Antenna design

Proposed antenna configuration

Figure 1 shows schematics of proposed antenna, printed on a FR-4 substrate (ε_r = 4.4, tanδ = 0.018, h _sub = 0.02λ_fa) with the overall volumetric dimension of 1.33λ_fa × 0.9λ_fa × 0.02λ_fa (i.e. λ_fa is the free-space guided wavelength at 5 GHz). Here, authors have shorted the partial ground plane with one of the twin PCS [PCS(L)] of 0.36λ_fa × 0.03λ_fa. It is separated by 0.019λ_fa from the upper edges of conventional partial ground plane by using a metallic copper strip, which is responsible for achieving CP radiation attributes. Then, the rectangular-MTS reflector with a dimension of 2λ_fa × 1.65λ_fa × 0.02λ_fa is placed below the Y-shaped radiating monopole at a height of 0.33λ_fa, which is supported by the four plastic spacers.

Fig. 1. Schematic configuration of the proposed broadband circularly polarized monopole antenna loaded with metasurface reflector. [The dimensions are as: W _SUB = 1.33λ₀, L _SUB = 0.9λ₀, W _GND = 0.83λ₀, L _GND = 0.4λ₀, L _PCS(R) = 0.36λ₀, L _PCS(L) = 0.36λ₀, W _PCS = 0.03λ₀, P _MCS = 0.05λ₀, W _A = 0.04λ₀, L _A = 0.6λ₀, L _F = 0.42λ₀, W _F = 0.05λ₀, W _RP = 1.78λ₀, L _RP = 1.48λ₀, and S ₁ × S ₂ = 0.13λ₀ × 0.09λ₀, where λ₀ = 60 mm.].

Parametric study

In this section, the design and its parametric analysis of proposed CP monopole antenna without MTS reflector is performed using CST microwave suite. Initially, the ground plane is optimized. It is observed that, if a large ground plane is used, then the S ₁₁ and AR will degrade, and there is a change of radiation pattern characteristics. The 3 dB axial ratio (AR) shows greater sensitivity to the width of the ground plane (W _GND), because of their dependency on horizontal components (horizontal currents). It is also sensitive to (a) length of the PCS (L _CS), (b) width of the PCS (W _CS), and (c) position of the metallic copper strip (P _MS). Thus, the current investigation dispenses that the variation of these geometrical parameters often puts significant impact on S ₁₁ and AR properties. The variations due to the L _CS on S ₁₁ and AR are highlighted in Fig. 2(a). At L _CS = 21.6 mm, the proposed antenna exhibits wider IBW and ARBW. A similar analogy is highlighted in Fig. 2(b), when W _CS = 1.16 mm is considered. With the due incorporation of twin PCS, impedance matching trait is improved. Furthermore, the positioning of metallic copper strip (P _MS) correlates with the phenomenon of shorting in between partial ground plane and PCS(L), which demonstrate the CP characteristics. In Fig. 2(c), the variation in P _MS and its impact on S ₁₁ and AR are highlighted. When P _MS = 2.5 mm, an optimum CP performance is observed in the desired operating bands.

Fig. 2. Effects on the antenna metrics due to variation in (a) L _CS, (b) W _CS, and (c) P _MS.

Since, the design intuition is to achieve effectiveness in its performance, mutual inductance effects of the geometry that stabilizes to an extent, and optimizes the antenna performance to the final optimum with the objective to render out a simple, compact, and low-profile characteristics without affecting the fundamental characteristics from its physical insights.

Analysis of CP characteristics

In this case, the understanding of CP mechanism is interpreted by utilizing surface current distribution phenomenon [approach-I] and far-field radiation pattern [approach-II]. In approach-I, the understanding toward the existence of CP waves is often confirmed by the simultaneous presence of both horizontal and vertical currents on the antenna surface. Subsequently, for approach-II, the orientation of CP is observed by considering far-field analogy in broadside direction (ϕ=0⁰ and θ = 0⁰), through maximum CP antenna gain [approach-II(a)] and relative power [approach-II(b)] obtained from the normalized radiation pattern. The analysis of CP mechanism is persuaded at f _a = 5 GHz, and the outcomes of both approaches are highlighted in Fig. 3. For approach-II(a), it is observed that left-hand CP (LHCP) gain is 2.32 dBic and right-hand CP (RHCP) gain of 2.2 dBic, which indicates that the proposed monopole antenna is of LHCP type. Similarly, in approach-II(b), it is observed that, LHCP is quite stronger than RHCP by − 22.5 dBic. These combined results do confirm the nature of antenna as LHCP orientation. From the above analysis, it is inferred that approach-I correlates with the existence of CP characteristics, whereas the approach-II gives idea about the nature of CP for the proposed antenna.

Fig. 3. Evaluation of CP mechanism at 5 GHz (a) surface current distribution [approach-I], (b) maximum realized gain [approach-II], and (c) relative power [approach-II] from normalized radiation pattern in the broadside direction for SYPMA.

A theoretical approach on analyzing CP characteristics

In the reported work, the theoretical insights [Reference Behera, Meher and Mishra39] of 3 dB ARBW (BW _3dB) and CP antenna gain (G _3dB) are presented for the complete evaluation of antenna characteristics, especially for analyzing the CP characteristics of any type of CP antennas, irrespective of the geometry and frequency of operation. To understand such phenomena, let us consider the criteria (C _r) in a general form, expressed as:

(1)$$C_{r} = {BW_{3{\rm dB}} \times G_{3{\rm dB}}\over 100}$$

Equation (1) is the basic form of the proposed criteria for analyzing CP attributes. It considers 3 dB axial bandwidth and CP antenna gain as the important parameters for RF front-end in RF-EH [Reference Behera, Meher and Mishra39]. With the segregation of CP antenna gain and its evaluation in terms of (a) average (G _avg), (b) maximum (G _max), (c) minimum (G _min), and (d) peak (G _peak), the proposed criteria (C _r) can be further derived as C _r1 to C _r4, shown in equations (2) to (5):

(2)$$C_{r1} = {BW_{3{\rm dB}} \times G_{3{\rm dB( {avg}) }}\over 100}$$

(3)$$C_{r2} = {BW_{3{\rm dB}} \times G_{3{\rm dB( {max}) }}\over 100}$$

(4)$$C_{r3} = {BW_{3{\rm dB}} \times G_{3{\rm dB( {min}) }}\over 100}$$

(5)$$C_{r4} = {BW_{3{\rm dB}} \times G_{3{\rm dB( {\,peak}) }}\over 100}$$

So, equations (2) to (5) are proposed for evaluating CP antenna characteristics. Their corresponding outcomes are shown in Table 2. Henceforth, an effective methodology is implemented for analyzing the RF front-ends based upon the bandwidth (ARBW) and CP gain characteristics.

Table 2. Examination of metasurface-inspired CP antennas [Reference Yue, Jiang and Werner24–Reference Huang, Yu-Xuan, Zhan-Wei, Shu-Ting, Xiao-Ming and Jing34] w.r.t. the proposed antenna (SYPMA-MTS) for the criteria: C _r1, C _r4

Experimental results

To validate the performance of a proposed MTS loaded monopole antenna, prototype is shown in Fig. 4(a) fabricated by using ETS-PCBMATE prototyping machine. The S ₁₁ (dB) ismeasured by using PNA X-series Microwave Network Analyzer (N5247A) from the Keysight Technologies, whereas the AR (dB), antenna gain, radiation pattern, and antenna efficiency are measured in the anechoic chamber. Figure 4(b) presents the S ₁₁ and AR plots. It exhibits ameasured IBW and ARBW of 48.45% (3.57–5.84 GHz) and 25.25% (4.21–5.42 GHz), which matches with that of the simulated outcomes.

Fig. 4. Characterization of proposed antenna in terms of (a) antenna prototype and far-field pattern setup in anechoic chamber, (b) S ₁₁ and axial ratio (AR), and (c) antenna gain and efficiency.

The average simulated and measured antenna gain are > 8.35 dBic and antenna efficiency > 75% over the operating frequency bands, as presented in Fig. 4(c). The proposed antenna has a peak gain variation from 7.5 to 8.5 dBi. It is also observed that, MTS layer has a better performance compared to earlier reported antennas in [Reference Mathew, Ameen, Jayakrishnan, Mohanan and Vasudevan5–Reference Pandey, Shankhwar and Singh37, Reference Behera, Meher and Mishra39, Reference Behera, Srikanth, Meher and Mishra40], and witness better reflection properties due to the presence of coupling effects [Reference Mohanty, Behera and N Nasimuddin41] of inductance and capacitance, which control the fringing fields to be effectively coupled out with the radiated fields. Here, the radiation pattern at different frequencies is measured in the anechoic chamber and their plots are presented in Figs 5(a) and (b). For the proposed MTS inspired antenna, centered at f _a = 4.5 and 5 GHz, radiation plots for the principal planes at ϕ = 0^° and ϕ = 90^° are highlighted with the directional pattern. Owing to the MTS reflector, it possesses stable FBR ranging from − 15 to − 21.5 dBic. Thus, the boresight radiation has a maximum intensity along +z-axis shown in Fig. 5(c). Hence, SYPMA-MTS has not only achieved performance trade-offs required from application perspective but also the proposed antenna design concept is a generic solution for achieving high CP performances.

Fig. 5. Far-field radiation traits of SYPMA-MTS at (a) 4.5 GHz, (b) 5 GHz, with (c) 3D pattern at 4.5 and 5 GHz respectively.

Key design aspects of proposed antenna

• • The size of printed monopole antenna (SYPMA) is less than the size of MTS reflector. Due to reduction in its size, the effective height is fixed at a gap of 0.33λ_fa, for better impedance matching and stable boresight radiations.

• • The radiator geometry in general reduces electrical footprint of antenna size. It is difficult to excite antenna with MTS reflector, that is why tuning of feeding mechanism and shorting process have been transformed, which transduces excitation of energy in an effective manner with the generation of CP characteristics.

• • Here, the MTS reflector initiates the effective coupling process [Reference Mohanty, Behera and N Nasimuddin41]. Due to the presence of such type of ground configuration, it emulsifies coupled energy into boresight radiation, so, there is no spurious nulls. It assists for radiation strength, as IBW, ARBW, and gain are enhanced with improved FBR. To demonstrate the low-profile and effectiveness, gap between the structures is fixed at 0.33λ_fa, for the significant improvement of CP antenna gain.

• • The proposed antenna satisfies impedance performances, antenna gain, radiation efficiency metrics from the application perspective.

Investigation toward RF energy harvesting

The proposed MTS antenna (SYPMA-MTS) is integrated with the CRLH and LC-based Greinacher voltage doubler circuits (GVDs), where RF-to-DC conversion efficiency (η₀, %) and DC output voltage (V _out, V) are calculated by using the ADS solver. The complete block diagram of RF-EH mechanism, and its corresponding outcomes for the two different designed rectifier circuits are shown in the Figs 6(a)–(c). In general, the selection of rectifier topology [Reference Surender, Khan, Talukdar, De, Antar and Freundorfer42–Reference Surender, Halimi, Khan, Talukdar and Antar47] is important in RF energy harvester design. Among the available and reported topologies of rectifier in literature, voltage doublers are considered as best choice for RF-to-DC rectification, due to simplicity of design and better power handling capability. In the another aspect, diode is considered as the main rectifying element in a GVD, and it influences the overall performance of the circuit. The parameters which play an important part in the selection of the diode/s are series resistance (R _s), junction capacitance (J _c), threshold voltage (V _t), and the breakdown voltage (V _b). To select the best diodes operating at 5 GHz frequency bands, some commercially available RF-diodes are listed along with their properties in Table 3.

Fig. 6. Application perspective. (a) Block diagram of RF energy harvesting mechanism, (b) CRLH-based GVD circuit (I), and (c) LC-based GVD circuit (II).

Table 3. Commercially available Schottky diodes for rectification

In Table 3, SMS-7630 has the minimum value for V _t, which implies that, it will turn ON for the minimum input power, and is considered as a best choice for harvesting DC power from the low-level ambient RF signals. In such a scenario, the impedance matching network is of huge significance for maintaining the overall efficiency (η₀) of the circuit. In this concurrent analysis, CRLH-TL and LC-based matching GVD circuits are proposed and investigated for the 5 GHz bands.

In general, CRLH type of matching circuit is a combination of microstrip line as RH part and lumped components as LH part, considered as the intermediate part of multi-stage GVD. In general, the component values associated with them are computed by the methodology followed in [Reference Jie, Nasimuddin, Karim and Chandrasekaran48]. A LC type of matching circuit is introduced to avoid the tedious calculations in reducing the complexity [Reference Behera, Meher and Mishra39].

Considering implementation, a large-signal S-parameter circuit simulation environment is setup in the ADS solver to determine the various outcomes associated with the RF-EH application. Here, the η₀ is calculated by considering equation (6), where the CRLH and LC-based multi-stage rectifier circuit/s reported for the first time in the literature are analyzed for the input power levels (P _in) from − 10 to + 20 dBm respectively, which covers the possibility of low-input power levels at 5 GHz.

(6)$$\eta_{0} ( \% ) = {P_{load}\over P_{incident}} = {V^2_{out}\over P_{in} \times R_{load}}$$

On the final note, at P _in = 5 dBm, V _out is 1.7 V, with η₀ as 43% for CRLH-based GVD circuit (I), whereas, V _out is 2.1 V, with η₀ as 61% for LC-based GVD circuit (II). Here, the maximum attainable V _out is 5.39 V (for I) and 5.4 V (for II), along with the maximum attainable η₀ as 56.28% (for I) and 73.82% (for II) are presented in Figs 6(b) and (c). By looking into the prospective of earlier RF-EH system designs [Reference Behera, Meher and Mishra39, Reference Behera, Srikanth, Meher and Mishra40, Reference Jie, Nasimuddin, Karim and Chandrasekaran48–Reference Chandrasekaran, Agarwal, Nasimuddin, Alphones, Mittra and Karim54], the proposed antenna + rectifier design reported here, especially LC-based GVD circuit (II), depicts higher outcomes over the existing literatures, as shown in Table 4.

Table 4. Performance characteristics of SYPMA-MTS integrated with the proposed rectifier circuits (I, II) over existing ones in [Reference Behera, Meher and Mishra39, Reference Behera, Srikanth, Meher and Mishra40, Reference Jie, Nasimuddin, Karim and Chandrasekaran48–Reference Chandrasekaran, Agarwal, Nasimuddin, Alphones, Mittra and Karim54]