Capacitively Loaded Loop-Based Antennas with Reconfigurable Radiation Patterns
A class of metamaterial-inspired antennas having reconfigurable radiation patterns is proposed. They consist of a driven monopole antenna with one- and two-capacitively loaded loop (CLL), near field resonant parasitic elements. Two configurations are studied by considering the state of these CLL elements as being either open or closed configurations. Simulation results explain the design features and demonstrate that the structure can change its beam direction simply by controlling the switched states. Two prototypes with one- and two-CLL elements were fabricated and tested. The measured impedance mismatch and radiation pattern results are presented and compared to the corresponding simulated values.
The interest in and demand for wireless and mobile platforms have grown dramatically in the past decade. Applications include, for example, communications, sensors, RFID tags, and health monitoring systems. The need to increase the functionality of the corresponding antenna systems within the same or smaller footprints in these systems has led to a host of investigations into nontraditional solutions.
One approach is the use of metamaterial-inspired antennas [1, 2]. Metamaterials are artificial materials that can be designed to exhibit unusual electromagnetic properties, such as simultaneous negative permittivity and permeability . These unusual properties have been shown to provide unconventional paths to novel and improved antennas with interesting and desirable performance characteristics [2, 4]. Different classes of metamaterial-inspired electrically small antennas, which are efficient and matched intrinsically to the source, have been designed to have multifunctional properties, for example, multiband and circular polarization, using near field resonant parasitic (NFRP) elements . A variety of these designs have included the introduction of capacitively loaded loop (CLL) and split ring resonator (SRR) elements as the NFRP elements [1, 2, 4–10], as artificial magnetic conductors (AMCs) to achieve hemispherical coverage , and as local resonators to achieve notched filters in UWB antennas [12–14].
A second approach is to introduce reconfigurable antennas [15, 16]. A variety of components have been investigated to achieve reconfigurability. These include mechanically movable parts ; electronic switches including semiconductor devices , such as MEMS  and varactors ; and optically activated switches . Numerous antenna performance characteristics have become tunable with reconfigurable elements.
In this paper, we introduce a class of metamaterial-inspired antennas having reconfigurable radiation patterns. It is achieved by combining two previous directive configurations. In the work , a family of efficient metamaterial-inspired antennas was studied that was composed of a monopole antenna with a SRR NFRP element. Four configurations of the antenna were studied by changing the position of the gap in the SRR loop. It was shown that, among the four studied configurations, only two exhibit high directivities. In the work , it was demonstrated that the location of the gap of the CLL NFRP element could be used to change the main beam directions of a similar electrically small metamaterial-inspired antenna. The structures presented here consist of a monopole antenna combined with one and two CLL-based NFRP elements that include two open-closed configurations, realized with switches, to achieve the reconfigurable patterns. It is demonstrated with simulation results that both of these structures change their main beam directions depending on the states of those switches. Prototypes of both antennas were fabricated and tested. The measured results confirm the predicted behaviors.
In comparison to a previous report of the same concept , the CLL elements employed here are much simpler than the SRR elements. As a result, the antennas reported here have less flexibility since they deal only with two switches rather than four. On the other hand, the CLL elements achieve much higher radiation efficiencies. In addition to simplifying the associated switching network, the emphasis here is simply to switch the directivity from one direction to the opposite one. Moreover, because they are assumed to have a (finite) ground plane, their intended applications are different. The antennas reported here, for instance, could be integrated into a variety of vehicles and other transportation systems. As a consequence of their geometry, the monopoles of our antenna systems are directly fed from a SMA connector rather than using an intermediate CPW feed. Furthermore, experimental results are provided to confirm the design concepts.
2. Antenna with One CLL Element
The single-CLL-based antenna was designed and simulated using the ANSYS-HFSS software. It was fabricated with a Rogers Duroid 5880 substrate with a thickness of 0.8 mm and relative permittivity , relative permeability , and 0.0009 loss tangent. The antenna was excited in the simulation models with a lumped port having a 50 Ω input impedance.
2.1. Antenna Design
The single-CLL NFRP antenna consists of a driven monopole of length 25.5 mm and width of 1.5 mm printed on the Rogers substrate (see Figure 1). The CLL element has a 25.5 mm length and 17 mm width. It includes two open gaps having mm2 dimensions as shown in Figure 1(a). The dimensions of the ground plane are mm2.
The antenna system was designed to operate at 1.55 GHz. Two configurations were considered that depend on the state of switches S1 and S2 in Figure 1(a). Details of the design parameters are given in Figure 1(b). The states ON and OFF of each switch correspond to a closed CLL (short circuit for the CLL-gap) and an open CLL (open circuit for the CLL-gap), respectively.
2.2. Simulation Results
The simulated impedance mismatch, that is, the values, of the single NFRP element reconfigurable antenna is presented in Figure 2. As shown, when switch S1 is ON and S2 is OFF (State 1), the structure operates at GHz. On the other hand, when the switch S2 is ON and S1 is OFF (State 2), the antenna resonance is at GHz. The −10 dB fractional bandwidth is 2% for State 1 and 0.6% for State 2.
Figure 3 illustrates the simulated surface current vectors of the two configurations at their resonance frequencies. We note from this figure that the current distributions depend on which switch is in its open position. In fact, this open switch location in the CLL element affects both the orientation and magnitude of the currents. These current distributions help explain the associated radiation pattern behaviors.
The corresponding simulated 3D radiation patterns of the antenna are given in Figure 4. These patterns correspond to the resonant frequencies 1.55 GHz and 1.72 GHz, respectively. One observes in Figure 4(a) that the antenna radiates more towards the negative -direction, opposite to the CLL element, at the lower resonance frequency. In contrast, Figure 4(b) shows that the structure radiates primarily towards the positive -direction at the higher resonance frequency. These two pattern changes are correlated to the major changes in the currents along the vertical strips (the driven monopole and the sides of the CLL element parallel to the -axis) shown in Figure 3. The simulated maximum radiation efficiency for State 1 and State 2 is, respectively, 80% and 60%, whereas the maximum realized gain is 1 dB and −1.5 dB, respectively.
Figure 5 gives the simulated 2D radiation patterns at both resonance frequencies in the () plane. When switch S1 is ON (S2 is OFF), the direction of the beam at the lower resonance frequency is toward the negative -direction. On the other hand, when the switch S2 is ON (S1 is OFF), the beam at the higher resonance frequency changes its direction toward the positive -direction.
2.3. Experimental Validation
A prototype of the single NFRP element antenna with the two open gaps was fabricated and characterized. Figure 6 presents a photo of the realized prototype. Both impedance mismatch and radiation patterns were measured.
The impedance mismatch, that is, the values, as a function of the source frequency was measured using a network analyzer Agilent N5230A over the frequency range 1.5–2.5 GHz. Figure 7 presents the measured values. According to Figure 8, when switch S1 is ON (S2 is OFF), the structure operates at 1.59 GHz, whereas when the switch S2 is ON (S1 is OFF), the resonance is at 1.76 GHz. The measured results are in very good agreement with the simulation values. For example, the simulated and measured fractional bandwidths corresponding to State 1 are, respectively, 2.0% and 1.9%. The measured value fell above −10 dB. The corresponding −3 dB are 2.3% and 1.7% for State 2.
Figure 8 presents the measured radiation patterns of the prototype antenna in the () plane at GHz and GHz. These measurements were carried out in a “SATIMO Stargate32” anechoic chamber. As shown in Figure 8, when switch S1 is ON (S2 is OFF), the direction of the beam, as predicted, is towards the negative -direction at 1.59 GHz. On the other hand, when the switch S2 is ON (S1 is OFF), the beam changes its direction, as predicted, towards the positive -direction at 1.76 GHz.
The corresponding measured 3D radiation patterns of the prototype single NFRP element antenna are shown in Figure 9. Again, the agreement with the predicted patterns given in Figure 4 is good. The simulated maximum values of the radiation efficiency are 80% and 60% for States 1 and 2, respectively.
3. Antenna with Two CLL Elements
The two-CLL-based NFRP antenna was also designed and simulated using the ANSYS-HFSS software. It was fabricated and measured with the same experimental conditions as the one-CLL-based NFRP antenna presented in Section 2. Some design parameters were changed compared to the previous structure with 1 CLL element in order to obtain lower operating frequencies.
3.1. Antenna Design
The antenna consists of a monopole of length 29 mm and width 2 mm coupled with two identical CLL elements of length 29 mm and the width 21 mm. Each CLL element is loaded with a gap of dimensions mm2. The other design parameters remained. One switch is located in each CLL element (Figure 10(a)) in order to ensure the antenna reconfigurability. Details of the design of this reconfigurable structure are given in Figure 10.
As shown in Figure 11, the antenna with either S1 or S2 open operates at the same resonance frequency GHz. A second resonance frequency occurs outside the band of interest. This primary frequency corresponds to the resonance of only one capacitor-loaded NFRP element. We note that the two sates operate at the same frequency because the structure is perfectly symmetric, which explains the coincidence of the curves corresponding to both States 1 and 2, respectively. The −10 dB fractional bandwidth for both States 1 and 2 is 2.6%.
The simulated surface current vectors of the two configurations at the same resonance frequency GHz are illustrated in Figure 12. We note from this figure that the current distributions depend on which switch is activated, that is, which CLL element is resonant. In fact, when the switch is ON, the CLL element corresponds to a closed loop and does not resonate, whereas when it is open, it resonates. We also notice that the current distributions, as would be expected, are symmetric. This behavior confirms the previous discussion of the coincident results at the same frequency.
Figure 13 illustrates the 2D simulated radiation patterns for the proposed structure. When switch S1 is ON (S2 is OFF), the direction of the beam is toward the negative -direction, whereas when the switch S2 is ON (S1 is OFF), the beam changes its direction toward the positive -direction. A major advantage of this structure, in contrast to the single-CLL NFRP element antenna, is the fact that this reconfigurability of the pattern occurs at the same frequency. The maximum values of the simulated radiation efficiency and realized gain, corresponding to both States 1 and 2, are approximately 80% and 0 dB, respectively.
3.2. Experimental Validation
A prototype of the two-CLL NFRP element antenna with each CLL element having one open gap was fabricated. The antenna was then characterized with only one of the gaps switched on. Figure 14 presents a photo of the realized prototype in the measurement chamber. The impedance mismatch, that is, the values, as a function of the source frequency was measured using an Agilent N5230A network analyzer over the frequency range 1–1.4 GHz.
Figure 15 gives the measured values as a function of the source frequency from 1 to 1.4 GHz. As shown in Figure 15, the structure with either S1 or S2 open operates at the same resonance frequency GHz. The measured −10 dB fractional bandwidth was 2.5% for State 1, in very good agreement with the simulated value: 2.6%. It is only 0.9% for State 2, indicating that fabrication errors were introduced with opening and closing the CLL elements.
Figure 16 presents the measured radiation pattern in () plane of the studied antenna at the resonance frequencies. These measurements also were carried out in the anechoic chamber “SATIMO Stargate32.” When switch S1 is ON (S2 is OFF), the direction of the beam is toward the negative -direction, whereas when the switch S2 is ON (S1 is OFF), the beam changes its direction toward the positive -direction. These results are in excellent agreement with the simulated ones.
The corresponding measured 3D radiation patterns of the prototype two-NFRP element antenna are shown in Figure 17. We notice in Figure 17 that the reconfigurability behavior of the two-CLL NFRP element antenna is obtained as predicted by activation/deactivation of only one of the switches. The maximum simulated and measured values of the radiation efficiency are 80% in comparison to 70% for both States 1 and 2, respectively. The maximum value of the measured gain is 4.1 dB for State 1 (Figure 17(a)) and 3.2 dB for State 2 (Figure 17(b)).
In this paper, a family of metamaterial-inspired antennas having reconfigurable radiation patterns was reported. The structures consist of a driven monopole antenna combined with one- and two-capacitively loaded loop (CLL) near field resonant parasitic elements. Two configurations were reported by considering the state of the gaps in the CLL elements, whether they were open or closed. Simulation results show that the structure can change its beam direction by controlling the switched states. Two prototypes with one- and two-CLL NFRP elements were fabricated and tested. The measured results in terms of impedance mismatch and radiation pattern results were presented and discussed. They were found to be in very good agreement with their simulated values. Since the simulations that were presented assumed ideal switches, the results for the radiation efficiencies and realized gains have to be considered as upper bounds. The actual implementation of the switches and the corresponding simulation models would depend on the commercially available technology selected, for example, pin-diodes or MEMs switches. The former could be incorporated with little impact of the overall antenna performance characteristics of the reported prototypes but would require modification of the actual designs and corresponding numerical models to account for their associated lumped element capacitance, inductance, and resistance values.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, under Grant no. 21-135-35-HiCi. The authors, therefore, acknowledge technical and financial support of KAU.
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