement compared to its component near-field resonant parasitic (NFRP) monopole antenna alone. Then, a set of compact filtennas with the NFRP element is simulated,

The degradation of the radiation pattern at higher frequency of the UWB range reveals a serious drawback for the planar design. For the purpose of decreasing this defect, some design methods have been published, such as adding electromagnetic band gaps (EBGs) [7], varying the radiating patches [8], reconstructing the ground planes [9], and turning to a trident-shaped strip integrated with a tapered imped-

Alternatively, through assembling an asymmetrical single-wing filter into a feedline section of a modified arc-slot UWB antenna, the broadside gain of the antenna in the upper portion of the UWB band is increased. For example, the simulated broadside gains at 10 GHz are increased from 3.89 to 4.16 dBi for the single-wing antenna. Moreover, integrating a filter element into the antenna strengthens the sharp cutoff performance at both edges of the frequency range for the UWB. Additionally, the developed co-design method makes the size compact for the whole system constituted by the filter and antenna effectively. Eventually, the experiment results in good agreement with simulations that could validate the

The geometries of a traditional patch UWB monopole antenna as a reference together with its arc-slot modified case are illustrated in Figure 1. The reference antenna is evolved from the reported printed planar UWB monopole antenna designs [11]. Its radiating patch is elliptical in shape, and its ground plane is designed with a rectangular slot at its upper edge for impedance matching. As its modified case, an arc-shaped slot is engineered to be symmetric within the radiating patch and to be close to the throat of the microstrip-feed strip. Both the reference antenna and the antenna with an arc-shaped slot have their comparison on Sparameters, and broadside realized gains are shown in Figure 2. It is shown that the reference antenna has a very wide 10 dB impedance bandwidth from 2.855 up to 14.0 GHz in the simulation. In contrast, the simulated (measured) bandwidth of the antenna with the arc-slot is shown to be from 2.615 (2.775 GHz) up to 14.0 GHz. Moreover, by etching the arc-shaped slot, the antenna with the arc-shaped slot achieves improved broadside realized gains, particularly at the high frequency side

fabricated, and analyzed to validate the filtennas' reliability.

2. Planar ultrawideband filtennas

Electromagnetic Materials and Devices

proposed strategy.

252

ance transformer connected to the feedline [10].

2.1 UWB antenna with an arc-shaped slot

The printed planar UWB antenna with an arc-shaped slot and its reference design. (a) Reference antenna, (b) top view, (c) side view, and (d) the fabricated prototype.

Figure 2. Comparisons between the reference antenna and the antenna with the arc-shaped slot. (a) |S11| values and (b) broadside realized gain.

of the UWB band, e.g., a 6 dB increase in the realized gain near 10 GHz. The reason is that the arc-shaped slot produces a parasitic element to resonate at TM10 mode around 10 GHz to remedy the radiation performance. However, as shown in Figure 3, the arc-shaped slot modification could ensure the broadside gain improvement in the upper portion of the UWB band. But there is no corresponding improvement in the H-plane, i.e., it does not exhibit an omni-directional radiation pattern.

#### 2.2 UWB multimode resonant filter

Numerous stub-loaded multimode-resonator-based UWB bandpass filters have been reported in recent years [12, 13]. We found one compact filter design that has several attractive features, including simple designs, compact sizes, low losses, flat group delays, enhanced out-of-band rejection, and easy integration with other microwave components in the UWB frequency range.

First, based on these advantages, a circular stub-loaded single-wing filter was designed, fabricated, and measured. In detail, its layout and the equivalent circuit network, together with the fabricated prototype and S-parameters, are shown in Figure 4. The filter is composed of a single-wing resonator and a pair of interdigitalcoupled lines. The resonator creates and adjusts several sequential modes within the UWB passband [13]. The interdigital-coupled lines are equivalent to two pairs of single transmission lines attached in their middle to a J-inverter susceptance. The simulated (measured) results demonstrate that the single-wing filter provides 3 dB passband bandwidth from 2.806 (2.824) to 10.892 GHz (10.760 GHz), which covers the entire UWB band. Moreover, the +10 dB return loss bandwidth is from 3.025

(10.817 GHz). Clearly, the measured lower frequency bound is downshifted by

The far-field realized gain patterns are presented in Figures 6 and 7. By comparing the results in Figure 7, it is clear that the integration of the single-wing filter further increases the broadside gain values in the higher frequency range, while maintaining its original radiation patterns in the lower frequency range. The broadside realized gain values of the single-wing version increase to 4.16 dBi in simulation and to 4.25 dBi in experiment. It must be noted that the single-wing filter antenna has very good omnidirectional radiation performance in the H-plane and

The circular stub-based single-wing multimode-resonator filter. (a) Design layout of the filter and its equivalent

Two electrically small, efficient planar monopole filtennas based on capacitively loaded loop (CLL) resonators are presented. Taking advantage of the characteristics of filters that are based on a pair of electrically coupled CLL resonators, the filtenna is designed, fabricated, and measured. The experimental results demonstrate that this electrically small system had a 6.27% fractional impedance bandwidth, high out-of-band rejection, and stable omnidirectional radiation patterns. An additional

46 MHz, and its upper frequency edge is downshifted by 230 MHz.

circuit network, (b) fabricated prototype, and (c) its simulated and measured |S11|.

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

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

exhibits some improvements in the cross-polarization values.

Figure 4.

255

3. Compact, planar, and wideband monopole filtennas

Figure 3. Realized gain of the arc-slot modified UWB antenna at (a) 3.0, (b) 6.5, and (c) 10 GHz.

(2.989) to 11.010 GHz (10.842 GHz). Two transmission zeros are generated at 2.12 GHz (2.085 GHz) and at 11.5 GHz (11.449 GHz).

#### 2.3 Integration of a UWB filter into an antenna with an arc-shaped slot

The single-wing filter was integrated into the arc-slot antenna as shown in Figure 5. The filter was connected directly to the microstrip feedline section. As shown in Figure 5, the UWB filter-antenna design was optimized, fabricated, and measured. As depicted, the simulated (measured) 10 dB impedance bandwidth of the antenna with the single-wing filter is from 2.995 (2.949) to 11.047 GHz

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

#### Figure 4.

The circular stub-based single-wing multimode-resonator filter. (a) Design layout of the filter and its equivalent circuit network, (b) fabricated prototype, and (c) its simulated and measured |S11|.

(10.817 GHz). Clearly, the measured lower frequency bound is downshifted by 46 MHz, and its upper frequency edge is downshifted by 230 MHz.

The far-field realized gain patterns are presented in Figures 6 and 7. By comparing the results in Figure 7, it is clear that the integration of the single-wing filter further increases the broadside gain values in the higher frequency range, while maintaining its original radiation patterns in the lower frequency range. The broadside realized gain values of the single-wing version increase to 4.16 dBi in simulation and to 4.25 dBi in experiment. It must be noted that the single-wing filter antenna has very good omnidirectional radiation performance in the H-plane and exhibits some improvements in the cross-polarization values.
