Dual Band Filter-Antenna with Fixed Low Band and Frequency- Agile High Band for Wireless Communications

This work proposes a design of compact filter-antenna with fixed and frequency-agile bands for wireless communications and cognitive radio (CR) systems. The filter-antenna is realized by attaching a bandpass filter with an independent fixed band and a tunable band to a UWB antenna. The dual bandpass filter consists of a folded open loop resonator loaded with single varactor diode at a suitable location to achieve reconfigurability operations for the high band while the low band remains fixed at 4.5 GHz. A reconfigurable broadband frequency range of 3.2 GHz from 7.8 to 4.6 GHz is achieved for the high band by changing the capacitance of the varactor from 0.16 to 2.25 pF, respectively. A prototype of the filter-antenna is fabricated and measured at a selected capacitance of 0.6 pF to verify the simulation results. Good agreements between simulated and measured results are obtained.


Introduction
Recent advancements in wireless communication systems and the growing request for frequency bands have caused congestions in the available electromagnetic (EM) radio frequency (RF) spectrum. Therefore, accessibility of various unlicensed bands in the crowded RF spectrum and compatibility of wireless devices with different standards represent the main objectives of current communication systems to operate with dual frequency bands [1], [2].
In cognitive radio (CR) and ultrawideband (UWB) communication systems [3], one of the keys issues is designing a compact antenna while providing wideband characteristics over the entire operating band and should have a wide reconfigurable band for CR applications [4]. In recent days, the RF front ends and antennas should be designed to operate at multiple frequency bands with frequency-agile characteristics to meet the growing demands and challenges in modern wireless systems and CR applications [4], [5], [6]. Electronic reconfigurability features are one of the most promising techniques to achieve the frequency-agile characteristics of the antennas [7] and filters [8] and represent a potential solution for future reconfigurable wireless communication systems. Therefore, there is an essential demand for designing reconfigurable antennas that can support the communication system operating with other devices through multi-channels. Also, achieving wide tuning ranges with steady radiation patterns are challenging features because of the matching problems and the switching devices limitations. In these systems, printed slot antennas have gained significant consideration due to their wideband matching characteristics and omnidirectional radiation patterns [9], [10]. However, they suffer from matching problems when converted to single tunable band operations with wide tunability range. The rapid development in designing antennas has produced many different techniques for realizing multi-band operations, compact, low priced, flexible, broadband frequency-Agile, and multiple bands antennas [11], [12].
Lately, reconfigurability for multi-band antennas has gained noteworthy devotion. Several applied approaches for realizing dual band antennas employ specific methodologies to alter one of the resonant modes of the structure to vary its operating frequency. For example, in [13], a 50 × 50 mm 2 dual reconfigurable antenna is achieved using a mechanical process for a movable superstrate. However, the system suffers from the low switching speed of the mechanical part. In [14], a multi-band antenna is proposed by using a fundamental patch and four sub-patches to produce five resonant frequencies. The sub-patches are loaded at the inputs with varactors for reconfiguration operations. The antenna has a size of 50 × 50 mm 2 . In [8], a dual band bandpass filter with fixed low-band and reconfigurable high-band resonant frequencies is suggested for reconfigurable applications. The suggested filter contains double split-ring resonators (SRRs) with stub and microfluidic channels. The tunability of the high resonant frequency is achieved by controlling the stub length by injecting liquid metal in the microfluidic channel. The filter has a size of 80 × 30 mm 2 . However, the injection process needs cautious treatments. In [15], a dual band filtering antenna using substrate integrated waveguide (SIW) cavity as a resonator with good isolation between the two bands. Besides, the filtering antenna has a size of 90 × 90 mm 2 . In [16], an improved structure of reconfigurable integrated SIW and patch antenna using the varactor diodes is proposed to achieve a tuning range of around 0.2 GHz with overall dimensions of 98.6 mm × 92.6 mm.
In this paper, a dual band filter-antenna with fixed and frequency-agile bands for wireless communications and CR applications is presented and discussed. Section 2 presents the configuration of the filter-antenna. Section 3 discusses the experimental measurements and observations. In conclusion, a summary of the work is summed up.

Filter-Antenna Design
This part presents and discusses the design of the filterantenna. Figure 1 displays the 2D layout of the suggested structure. Obviously, the antenna is composed of tunable bandpass filter connected and matched to the radiating patch. The bandpass filter is composed of a folded arms open loop resonator (FAOLR). The FAOLR is loaded with a varactor diode for reconfiguration operation of the tunable band as we did in [17] as a summary of the simulation results of this study. The filter-antenna has dimensions of 40 mm × 40 mm. The strip W2 × L2 has dimensions of 12 mm × 2 mm which is responsible for the impedance matching. The patch dimensions are W = L = 18 mm. The transmission line dimensions are W1 = 1.9 mm, Wt = 0.8 mm, gt = 0.7 mm, and L1 = 19 mm. The resonator dimensions are Xr = 7.5 mm, Yr = 4.5 mm, Cr = 0.4 mm, and Lr = 0.5 mm. The reduced ground plane has a length of Lg = 19 mm while the slot defected ground structure (DGS) has dimensions of 2.8 mm × 0.6 mm with 1.5 mm separation distance between the two etched slots. The etched slots are corresponding to a resonant circuit and considered as a parallel LC circuit. At any frequency lower than the resonant frequency of the slots, the parallel circuit acts as an inductor [18], [19], [20]. Therefore, they improve the coupling and the matching between the feed line and the FAOLR resonator. Furthermore, the dimensions of the DGS does not have any significant effect on the desired dual frequencies of the capacitively loaded FAOLR. A varactor is attached between the FAOLR and the ground to accomplish miniaturization and reconfiguration. The varactor diode is realized with its equivalent circuit model [17], [18]. The reconfiguration operation of the proposed design is achieved through changing the capacitance of the varactor (C) from 2.67 to 0.63 pF which will tune the second resonance of the FOLAR. The antenna is designed to operate at a continuous tuning range of 3.18 GHz and a fixed band of 4.3 GHz, simultaneously. The antenna is designed using the CST microwave studio software using RO3003 substrate with = 3 and ℎ = 0.762 . The simulated |S11| of the proposed filter-antenna accomplishes a continuous reconfiguration band of 3.18 GHz from 4.6 to 7.8 GHz (|S11| ≤ -10 dB) for a capacitance of 2.25 to 0.16 pF with 52 % frequency tuning range. The desired wideband matching characteristic is achieved through the two rectangular etched slots in the ground plane as explained in [18]. It should be noticed that the reverse voltage increases, the filter-antenna resonates at high frequencies as indicated in Fig. 1(c). For studying the influence of the gap gt on performance of the proposed design, parametric study for different values of the distance gt has been done as shown in Fig. 1(d). It is noticed that the distance gt has a slight effect on the |S11| parameters of the proposed filter-antenna as shown in Fig. 1(d). The filterantenna is simulated with and without the SMD capacitor C = 0.6 pF as it will be tested at this value at the measurement as shown in Fig. 2. Noticeably, the attached capacitor miniaturizes the second resonant frequency from 9.5 GHz to 6 GHz which achieves the compactness of the design.

Experimental Measurements and Discussions
The suggested design is fabricated and tested at one capacitance for confirmation. The measurements are conducted using N5222A PNA microwave network analyzer with a maximum frequency range of 26.5 GHz. The pictures of the fabricated sample are shown in Fig. 3. A prototype of the filter-antenna is fabricated and measured at one selected capacitance of 0.6 pF to confirm the simulation results. Moreover, the filter-antenna is tested and measured without the capacitor. All measured results are compared with the simulated results as shown in Fig. 3. Good agreement between the measured and the simulated |S11| of the proposed filter-antenna is observed as shown in Fig. 3. As a result, these outcomes confirm that the antenna is appropriate for fixed and frequency-agile bands communication applications. Moreover, the simulation and measurement data of the realized gains are plotted and they are in acceptable agreements as shown in Fig. 4. Figure 4 indicates that the antenna has two peaks at the two operating frequencies of 4.3 and 6 GHz. This confirms the proper operation of the proposed filtering antenna at both working bands. As well, it has low gains at the non-operating bands. For more confirmation, the normalized radiation patterns in the E and H planes at the selected capacitance (C = 0.6 pF) for the operating frequencies (3.4 and 6 GHz) are plotted in Fig. 5. Furthermore, the measured patterns are compared to the simulated ones. The filter-antenna exhibits stable omnidirectional radiation patterns at the two frequencies.
However, some minor deviations are observed in the E plane at 6 GHz which is due to the soldering effects of the capacitance and the coaxial cable.

Conclusions
In this paper, the experimental work of a low-profile filterantenna with a fixed band and a reconfigurable frequency band for wireless communications and CR applications is presented and discussed. The filter-antenna is realized by attaching a tunable bandpass filter to the feed line of a UWB antenna. A wide reconfigurable band of 3.18 GHz from 4.6 to 7.8 GHz is effectively achieved by changing the capacitance of the varactor diode. Furthermore, the filter antenna is tested at 0.6 pF for validation. The radiation patterns are steady at the dual band of operation at 4.3 and 6 GHz. The agreement between the simulated and measured data confirms the operation of the proposed design.