Low-cost, compact, and reconfigurable antennas using complementary split-ring resonator metasurface for next-generation communication systems

Abstract

An innovative and simple method for attaining broadband frequency reconfigurable antenna structure is presented using low-cost materials and a compact design. The frequency reconfigurability is attained by the “OFF” and “ON” mechanisms of three PIN diodes. Many performance observations are carried out such as reflectance coefficient, bandwidth (BW), resonance frequency, electric field, feed position variation, and gain among different configurations. The suitability of the presented work for the different applications lies in the X frequency band. The resonating frequency for all switch OFF modes is achieved at 13.2 GHz and one switch ON mode at 10.7 GHz. The proposed design yields the frequency tunability behavior over the broadband of 2.5 GHz. The design yields the directivity of 5.58 dB, the minimum reflectance coefficient of −17.27 dB, and a total gain of 3.87 dB. This design offers an electric field of 46 558 v/m, a BW of 340 MHz, and a normalized directivity of 87° using low-cost substrates. The results of the presented design were also fabricated and related to simulated results. Performance observation of the proposed work with previously published research work is also included. The presented design provides the solution of the low-cost, compact, reconfigurable antennas, which can be used for next-generation communication systems.

Introduction

Antenna design has come a long way; previously, simple antennas were designed with high gain or high bandwidth (BW), but now antennas have improved. They have high gain, high BW, small size, etc. [1–4]. Antenna design has been improved to accommodate more applications in a single antenna. One of the essential features required nowadays is reconfiguration [5]. Frequency reconfiguration is significant nowadays in antenna design to be applicable in the military and other applications. Radiation and polarization tuning are also essential in radar applications [6]. This requirement has created the need for an antenna having high gain and reconfiguration capability. This requirement can be met by incorporating metamaterials and PIN diode switches.

Metamaterials have unique properties that do not lie in natural materials. These unique properties are the negative effect of permeability and permittivity. Antenna performance is enhanced due to the metamaterial property. The metamaterial concept is added to the standard patch by etching strip lines in the ground plane, which improves the gain and BW [7]. Metamaterial antennas with its unique properties are also applicable for 5G applications [8]. The superstrate can be applied with different materials to improve gain, BW, and directivity. The superstrate with a split-ring resonator improves the BW and gain [9]. The design with metamaterials is applicable in LTE/WiMax/Bluetooth [10–12]. Microstrip patch antenna (MPA) design with liquid metamaterials enhances radiation [13, 14]. The antenna's radiation is improved with a truncated corner MPA loaded with metamaterials. Antenna loaded with metamaterials is applicable in Doppler radar with improved antenna parameters [15]. Split-ring resonators and the complementary split-ring resonator are used for making the metamaterial component. Therefore, the antenna operates on multifrequency bands with high BW.

The reconfiguration is significant and can be attained by applying a switch to a patch antenna. The PIN diodes [16] and RF-MEMS [17] can be used as a switch. Frequency reconfiguration can be achieved using metamaterials and PIN diode switches [18, 19]. Superstrate and metamaterial are incorporated into the MPA to enhance the gain and achieve vast frequency operation [20]. Pattern reconfiguration can also be possible to achieve using switching in an MPA. This reconfiguration in radiation patterns is applicable in vehicular applications [21]. The defected ground structure reconfigures the antenna frequency [22]. The slot antenna is tuned for frequency and pattern characteristics. The wideband response is also achieved by reconfiguring this slot antenna using a PIN diode [23]. MIMO slot antennas can also be frequency reconfigured using the same concept [24]. Varactor diodes can also be used for achieving frequency tunability [25]. Antenna design needs reconfiguration, high gain, and high BW.

Antennas with high gain, BW, and tunability are the requirement of today's world. We propose an antenna having high gain, high BW, and frequency reconfiguration with switching “ON” and “OFF” three PIN diodes. The reconfiguration is achieved by varying the switching of different diodes. The resonance frequency of all switch OFF designs was attained at 13.2 GHz, which helps target the point to point communication application. The second diode with ON configuration resonating at 10.7 GHz is suitable for the satellite downlink communication. The two diodes with ON condition resonating at 11.3 GHz are suitable for the armature radio telescope application. The fourth switching configuration provides the resonance at 11.5 GHz, which helps atmospheric wave attenuation [26, 27].

Moreover, the new design's findings are compared to those of past published designs to demonstrate its progress. Finally, the design is built and tested, and the simulation results are used to verify its correctness. The antenna design in detail is discussed in Section “Design and modeling.” The results of the fabricated prototype and simulation model are discussed in Section “Simulation and fabricated design results.” Finally, “Conclusion” section contains the author's closing observations and recommendations.

Design and modeling

Figure 1 presents the three-dimensional view of the planned antenna structure. The substrate layer is designed using a low-profile material (FR-4). The dielectric constant is 4.4 for FR-4 material [19]. Three PIN diodes were used for the charge distribution among different sections in the patch region, leading to reconfigurability. Table 1 shows the four switching modes. Due to the switching mechanism, the tunability of the frequency can be attained. In the first mode, all the PIN diodes are in the OFF condition. The second mode has one diode in the ON state and the rest of the two switching diodes in the OFF condition. The third mode has PIN diode-1 and PIN diode-2 in ON the condition, and PIN diode-3 in the OFF condition. Finally, the fourth model has all the PIN diodes in one condition.

Fig. 1. Three-dimensional representation of the planned frequency reconfigurable patch antenna system. The bottom layer is made of FR-4 material. The breadth of the substrate material is 1.5 mm. The ground and patch layer are made of copper. The height of both these layers is 0.35 mm.

Table 1. Switching states of the three PIN diodes for achieving frequency reconfigurability

Figure 2 reveals the designed view of the proposed structure. Figure 2(a) presents the top view of the simple cropped patch without the PIN diodes. The top view of the patch with the three connected PIN diodes is shown in Fig. 2(b). The lateral view is observed in Fig. 2(c). Figure 2(d) shows an anechoic chamber. It is used for directivity measurement.

Fig. 2. Designed model of the planned reconfigurable MPA. (a) The top view of the patch without attaching the PIN diodes. (b) The top view of the three PIN diode attached antenna structure. (c) The lateral view of the prototype with three PIN diodes. (d) Directivity finding using an anechoic chamber.

The top view of the MPA is shown in Fig. 3(a). Figure 3(b) shows the lateral view of the presented structure. The copper material is used in the patch of antenna and ground layer; both have a thickness of 0.35 mm. In this design, the size of the copper layer is 1 oz; 1 oz corresponds to 14 mils (0.3 mm). The size of the ground and substrate are 14 mm × 14 mm. The dimension of the patch is 12 mm × 12 mm. The patch area is cropped for attaching the PIN diode, which changes the energy distribution, resulting in frequency reconfigurability. Two gaps in the cropped area are 1 mm. The dimension of the inner area of the rectangle patch is 2 mm. After cropping, the inner area is 4 mm. After cropping the outer area, the dimensions are 8 mm.

Fig. 3. (a) Design structure's top level schematic diagram. (b) The side view of the design. The coaxial feed mechanism is used for the energies of the patch. All dimensions are represented in mm.

The dimensions of the proposed antenna structure are calculated using equations (1)–(11) [20]. The resonance frequency of Split ring resonator (SRR) can be calculated using equation (1). Here, L represents the inductance and C represents the capacitance:

(1)$$f = \displaystyle{1 \over {2\pi \sqrt {LC_s} }}$$

The effective relative permittivity of a metamaterial antenna is calculated as below:

(2)$$\varepsilon _{eff}( f ) = \varepsilon _r-\displaystyle{{\varepsilon _r-\;\varepsilon _{es}} \over {1 + G{( {\,f/f_d} ) }^2}}$$

The coefficients are calculated as per the following equations [18,28]:

(3)$$f_d = \displaystyle{{Z_c} \over {2\mu _0h_1\;\;}}$$

(4)$$Z_c = \displaystyle{1 \over {2\pi }}\sqrt {\displaystyle{{\mu _0} \over {\varepsilon _{es}\varepsilon _0}}} \log \left[{F_1\displaystyle{h \over {w_1}} + \sqrt {1 + {\left({\displaystyle{{2h} \over {w_1}}} \right)}^2} } \right]$$

(5)$$G = 0.6 + 0.0009Z_c$$

(6)$$\varepsilon _{es} = \displaystyle{{\varepsilon _r + 1} \over 2} + \left({\displaystyle{{\varepsilon_r-1} \over 2}} \right)\left[{1 + 10\left({\displaystyle{t \over {w_1}}} \right)} \right]^{{-}a_1b_1}$$

The coefficients F ₁, a ₁, b ₁, and relative electrostatic permittivity (ɛ _es) are calculated as follows:

(7)$$F_1 = 6 + ( {2\pi -6} ) \exp \left[{-\left({30.666\displaystyle{t \over {w_1}}} \right)} \right]^{0.7528}$$

(8)$$a_1 = 1 + \displaystyle{1 \over {49}}\log \left[{\displaystyle{{{( {w/t} ) }^4 + {( {w/52t} ) }^2} \over {{( {w/t} ) }^4 + 0.432}}} \right] + \displaystyle{1 \over {18.7}}\left[{1 + {\left({\displaystyle{1 \over {18.1}}\displaystyle{w \over t}} \right)}^3} \right]$$

(9)$$b_1 = 0.564\left({\displaystyle{{\varepsilon_r-0.9} \over {\varepsilon_r + 3.0}}} \right)^{0.053}$$

The S-parameters are required to analyze reflectance responses for the antenna structure in the gigahertz frequency range. The impedance and refractive index are required to calculate the transmittance (S ₂₁) and reflectance (S ₁₁) [29]:

(10)$$z = \sqrt {\displaystyle{{( {1 + S_{11}^2 } ) -S_{21}^2 } \over {( {1-S_{11}^2 } ) -S_{21}^2 }}} $$

(11)$$n = \displaystyle{1 \over {kd}}\cos ^{{-}1}\left[{\displaystyle{1 \over {2S_{21}}}( {1-S_{11}^2 + S_{21}^2 } ) } \right]$$

where n represents the refractive ratio, d represents the layer thickness, and z represents the wave impedance [30].

The switch OFF and ON mechanism of the presented architecture reflects frequency reconfigurability by switching the ON/OFF direction. The PIN diode is used as an RF switch. Figure 4 presents the equivalent electric circuit model of the PIN diode for the HFSS tool. Figure 4(a) presents two segments of a patch connected by the component R–L–C. The ON condition is presented in Fig. 4(b). In the ON configuration, resistor (Rs) and inductor (Ls) are connected in the series. The OFF condition is presented in Fig. 4(c); it is a series combination of resistor (Rp) and capacitor (Cp). The equivalent of both is connected with the inductor (Ls). The HPND 4005 model of PIN diode is used in the presented design. Planar beam lead PIN diode HPND-4005 has a high lead strength and excellent electrical performance. The PIN diode provides the resistance of 4.7 Ω and the smaller capacitance value of 0.017 pf. The biasing circuit is presented in Fig. 4(d). Furthermore, this PIN diode is very rugged. For simplicity, the antenna's reconfigurability was examined in terms of resistance, using the idea that the PIN diode functions as an open circuit for high resistor values and as a closed circuit for low resistor values [30] (Table 2).

Fig. 4. Equivalent model of PIN diode: (a) the lumped R–L–C model for the HFSS tool, (b) ON state model, (c) OFF state of the model, and (d) PIN diode biasing circuit.

Table 2. Frequency tunability examination for different switching conditions

Simulation and fabricated design results

The graphical representation of reflectance response for the simulated and measured results for the switch ON and OFF configuration is presented in Fig. 5. There is some tolerance in both the results due to the limitation in the fabrication process. The large soldering area modified the path of charge propagation in the structure, affecting the design's characteristic impedance. The excessive path will also increase the effective length of the inductance. The capacitance is contingent on the gap between the two conducting regions. The gap between the two conducting regions decreases, affecting that region's capacitance [31]. Due to the variation in the capacitance and inductance, the resonance frequency is also affected – the resonance frequency change in the simulated and fabricated reflectance responses is shown in Fig. 5. The efficiency and thermal loss is affected due to the soldering. The thermal loss has been affected up to 0.3 dB. The effect in the high Q antenna for the high current has a more negligible effect on the thermal loss. The resistance loss of ~0.25 Ω is estimated [32].

Fig. 5. Reflectance coefficient plots for the three PIN diodes, switch-ON and OFF modes: (a) all switch OFF mode configuration, (b) one switch ON mode configuration, (c) two switches ON mode configurations, and (d) three switches ON configuration.

All switch OFF mode shows the reflectance response of −11.27 dB for the resonance of 13.2 GHz with the BW of 250 MHz as illustrated in Fig. 5(a). One switch ON mode signifies the first S ₁₁ of −17.27 dB achieved at the resonating frequency of 10.7 GHz along with the BW of 250 MHz, and the second S ₁₁ of −11.2 dB achieved at the resonating frequency of 12.5 GHz with the BW of 170 MHz, as illustrated in Fig. 5(b). Two switches ON mode represent the reflectance response of −16.75 dB achieved at the resonating frequency of 11.3 GHz with the BW of 340 GHz as illustrated in Fig. 5(c). All switch ON mode signifies the S ₁₁ of −13.18 dB achieved at the resonating frequency of 11.5 GHz with the BW of 290 MHz as illustrated in Fig. 5(d). Table 3 presents the data in the tabular form for plot represented in (Fig. 6).

Fig. 6. Reflectance plot for three switching conditions over the frequency range of 10–14 GHz. All switch OFF mode represents the reflectance of −11.27 dB at the 3.2 GHz frequency with a BW of 250 MHz. One switch ON mode shows the first reflectance of −17.27 dB at the 10.7 GHz frequency with a BW of 250 MHz and the second reflectance of −11.2 dB at the 12.5 GHz frequency with a BW of 170 MHz. Two switches ON mode represent the reflectance of −16.75 dB at 11.3 GHz frequency with a BW of 340 MHz. Finally, all switch ON mode shows the reflectance of −13.18 dB at 11.5 GHz frequency with a BW of 290 MHz.

Table 3. Tabular illustration of switching modes, reflection response, frequency of resonance, BW, gain and the electric field for the performance analysis

The one switch-ON mode yields the reflectance coefficient at 10.7 GHz, and all switch-ON mode provides the reflectance coefficient at 13.2 GHz. These represent that the maximum frequency tunability is 2.5 GHz. The radiation pattern behavior is analyzed based upon the directivity. Figure 7 illustrates the measured and simulated directivity plot of the switch OFF and ON modes over −180° to +180°. The directivity of mode-1 (all switch OFF mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) is 4.46, 5.58, 4.08 and 4.22 dB, respectively. Good directivity is shown in all the mode configurations. Figure 8(a) shows the measured and simulated directivity plot. The normalized directivity for mode-1 (all switch OFF mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) is respectively 56° (−26° to +30°), 55° (−30° to +25°), 85° (−15° to +70°), and 87° (−12° to +75°) as shown in Fig. 8(b).

Fig. 7. Directivity plot for four modes. (a) All switch-OFF mode represents the highest directivity of 4.46 dB. (b) One switch ON mode represents the highest directivity of 5.58 dB. (c) Two switches ON modes represent the highest directivity of 4.08 dB. (d) All switch ON mode represents the highest directivity of 4.22 dB.

Fig. 8. (a) Directivity plot for mode-1 (switch-off mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) are respectively 4.46, 5.58, 4.08, and 4.22 dB over −180° to +180°. (b) The −3 dB down directivity for mode-1 (switch OFF mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) are respectively 56°, 55°, 85°, and 87°.

Figure 9 illustrates the Fermi (contour) plot of reflectance response for the different feed positions over the different PIN diode configurations. Figure 9(a) shows the feed variation in the X direction (1–11 mm) to identify the S ₁₁ over the 10–13 GHz for mode-1. Three bands of S ₁₁ are detected at 10.5, 11.1, and 12.5 GHz. Figure 9(b) shows the feed variation in the Y direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-1. There is one band of S ₁₁ detected at 13 GHz. Figure 9(c) shows the feed variation in the X direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-2. Three bands of S ₁₁ are detected at 10.8 and 12.5 GHz. Figure 9(d) shows the feed variation in the Y direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-2. Three bands of S ₁₁ are detected at 10.53, 12.5, and 13 GHz for mode-2. Figure 9(e) shows the feed variation in the X direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-3. There is one band of S ₁₁ observed at 11 GHz. Figure 9(f) shows the feed variation in the Y direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-3. There is one band of S ₁₁ observed at 11.2 GHz. Figure 9(g) shows the feed variation in the X direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-4. There are three bands of S ₁₁ detected at 10.25 and 13 GHz. Figure 9(h) shows the feed variation in the Y direction (1–11 mm) to observe the S ₁₁ over 10–13 GHz for mode-4. There is one band of S ₁₁ observed at 10.25 and 13 GHz. It is observed that reflectance responses also shift for the varying feed position [9].

Fig. 9. Reflectance coefficient analysis using Fermi plot for the different feed positions under the switch ON and OFF configurations. Variation of feed position in the X and Y directions is carried out for 1–11 mm over the 10–13 GHz frequency range.

The electric field distribution for all modes is presented in Fig. 10. Electric fields for mode-1 (switch-OFF mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) are respectively 16 993, 21 221, 46 558, and 15 051 v/m. The maximum electric field is observed in mode-3 (two switches ON mode). The efficiency of the structure can be observed based upon higher gain. The two-dimensional and three-dimensional gains for the different structures are illustrated in Fig. 11. Maximum gain for mode-1 (switch-OFF mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) are respectively 3.38, 2.03, 3.87, and 0.9 dB. Table 4 compares the proposed work's performance to previously published work.

Fig. 10. Electric field for mode-1 (switch-off mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) are respectively 16 993, 21 221, 46 558, and 15 051 v/m.

Fig. 11. Two-dimensional and three-dimensional gain patterns are presented. The maximum gain for mode-1 (switch-OFF mode), mode-2 (one switch ON mode), mode-3 (two switches ON mode), and mode-4 (all switch ON mode) are respectively 3.38, 2.03, 3.87, and 0.9 dB.

Table 4. Performance observation of the presented work with earlier published work

Conclusion

The miniaturized and broadband frequency reconfigurable MPA design structure is presented in this study. The frequency tunability is attained using the three PIN diodes. The assessment of simulation and fabrication was carried out for reliability. The three PIN diodes are located on the patch area, and by switching them ON and OFF, four modes are analyzed and the results are presented. Many performance parameters are analyzed such as reflectance coefficient, BW, resonance frequency, electric field, feed position variation, and gain among different configurations. The unique features attained by the proposed design structure are the BW of 340 MHz, maximum frequency reconfigurability of 2.50 GHz, maximum directivity of 5.58 dB, normalized directivity of 87°, the electric field of 46 558 v/m, and the maximum gain of 3.87 dB. Performance observation of the presented work with previously published research work is also included. The presented design is used for radar, short-range tracking, missile guidance, and many more.