Frequency Reconfigurable UWB Antenna Design for Wireless Applications

A frequency band reconfigurable antenna is designed and experimentally analyzed. Proposed design achieve UWB (3.1–10.6 GHz), two dual band and two narrow band switching states by implementation of PIN diodes inside the slotted ground. Antenna covers the various wireless standards WLAN, WiMAX, WiFi and UWB with return loss S11 < −10 dB. Proposed antenna is also operating at 9.2 GHz to include the airborne radar applications. Simulated and experimental results are compared and found to be in good agreement.


Introduction
Present scenario of wireless communication system required compact and multiple band antenna design. Since many systems are operating at multiple frequency range, requiring dual and triple band antenna for various applications such as WLAN, WiMAX, RFID, satellite communication, etc. Presently, many printed monopole antenna are proposed. Serve for wireless applications to cover the wireless standards for Wireless local area network (WLAN: 2.4-2.48, 5. 15-5.35, and 5.75-5.825 GHz) and worldwide interoperability for microwave access (WiMAX: 3.4-3.69 GHz) are two among the available wireless standards which allow interconnections of devices for communication. To achieve multi-functionality, various parameters of antenna such as polarization characteristics, resonant frequency, patterns and impedance bandwidth etc., are reconfigured as per requirements [1].
Presently wireless communication systems are adopting the concept of cognitive radio system where using a sensing antenna performing the monitoring of the spectrum, and can be reconfigured to operate over a desired frequency band. This system is required a frequency band reconfigurable antenna as a sensing element [2]. Most of frequency band reconfigurable antennas providing the band switching between narrowband modes [3][4][5].
Antenna obtained the quad-band switching by implementation of microelectromechanical systems (MEMS) switch [3]. Another frequency band reconfigurable patch antenna is proposed that operate in four different modes with the help of switching elements [4]. Recently, many microstip patch antenna have been designed that indicate the switching facility in narrowband as well as wideband modes [6][7][8][9][10][11]. In [6], a Vivaldi antenna is designed that provide the wideband and narrowband mode switching facility. A frequency band reconfigurable antenna with four photoconductive switches is proposed that operating with switching between the three narrowband modes and UWB mode [7]. In [9], antenna has been proposed with narrowband and wideband functionality with reconfigurability characteristics is achieved with the implementation of p-i-n and varactor diodes. Tunable EBG structure are analyzed with active switching devices FET and obtained the transmission characteristics of the structure [12,13]. Many techniques such as defective ground [14], etching slots [15,16], metamaterial loading [17][18][19][20][21][22][23], dielectric resonator [24], fractal geometry [25,26], etc., are applied to accomplish multiband reconfigurable operation to cover various wireless applications.
In this chapter, firstly design the octagonal shape patch antenna and implementing the inverted L shaped switchable slotted ground yielded switchable resonant modes such as, two narrowband modes (5.05-5.89 and 8.76-9.80 GHz), two dual band modes (2.21-2.52 GHz and 5.07-5.89 GHz and 2.18-2.52 GHz and 8.78-9.71 GHz) and UWB mode (2.87-16.56 GHz) for wireless applications. As per requirement to design antenna to frequency band reconfigurability introducing the five switching elements p-i-n diodes placed inside the slotted ground. The proposed design is compact in size as compared to antennas are discussed in published literature [6][7][8][9][10][11]. The simulation work of antenna is done by using CST Microwave Studio (CST MWS) software [27] and measurement is performed with the help of VNA (vector network analyzer-E5071C (300 KHz-20 GHz) ENA series Agilent Technologies). The fabrication of proposed structure is executed by using of PCB prototype machine (Caddo-71).
Following sections focused on the antenna designing with parametric study and switchable modes analysis with results in simulated as well as measurement modes.

Ultra wideband (UWB) antenna design
In this section initially octagonal shape monopole antenna size of 40 × 40 × 0.40 mm 3 is designed, as represented in configuration "a" of Figure 1. The proposed design is constructed on 0.40 mm thick Roger RT 5880 substrate with the relative permittivity 2.2, fed with 50 ohm microstrip feed line. Octagonal shape radiating element has the dimensions ab = ef = 10 mm, bc = ha = 5.14 mm, cd = gh = 6.0 mm and de = fg = 5.14 mm attached with feedline of dimensions 22 × 1.25 × 0.01 mm 3 . On back side of antenna, ground plane  exist with length of 21.1 mm, width of 40 mm and thickness of 0.01 mm. Figure 1 shows the configuration of the proposed antenna with a top view and bottom view.
The operational performance of the antenna is analyzed with variations in its parameters such as ground slot dimensions (length l 1 and width W 1 ) are known as the parametric study of the proposed design. This study is carried out by variations in slot length l 1 and width W 1 while keeping other parameters constant. Slot length l 1 and width W 1 is varied from 0 to 7 mm and 0.6 to 1.4 mm respectively, as depicted in Figure 2. It is observed that, as variation are done in the value of l 1 and W 1 , the reflection coefficient (S 11 ) is changes and the respective frequency band is also changes accordingly. The optimized impedance matching for UWB band characteristics is achieved at length l 1 = 6.1 mm and width W 1 = 1 mm.  From Figure 2, it is indicated that at lower frequencies (2-4 GHz) that the impedance matching is improved when the slot dimensions are reduced (either by reducing l 1 or W 1 ). At higher frequencies (above 5 GHz), the impedance matching is enhanced when the slot dimensions are increased. The input reflection coefficient S 11 (below −10 dB) of UWB antenna is achieved at the optimized value of l 1 = 6.1 mm and W 1 = 1 mm. The impedance bandwidth of 141% (2.87-16.56 GHz) under simulation and 140% (2.85-15.85 GHz) in measurement is obtained as shown in Figure 3.

Narrowband and dual band antenna design
The UWB monopole antenna can be reconfigured to others frequency bands by using an inverted L and rectangular shaped slotted structure placed on the ground plane, as shown in Figure 4. This inverted L-shaped slot in ground plane generating an additional current path due to the perturbation of the current flow in antenna structure that leads to the filter characteristics, responsible to suppress the frequencies outside the desired frequency band. These ground slots are generating the stop bands in the UWB frequency range [9]. Figure 5 represents the different filter structures of the proposed design by variation in the inverted L-shaped slot. Figure 6 represents the input reflection coefficient S 11 (below −10 dB) of the antenna for different filter structures. Structure I and II are creating the dual bands whereas structure III and IV are responsible for obtaining the single bands only.
Moreover, the bandwidth of each filtering structure in Figure 6, is controllable with changing the length l 2 and width W 2 of parallel vertical arms (in Figure 4). By the variations of these arms dimensions impedance bandwidth changes accordingly, as shown in Figure 7. While increasing the slot length l 2 and width W 2 ,  the bandwidth of antenna decreases from 30 to 12% in structure III. The desired resonant band with the input reflection coefficient S 11 (below −10 dB) is achieved at the optimized value l 2 = 2.5 mm and W 2 = 1 mm respectively.

Frequency reconfigurable antenna design
In this section analyzed the reconfiguration of UWB mode of proposed design to another narrowband and dual band modes. This reconfiguration is done by implementation of filter structures on the ground plane by placing of five switching elements p-i-n diodes inside it, as indicated in Figure 8. The switches D 1 -D 5 are positioned in such a way to obtain the required structures I-IV for desired frequency bands. If diode D 1 is on and remaining are off, we will get a filter structure like structure-I for dual band.
For biasing of p-i-n diodes, apply the dc voltage across the p-i-n diodes with the help of metal strips dimension of 2 × 0.6 mm 2 , as indicated in Figure 8. As shown in Figure 8, blocking capacitor of 100 pF is also connected with diode, to provide the isolation between the dc and the RF signal. A beam lead p-i-n diodes (ALPHA-6355) are placed inside the ground slot, where 0.7 V dc is required for biasing of diode. During ON state (forward bias) of diode, it exhibit resistance of 2.6 ohm while in case of OFF state (reverse bias) it represents 0.081 pF [28]. Figure 9 shows the electrical equivalent circuit of the diode for both states (ON/ OFF state). For On state, it represents a series combination of fixed inductor (L s ) and a current-controlled resistor (R s ), whereas for OFF state, it indicates a shunt combination of intrinsic-layer capacitance (C p ) and the resistance (R s ) in series with fixed inductance (L s ). The intrinsic-layer capacitance (C p ) is a combination of the stray capacitance C s and the junction's capacitance C j .
As per Table 1, narrow bands, dual bands and UWB band are obtained by changing the states of diodes and compare the frequency bands and 10-dB bandwidth in simulation and measurement mode. The proposed antenna is initially simulated with the help of simulation software CST Microwave Studio (MWS) [27] and thereafter, fabricated on FR4 substrate with optimized values. Figure 10 shows the fabricated prototype of the proposed antenna.
A setup is used for frequency band reconfigurable structure to observe the measured reflection coefficient (S 11 ) with the help of vector network analyzer (VNA) and radiation characteristics by using anechoic chamber, shown in Figure 11.  The simulated and measured reflection coefficients S 11 for all five states are shown in Figure 12. Comparison of simulated S 11 with measured ones is indicated as a good agreement between them. From Table 1, for narrowband states I achieve the bandwidth of 16% (5.05-5.89 GHz) and 14% (5.01-5.79 GHz) in simulation and measurement mode respectively. For state II (narrowband), obtained bandwidth of 11% (8.76-9.80 GHz) and 10% (8.68-9.69 GHz) in simulation and measurement        Table 1).

Figure 13.
Surface current distribution of the proposed antenna for different frequencies.
and 5.15-5.85 GHz and 3.1-10.6 GHz respectively. Proposed design also covers the airborne radar applications works at 9.2 GHz. From Figure 12(e), the resonance is identified at the frequency of 3.0, 5.0, 7.0 and 9.6 GHz. As per the observation of Figure 13, it is found that the first resonance is controlled by the inverted L shaped slot dimensions since the maximum surface current is present across it. Second resonance 5.0 GHz is obtained due to the octagonal shape of radiating element and the feedline attached to the patch. Third resonance at 7.0 GHz is obtained due to the rectangular slot created on the ground plane. The two rectangular slits dimensions of l 2 and W 2 are responsible to generate tank circuit causes the fourth resonance at 9.5 GHz. The surface current distribution is observed at theses resonance frequencies as shown in Figure 13.
From Figure 14, the 3D-gain of the antenna is observed at different resonant frequencies, where the maximum radiation is identify at the various values of angles (theta and phi). It is noticed that at higher frequency, the directivity is improved so that the gain is increased. Figure 15 represents the measured antenna gain in single band, dual band and UWB modes for various switching states. It is analyzed that at lower frequency range gain is reduced whereas at higher frequencies (above 6 GHz) gain is improved as compared to reference gain level of 4 dB. It is also observed that antenna exhibit the acceptable gain in narrowband and dual-band modes. Figure 15(b) shows the average gain of 3.9 dB is achieved for UWB mode of the proposed antenna. Figure 16(a) indicates the variation of the simulated radiation efficiency from 96.9 to 79.5% and from 97.2 to 70.3% for switching states I and II respectively, for the proposed antenna. The variation of the simulated radiation efficiency from 95.5 to 73.3% and from 94.0 to 78.1% is observed for state III and IV respectively. Figure 16(b) shows the simulated radiation efficiency variation from 98.9 to 85.8% for UWB mode. It is noticed that the radiation efficiency is stay above the 70% in all the narrow band, dual band and UWB band. Another observation is that at higher frequency range the simulated radiation efficiency is decreases.  The axial ratio (AR) measurement of the proposed antenna is done inside an anechoic chamber by using antenna measurement system with VNA. To obtain maximum ARBW (axial ratio bandwidth), the antenna measurement system is aligned along the directions of θ = 78° and Ф = −89°, where AR is stay below the 3 dB reference level. The simulated ARBW of 38% is achieved for frequency range from 4.65 to 6.85 GHz as shown in Figure 17. The measured ARBW is slightly less than the simulated one at the center frequency 5.65 GHz.   Patterns are analyzed at operating frequencies 2.4, 5.4, 7.5, and 10 GHz for E and H plane (principal plane). From Figure 18, there is dumb bell shape and quasiomnidirectional like radiation patterns in H-Plane and E-Plane respectively, which represents that the proposed design is a good candidate for wireless communication. There is a good agreement seen between the measured and simulated radiation patterns for E and H plane with the slight difference caused due to assembly  misalignments. A consistent omnidirectional radiation is observed in the E plane and a nearly bi-directional pattern is observed along the H plane for all the operating frequencies.
Both the measured and simulated E and H plane radiation patterns are appear reasonably stable with respect to resonant frequency. It is also observed that there is pinch-off along the end fire directions (θ = ±90°) at lower frequencies for H-plane pattern. The E-plane pattern shows the unidirectional nature at higher frequencies because at these frequencies the back lobes (along θ = 180°) are considerably decreases. Table 2 shows the comparison of the proposed antenna characteristics, like as antenna size, impedance bandwidth, gain, radiation efficiency and operating modes, with reported multiband antennas for wireless standard. Proposed design also covers the airborne radar applications works at 9.2 GHz. The radiating element of octagonal shape and switchable slotted ground is implemented to achieve the frequency band reconfigurability between wireless standards. The switching between the narrowband, dual band and UWB modes is obtain by using five p-i-n diodes placed inside the inverted L shaped ground slot. The proposed design is provides the facility of easily integration with cognitive radio and multi radio wireless terminal devices. Proposed design achieve the bandwidth of 16% (5.05-5.89 GHz) and 14% (5.01-5.79 GHz) in simulation and measurement mode respectively for narrowband states I. Next it obtained bandwidth of 11% (8.76-9.80 GHz) and 10% (8.68-9.69 GHz) in simulation and measurement mode respectively for narrowband states II. Antenna resonant in dual band mode and achieve impedance bandwidth of 13% (2.21-2.52 GHz) and 15% (5.07-5.89 GHz) under simulation and 12% (2.20-2.50 GHz) and 15% (5.05-5.90 GHz) during measurement for next state III. For next state IV, antenna identifies the operating bandwidth of 14% (2.18-2.52 GHz) and 10% (8.78-9.71 GHz) and 13% (2.19-2.50 GHz) and 9% (8.70-9.60 GHz) during simulation and measurement mode respectively. For UWB mode of V state, antenna indicate the operating bandwidth of 141% (2.87-16.87 GHz) and 140% (2.97-16.80 GHz) under simulation and measurement mode respectively. The average gain of 3.9 dB is achieved for UWB mode of the proposed antenna. The radiation efficiency is stay above the 70% in all the narrow band, dual band and UWB band. Radiation characteristics of the proposed antenna are achieved with good impedance matching at these resonant frequencies. The radiation pattern, gain and efficiency are consistent over all the operating bands making the proposed antenna a good choice for wireless applications.

Conflict of interest
The author(s) declare(s) that there is no conflict of interest regarding the publication of this paper.
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