Patch antenna design

Figures 1(a) and (b) show the suggested antenna, which consists of a pentagonal-shaped patch fed through a coaxial feed. The pentagon shape is chosen as it offers a more compact design than conventional shapes such as rectangular, circular, or elliptical. Also, the pentagon geometry offers flexibility to incorporate extra elements such as slots, notches, or loading mechanisms, allowing for performance optimization. The side length of the pentagonal-shaped patch antenna is calculated as

(1)\begin{equation}s = {\text{ }}{a_{eff}}\sqrt {5 - 2\sqrt 5 } \end{equation}

(2)\begin{equation}{a_{{\text{eff}}}} = \frac{{1.8412c}}{{2\pi {f_{\text{r}}}\sqrt {{\varepsilon _{{\text{eff}}}}} }},\end{equation}

(3)\begin{equation}{\varepsilon _{eff}} = {\text{ }}\frac{{{\varepsilon _r} + 1}}{2} + \frac{{{\varepsilon _r} - 1}}{2}{\left( {1 + 12\frac{h}{{{D_p}}}} \right)^{ - 1/2}}\end{equation}

where c is the velocity of light in vacuum, f _r is the resonance frequency, ε _r is the relative permittivity, h is the thickness of the substrate, and D _p is the diameter of the pentagon.

The pentagonal-shaped radiating antenna is incorporated with two slots to enhance bandwidth and enable circular polarization. The antenna is modelled using CST Microwave Studio software and fabricated on the FR-4 substrate with a dielectric constant (ε _r) of 4.4, loss tangent (tan δ) of 0.02, and a thickness (h) of 1.6 mm. Figures 1(c) and (d) show the prototype of the designed pentagonal-shaped antenna.

FSS layer

Figure 2(a) shows the schematic of the designed FSS array. In the proposed pentagonal-shaped patch CP antenna, FSS plays an important role in increasing the antenna gain. The FSS unit cell is made up of two concentric hexagonal-shaped rings, as illustrated in Figure 2(c). Traditional rectenna configurations are often inefficient in capturing RF signals due to their inherent polarization sensitivity, as noted in previous studies [Reference Ferreira, Sismeiro, Ferreira, Caldeirinha, Fernandes and Cuiñas21]. The length (λ _r) of FSS elements can be calculated as

(4)\begin{equation}{\lambda _{\text{r}}} = 2p - 2g + w.\end{equation}

Figure 2.

Proposed FSS layer: (a) schematic, (b) photograph of the FSS layer prototype, and (c) unit cell (L ₁ = 4 mm, L ₂ = 2 mm, L ₃ = 1 mm, L ₄ = 0.5 mm).

The distance between the centers of adjacent hexagonal unit cells is referred to as pitch (p). The outer hexagon size (L ₁) has been used to derive the pitch, which is p = 2 × L ₁. The gap (g) denotes the space between two adjacent unit cells, and (w) is the trace width of the metallic conductive strip used to make hexagonal ring.

The FSS elements, which have been carefully engineered for maximum performance, allow for selective alteration of the frequency response and polarization of incoming electromagnetic waves. The FSS layer is printed on the FR-4 substrate with dimensions (W _f = 50 mm × L _f = 40.4 mm) and a height of 1.6 mm, designed to function efficiently at ISM (2.4 GHz) band. The precise engineering ensures that the FSS layer within the rectenna system functions optimally, enhancing its ability to capture ambient signals. Figure 2(b) shows the prototype of the designed FSS layer.

Integration of patch antenna and FSS

Figure 3(a) depicts the proposed pentagonal-shaped patch antenna integrated with the FSS layer. The optimal gap between the FSS layer and the proposed CP antenna is calculated using rigorous simulations. Maintaining a space between the FSS and the suggested antenna is critical to ensuring the seamless integration and functionality of the FSS-based antenna, allowing for the achievement of desired operational results. Figures 3(b) and (c) display the side and back views of the fabricated antenna with the FSS layer. Figure 3(d) depicts measurement of the antenna in an anechoic chamber with and without FSS.

Figure 3.

(a) Schematic of reported antenna with FSS plane, (b) side view of prototype with FSS plane, (c) back view of designed prototype with FSS layer, and (d) measurement of reported prototype in an anechoic chamber with and without FSS.

Figure 3 Long description

A pentagonal-shaped patch antenna integrated with a Frequency Selective Surface (FSS) plane. The FSS plane is depicted with a grid pattern and the antenna is positioned below it. The image B displays a side view of the prototype, where the antenna is mounted on a stand with the FSS layer visible beneath it. The image C presents a back view of the prototype, showing the antenna mounted on a flat surface with the FSS layer underneath. The image D illustrates the measurement setup in an anechoic chamber, with two separate views. On the left, the prototype is shown with a horn antenna labeled as a reference and a close-up of the pentagonal patch is highlighted. On the right, the setup is similar, with the horn antenna reference and a close-up of the FSS layer. Both setups are enclosed in red dashed lines to emphasize the measurement configuration.

Figures 4(a), (b), and (c) present the measured and simulated reflection coefficients, axial ratio, and gain of the antenna with and without FSS layer, respectively. The operational frequency does not significantly shift with the FSS layer, indicating no changes in the resonant properties of the antenna. This observation confirms the ability of the FSS structure to interact with incident waves at various angles of arrival [Reference Khajevandi, Oraizi, Amini and Poordaraee22, Reference Sung23]. The predicted bandwidth of the antenna is 140 MHz, demonstrating wide performance.

Figure 4.

Results of the antenna with and without FSS layer: (a) reflection coefficients, (b) axial ratio, and (c) gain.

Figure 4 Long description

The line graph A has four curves and a legend listing, Simulated S subscript 11 With Frequency Selective Surface, Measured S subscript 11 With Frequency Selective Surface, Simulated S subscript 11 Without Frequency Selective Surface and Measured S subscript 11 Without Frequency Selective Surface. The x-axis label is, Frequency, in gigahertz, with tick labels from 2.0 to 3.0. The y-axis label is, S subscript 11, in decibel, with tick labels from 0 to minus 35. All four curves form a narrow dip centered near 2.4 gigahertz. The lowest point of the dip is near minus 30 decibel for the without frequency selective surface simulated curve, near minus 25 decibel for the without frequency selective surface measured curve, near minus 22 decibel for the with frequency selective surface simulated curve and near minus 20 decibel for the with frequency selective surface measured curve. Away from the dip, the curves rise toward values near 0 decibel. The line graph B has four curves and a legend listing, Without Frequency Selective Surface, With Frequency Selective Surface, Measured Without Frequency Selective Surface and Measured With Frequency Selective Surface. The x-axis label is, Frequency, in gigahertz, with tick labels from 2.30 to 2.50. The y-axis label is, Axial Ratio, in decibel, with tick labels from 1 to 19. All four curves form a V shape with a minimum near 2.40 gigahertz. The minima are near 1 decibel for the with frequency selective surface curve and the measured with frequency selective surface curve and near 2 decibel for the without frequency selective surface curve and the measured without frequency selective surface curve. At 2.30 gigahertz and 2.50 gigahertz, the curves rise to values in the mid to high teens in decibel. The line graph C has four curves and a legend listing, Simulated Without Frequency Selective Surface, Simulated With Frequency Selective Surface, Measured Without Frequency Selective Surface and Measured With Frequency Selective Surface. The x-axis label is, Frequency, in gigahertz, with tick labels from 2.30 to 2.50. The y-axis label is, Gain, in decibel, with tick labels from 0.0 to 3.0. Two curves for without frequency selective surface rise from about 1.5 decibel near 2.30 gigahertz to about 2.8 to 2.9 decibel near 2.50 gigahertz. Two curves for with frequency selective surface rise from about 0.2 decibel near 2.30 gigahertz to about 1.6 to 1.7 decibel near 2.50 gigahertz.

Additionally, it is clear that the addition of the FSS layer has no discernible effect on the axial ratio (dB) of the suggested antenna. At 2.4 GHz, the axial ratio is consistently less than 3 dB, as shown in Figure 4(b), confirming the circular polarization characteristics. Also, the FSS layer increases antenna gain significantly, from 1.12 dBi to 2.22 dBi, as demonstrated in Figure 4(c). Moreover, due to dielectric and conductor losses in the FR-4 substrate, the antenna without the FSS achieves a maximum radiation efficiency of 65%, whereas with the FSS layer, the efficiency increases to 74% at 2.4 GHz because the FSS suppresses surface-wave modes and enhances broadside radiation. Thus, the FSS compensates for the FR-4 losses, improving overall radiating performance at the operating frequency. FR-4 was chosen for this work because it is inexpensive and widely available, making the design suitable for low-cost applications.

Figure 5 depicts the surface current of the pentagonal-shaped radiating element at 2.4 GHz at angles φ = 0°, φ = 90°, φ = 180°, and φ = 270°. As the electric field vector rotates in a clockwise direction, the antenna demonstrates a right-hand circular polarization (RHCP) pattern, as illustrated in Figures 5(a–d). The suggested antenna design provides robust circular polarization performance at ISM 2.45 GHz band, validating its reliability as a sensing antenna for RF energy harvesting [Reference Shakhirul, Jusoh, Ismail, Kamarudin, Yahya, Yasin and Sabapathy24]. This eliminates the need to align the proposed antenna with the RF sources, which is important for collecting energy from different sources of variable polarization.

Figure 5.

Surface current of the antenna at 2.4 GHz: (a) φ = 0°, (b) φ = 90°, (c) φ = 180°, and (d) φ = 270°.

Figure 5 Long description

Four vector plots labeled a, b, c and d illustrate the surface current distribution on a pentagonal-shaped antenna at angles zero degrees, ninety degrees, one hundred eighty degrees and two hundred seventy degrees. Each plot shows a pentagon with a large central circular cutout and a smaller circular cutout near one vertex. The surface currents are represented by lines forming a grid-like pattern across the pentagon, indicating the flow direction and intensity. No scale or units are shown. The patterns rotate with the angle, demonstrating how the current distribution changes. The currents appear concentrated around the edges and near the circular cutouts, with variations in density visible across different angles. The plots do not include any legends or color scales and the color does not indicate magnitude. The main takeaway is the rotation of the current distribution, highlighting areas of concentration and variation across the antenna surface.

The radiation patterns are measured with a reference standard horn antenna in an anechoic chamber, as displayed in Figure 3(d). The controlled environment provides immunity from external electromagnetic interference while also reducing the emission of electromagnetic waves into the environment. Figures 6(a) and (b) display the measured and simulated RHCP and left-hand circular polarization (LHCP) radiation shape of suggested pentagonal-shaped antenna at 2.4 GHz frequency, φ = 0° and φ = 90°, respectively. The 2D pattern results show that the obtained values of the LHCP curves are less than the values of the RHCP curves, indicating that the proposed antenna is RHCP.

Figure 6.

Measured and simulated radiation patterns at 2.4 GHz: (a) φ = 0° and (b) φ = 90°.

Figure 6 Long description

A polar plot with angular labels 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 and 330. The radial scale is labeled 0, minus 20 and minus 40. A legend lists four series: Simulated left hand circular polarization, Simulated right hand circular polarization, Measured left hand circular polarization and Measured right hand circular polarization. The radial quantity name and unit are not shown. The plotted traces include an outer near circular curve close to the 0 ring over most angles and an inner curve that drops inward most strongly around 180 degrees, reaching near the minus 40 ring. The image B showing a polar plot with angular labels 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 and 330. The radial scale is labeled 0, minus 20 and minus 40. A legend lists four series: Simulated left hand circular polarization, Simulated right hand circular polarization, Measured left hand circular polarization and Measured right hand circular polarization. The radial quantity name and unit are not shown. The plotted traces include an outer near circular curve close to the 0 ring and an inner curve that stays near the minus 20 to minus 40 region across a broad angle range, with the lowest region centered around 180 degrees and approaching the minus 40 ring.

Measurement results

The RF-DC efficiency can be expressed as a ratio of output DC power to input RF power. The increase in DC power enhances the voltage drop through the load resistance at low RF input power, thus increasing the efficiency, which is directly proportionate to the output DC power. The efficiency ( $\eta $) of the rectifier is calculated as

(5)\begin{equation}\eta = \frac{{{P_{{\text{DC}}}}}}{{{P_{{\text{in}}}}}},\end{equation}

(6)\begin{equation}{P_{{\text{DC}}}} = \frac{{V_{{\text{DC}}}^{\text{2}}}}{{{R_{\text{L}}}}},\end{equation}

where P _in is the RF input power, V _DC is the voltage across load resistance, P _DC is the output DC voltage and R _L is the load resistor.

Figure 9 illustrates the real and imaginary impedance plots with FSS structure. The graph displays the frequency-dependent impedance behavior, indicating that the antenna does not maintain 50 Ω across the band. The real impedance ranges from 30 Ω to 55 Ω, while the imaginary impedance ranges from −40 Ω to +10 Ω, demonstrating reactive behavior and validating impedance matching across the operating band.

To achieve optimal performance, the impedance of the RF rectifier should match that of the antenna, typically set to 50 Ω. Impedance matching confirms optimal power transfer from the antenna to the rectifier, while also reducing signal reflections and increasing energy harvesting efficiency. To accurately capture the nonlinear behavior of the rectifier impedance under varying input power levels using a frequency sweep, Figure 10 shows the variation of rectifier impedance with frequency and input power. Figure 10(a) shows that the rectifier impedance is frequency dependent, with 225 Ω at 2.1 GHz, ∼50 Ω at 2.4 GHz and gradually decreasing to below 50 Ω above 2.6 GHz as diode conduction increases. This behavior confirms the nonlinear nature of the rectifier and the importance of frequency-optimized matching.

Figure 8.

Proposed rectifier: (a) dimensions and design specifications (W = 25 mm, L = 19 mm, L1 = 2 mm, W1 = 3 mm, L2 = 10 mm, W2 = 3 mm, L3 = 5 mm, W3 = 3 mm, L4 = 2 mm, W4 = 2 mm, L5 = 7 mm, W5 = 3 mm, L6 = 3 mm, W6 = 3 mm) and (b) photograph of the rectifier prototype.

Figure 8 Long description

Image (a) shows a circuit diagram labeled ′T-Matching Network′ with components including an HSMS2860 diode, resistors labeled R, capacitors labeled C and inductors labeled L subscript 1 to L subscript 6. The diagram includes via holes and connections marked W subscript 1 to W subscript 6. The RF input is indicated and the layout is enclosed in a dashed red line. Image (b) is a photograph of the rectifier prototype, featuring a mounted connector, soldered components and red wires attached to a brown base. The physical layout corresponds to the schematic in image (a).

Figure 10(b) shows the variation of input impedance of the rectifier with input power level, emphasizing the nonlinear behavior typical of diode-based rectifier circuits. When the input power ranges from −30 dBm to −20 dBm, the impedance is high, ranging from 150 Ω to 180 Ω due to the diode not fully turned on condition. The rectifier has an impedance of ∼50 Ω when the input power is −10 dBm. As input power increases, diode conduction reduces impedance to 35–45 Ω at 0 to +5 dBm. This power-dependent impedance variation confirms that rectifier matching must be designed considering nonlinear operation instead of a single fixed impedance value.

Figure 9.

Real and imaginary impedance of the proposed antenna.

Figure 10.

Rectifier circuit impedance matching: (a) variation of impedance and frequency and (b) variation of impedance and input power level at 2.4 GHz.

Figure 11.

(a) Simulated and measured S-parameters, (b) simulated DC output voltage versus input power levels for various load resistances, and (c) simulated PCE versus load resistance at various input power levels.

Figure 11 Long description

The line graph in image A has the horizontal axis labeled, Frequency (GHz) and the vertical axis labeled, S subscript 1 1 (dB). The horizontal axis ranges from 2.2 to 2.8. The vertical axis ranges from 0 to minus 60. A legend lists S subscript 1 1, Simulated and S subscript 1 1, Measured. The simulated curve reaches its lowest point at about 2.4 with a value near minus 55. The measured curve reaches its lowest point at about 2.4 with a value near minus 30. The line graph in image B has the horizontal axis labeled, Input power level (dBm) and the vertical axis labeled, DC output voltage (V). The horizontal axis ranges from minus 25 to 5. The vertical axis ranges from 0 to 3.0. A legend lists 500 Ohms, 1000 Ohms, 1500 Ohms and 2000 Ohms. At input power level minus 25, all curves are near 0. At input power level 0, the curves are about 1.5 for 500 Ohms, about 1.7 for 1000 Ohms, about 2.0 for 1500 Ohms and about 2.2 for 2000 Ohms. At input power level 5, the curves are about 1.6 for 500 Ohms, about 1.9 for 1000 Ohms, about 2.3 for 1500 Ohms and about 2.6 for 2000 Ohms. The line graph in image C has the horizontal axis labeled, Load R subscript L (Ohms) and the vertical axis labeled, PCE percent. The horizontal axis ranges from 500 to 4000. The vertical axis ranges from 15 to 70. A legend lists 0 dBm, minus 10 dBm and minus 20 dBm. The 0 dBm curve is about 45 at 500, about 65 at 1000, about 62 at 2000 and about 45 at 4000. The minus 10 dBm curve is about 35 at 500, about 55 at 1000, about 50 at 2000 and about 35 at 4000. The minus 20 dBm curve is about 25 at 500, about 35 at 1000, about 30 at 2000 and about 22 at 4000.

Figure 12.

(a) RF-to-DC conversion efficiency at an input power sweep, (b) DC output voltage versus input power at 1000 Ω load resistor, and (c) variation of DC output current, PCE, and DC output voltage.

Figure 12 Long description

A line graph with the vertical axis label, PCE percent and tick marks from 10 to 60. The horizontal axis label is Frequency gigahertz, with tick marks from 2.25 to 2.55. A legend lists Sim PCE minus 10 decibel milliwatt, Mea PCE minus 10 decibel milliwatt, Sim PCE 0 decibel milliwatt, Mea PCE 0 decibel milliwatt, Sim PCE minus 20 decibel milliwatt and Mea PCE minus 20 decibel milliwatt. At Frequency 2.25, Sim PCE minus 10 decibel milliwatt is about 35 and Mea PCE minus 10 decibel milliwatt is about 33. At 2.30, about 40 and 38. At 2.35, about 48 and 45. At 2.40, about 56 and 55. At 2.45, about 59 and 57. At 2.50, about 50 and 45. At 2.55, about 42 and 35. At Frequency 2.25, Sim PCE 0 decibel milliwatt is about 32 and Mea PCE 0 decibel milliwatt is about 30. At 2.30, about 36 and 34. At 2.35, about 44 and 41. At 2.40, about 55 and 53. At 2.45, about 58 and 55. At 2.50, about 48 and 42. At 2.55, about 40 and 32. At Frequency 2.25, Sim PCE minus 20 decibel milliwatt is about 28 and Mea PCE minus 20 decibel milliwatt is about 26. At 2.30, about 32 and 30. At 2.35, about 40 and 38. At 2.40, about 36 and 35. At 2.45, about 32 and 31. At 2.50, about 29 and 28. At 2.55, about 24 and 22. The image B showing a line graph with the vertical axis label, DC output voltage V, with tick marks from 0 to 2.5. The horizontal axis label is Input power level decibel milliwatt, with tick marks from minus 25 to 5. A legend lists Simulated and Measured. The Simulated curve rises from about 0.0 at minus 25 to about 0.1 at minus 20, about 0.2 at minus 15, about 0.4 at minus 10, about 0.8 at minus 5, about 1.5 at 0 and about 2.0 at 5. The Measured curve rises from about 0.0 at minus 25 to about 0.1 at minus 20, about 0.2 at minus 15, about 0.35 at minus 10, about 0.7 at minus 5, about 1.3 at 0 and about 1.8 at 5. The image C showing a line graph with the horizontal axis label, DC output Voltage millivolt, with tick marks from 0 to 600. The left vertical axis label is DC output Current microampere, with tick marks from 0 to 250. The right vertical axis label is Rectenna Efficiency percent, with tick marks from 0 to 60. A black line with square markers decreases from about 170 microampere at 0 millivolt to about 150 at 100 millivolt, about 110 at 200 millivolt, about 80 at 300 millivolt, about 50 at 400 millivolt, about 20 at 500 millivolt and about 0 at 600 millivolt. A red curve labeled PCE percent maximum rises from about 0 percent at 0 millivolt to about 35 percent at 100 millivolt, about 55 percent at 200 millivolt, peaks near about 55 percent around 250 millivolt, then falls to about 45 percent at 350 millivolt, about 30 percent at 400 millivolt, about 15 percent at 500 millivolt and about 0 percent at 600 millivolt. Two vertical lines are labeled V load 300 and V open 600.

Figure 11(a) shows the measurement and simulated return loss parameters of the suggested rectifier. Inconsistencies in the measurement data are attributed to environmental losses such as absorption, scattering, and reflection. The propagation of electromagnetic waves is influenced by a variety of factors, including material properties, interference, and spatial configuration, all of which can cause performance losses in rectifiers. Shielding techniques, improved antenna design, and rectifier circuitry refinement can all help to reduce environmental losses while increasing system performance.

Figure 11(b) shows that the DC output voltage increases nonlinearly with input power. Higher load resistances (1500–2000 Ω) result in significantly higher output voltage due to improved power-to-voltage conversion. The 1000 Ω load consistently produces a higher DC output voltage than 500 Ω and slightly lower than 1500–2000 Ω, indicating near-optimal voltage conversion with minimal voltage drop or mismatch.

The PCE versus load resistance graph shows that all three input power levels (0 dBm, −10 dBm, and −20 dBm) achieve their maximum efficiency at 1000 Ω, indicating that this load achieves the most effective power extraction from the rectifier in Figure 11(c). The optimum load is 1000 Ω, which provides optimal impedance matching, minimizes conduction losses, and maintains the voltage–current operating point of the rectifier across different input power levels.

Figure 12(a) depicts simulated and measured RF-to-DC conversion efficiencies at various input power levels. At 2.4 GHz and an input power of −10 dBm, the rectifier achieves a measured PCE of around 56.5%, while the simulated value is slightly higher at 62.5%. When the input power is reduced to −20 dBm, the conversion efficiency drops significantly, reaching only around 40%, highlighting the rectifier’s reduced performance at lower power levels.

Figure 12(b) shows the simulated and measured DC output voltages versus the available input power at 2.4 GHz. At −10 dBm input power, the rectifier generates approximately 0.238 V. As the input power increases from −10 dBm to 0 dBm, the output voltage increases significantly, reaching nearly 1.5 V, demonstrating a nonlinear improvement in rectification efficiency at higher power levels.

Figure 12(c) shows the DC output performance of the rectifier under varying load conditions, focusing on the internal behavior of the rectenna system. The DC I–V curve (black) shows a nearly linear decline in output current as the output voltage increases, which is consistent with the behavior of a diode rectifier under RF excitation. When the load resistance is low, the rectifier produces high current but low voltage. As the load resistance increases, the operating point shifts toward higher voltage at the expense of current, eventually reaching the open-circuit voltage (≈575 mV) where the current is zero. However, the PCE curve (red) has a distinct maximum that corresponds to the maximum power point. The peak occurs at V _DC ≈ 238 mV, when the DC output power reaches its maximum voltage and current. The location of this peak indicates that the optimum load voltage is close to half of the open-circuit voltage, which is consistent with the theoretical condition for maximum power transfer in nonlinear rectifying circuits, where the effective load resistance matches the rectifier’s internal dynamic resistance.

This relationship confirms proper impedance interaction between the rectifier, matching network, and load and emphasizes the importance of selecting a load value that positions the operating point near the maximum power point to maximize harvesting efficiency.

Table 1 compares the properties of the suggested rectifier to rectenna designs published in the literature. The comparison shows that the proposed rectifier outperforms the reported rectifier designs, making it more advantageous and useful for RF energy harvesting.

Table 1.

Comparison of the suggested RF rectifier to related works

Table 1 Long description

Rows compare references 25 through 29 and a proposed design across antenna size, resonant frequency in GHz, efficiency eta, topology, polarization, gain in dBi, HPBW in degrees, PCE percent, and input power for PCE in dBm. Reference 25 reports antenna size 0.92 by 0.744 in lambda naught units, 2.4 GHz, efficiency 52, topology modified bridge rectifier, linear polarization, gain 4.7, HPBW not given, PCE 52, and input power about 10 dBm with 0.15 milliwatts per square centimeter noted. Reference 26 reports 0.76 by 0.76, 2.4 GHz, efficiency 56, topology not given, circular polarization, gain 4.7, HPBW not given, PCE 69 marked as peak efficiency, and input power about 20 dBm with power density context noted. Reference 27 reports 0.4 by 0.092, 2.4 GHz, efficiency 40, topology matching network, linear polarization, gain 0, HPBW not given, PCE 58, and input power 0 dBm. Reference 28 reports 0.84 by 0.84, 2.2 GHz, efficiency 50, topology voltage doubler, circular polarization, gain 7, HPBW not given, PCE 40, and input power not given. Reference 29 reports 5.6 by 2.22, 31.5 GHz, efficiency 12, topology voltage doubler, linear polarization, gain 8, HPBW not given, PCE 12, and input power 9.5 dBm. Proposed design reports antenna size 0.4 by 0.32, 2.4 GHz, efficiency 56.5, topology series, circular polarization, gain 6.5, HPBW 73, PCE 56.5, and input power minus 10 dBm.

Note: LP = linearly polarized; CP = circularly polarized.

Pentagonal antenna array

Figure 13 depicts the proposed 3D pentagonal antenna array structure, along with FSS. The array consists of five antenna elements and an FSS, each with a 72° coverage angle. The 3D pentagonal antenna array allows for 360° collection of ambient signals, while the CP antenna ensures RF signal sensing in the full azimuth plane, making it suitable for real-world implementation.

Figure 13.

Proposed 3D pentagonal antenna array: (a) side view and (b) top view.

Figure 14(a) depicts the reflection coefficients of the four array elements at 2.4 GHz, demonstrating good impedance matching at all ports. Figure 14(b) shows an axial ratioof less than 3 dB at 2.4 GHz frequency, indicating that the pentagonal antenna array has good circular polarization. The pentagonal radiating element at 2.4 GHz has a peak gain of 6.5 dBi, indicating that there is no hidden spot in the antenna’s coverage and signal is effectively received from all directions, enhancing its suitability for energy harvesting applications.

Figure 14.

3D pentagonal antenna array: (a) S-parameters and (b) axial ratio.

Figure 14 Long description

A line graph with the y axis label, S parameters left parenthesis decibel right parenthesis, ranging from negative 30 to 0 and the x axis label, Frequency left parenthesis gigahertz right parenthesis, ranging from 2.0 to 3.0. A legend lists S subscript 11, S subscript 22, S subscript 33, S subscript 44 and S subscript 55. The plotted curve forms a deep notch near 2.4 on the frequency axis, reaching about negative 25 on the S parameters axis, then rises toward 0 as frequency approaches 3.0. The image B showing a line graph with the y axis label, Axial Ratio, ranging from 0 to 39 and the x axis label, Frequency left parenthesis gigahertz right parenthesis, ranging from 2.0 to 3.0. Text above the plot reads, Simulated at 2.4 gigahertz. The curve decreases from about 25 at 2.0 to a minimum near 2.4 at about 3, then increases to about 35 by 3.0.