A High-Gain Reconfigurable Beam-Switched Circular Array Antenna Based on Pentagonal Radiating Elements Fed by Mutual Coupling for Sub-6 GHz Wireless Application Systems

Abstract

This paper presents the design and development of a reconfigurable circular array antenna capable of producing ten distinct radiation beams, intended for wireless systems in the sub-6 GHz frequency band. The antenna structure is based on four pentagon-shaped radiating elements arranged symmetrically around a central circular patch, which is excited through a coaxial feed. These radiating elements are linked by four circular segments, ensuring mutual coupling for effective operation. A systematic dimensional analysis has been conducted to optimize electromagnetic performance, resulting in a compact and efficient architecture. The beam reconfiguration is achieved through the control of four PIN diodes, which allow the main radiation beam to switch among ten different orientations in the azimuth plane. Specifically, the antenna supports eight directional states, oriented at 45° intervals, and two additional bidirectional states covering opposite directions. A prototype has been fabricated and experimentally validated, confirming the steering capability of ±40° in both the XZ and YZ planes. Performance evaluation shows a maximum gain of 9.29 dBi and efficiency levels ranging from 91% to 97%. Bandwidth varies across states, with 9.72% for S1–S7, 7.45% for S2–S8, and 4.61% for S9–S10. Overall, the proposed design demonstrates optimized bandwidth, gain, efficiency, and complete azimuthal coverage.

1. Introduction

In general, traditional antennas have been widely used in wireless communication systems for many years. Although they are reliable and relatively inexpensive, they can present key issues that can affect their performance. Thus, the rapid evolution of wireless data traffic and the requirements of future generations of communications in terms of latency, reliability and productivity have contributed to the search for an alternative to traditional antennas. Reconfigurable Beam-Switched (RBS) antennas are seen as an ideal solution, more efficient in terms of coverage, adaptation to propagation environments, interference cancellation, improved wireless networks security and optimized power consumption [1,2,3,4,5]. Transparent antennas for 5G applications integrated into OLEDs have been developed and studied ([6,7]), including antennas made from fabrics with high transparency, allowing applications towards flexible screens [8]. To name just a few examples, reconfigurable antennas can be integrated into devices such as OLED (Organic Light Emitting Diode) displays, Wi-Fi technologies, vehicular ad hoc networks (VANET), Vehicle-to-everything (V2X) networks, Wireless Sensor Networks (WSN), and 5G and 6G wireless systems [9,10,11,12,13]. They enable parameters such as polarization, frequency and radiation pattern to be dynamically adjusted to meet the specific requirements of wireless communications [14,15,16].

RBS antennas offer a significant reduction in interference and help improve the capacity of wireless systems [17,18]. Their integration makes it possible to add new functionalities and increase flexibility, while maintaining or even reducing the footprint compared with conventional smart antenna arrays [19]. In addition, to meet the future requirements of users and emerging technologies, beamforming subsystems and components must satisfy essential criteria such as low loss, high compactness and easy manufacture. They must also overcome the challenges of cost-effective design [20].

Various techniques are used in the literature to improve radiation efficiency, antenna gain and bandwidth, while maintaining the reconfigurability of the radiation pattern [21,22,23]. Antenna reconfiguration is a vast field that encompasses several methods, including the use of localized active components, which enable near-instantaneous modulation of currents or impedances [24,25]. Electroactive elements are often used for this purpose. In [10], the proposed antenna radiates in six cardinal directions using four PIN diodes, achieving peak gains of 6.68 dBi and 8.25 dBi over bandwidths of 8.86% and 7.83%, respectively. The antenna reported in [26] demonstrates beam steering capability in four distinct azimuthal directions by utilizing two feed ports in conjunction with state switching of two PIN diodes. For all operating states, it exhibits a bandwidth of 9.86%. The proposed design reaches a peak gain of 7.74 dBi and sustains a high efficiency, ranging from 75% to 84%, across the 5.41–5.97 GHz band. Reference [27] presents a K/Ka-band reconfigurable waveguide phased-array antenna based on a 1-to-4 feeding network. The proposed design is well suited for satellite communication applications employing separate transmit and receive systems. The antenna in [28], controlled through MEMS states, is capable of steering the THz beam in multiple directions, thereby confirming its feasibility for terahertz beam-steering applications. In [29], the proposed antenna—comprising a reflector, a director, and a tunable dipole—achieves bidirectional beam steering (separated by 180°) through the on/off switching of two PIN photodiodes. Measurements confirm that the antenna resonates at 800 MHz and achieves maximum gains of 6.3 dBi and 5.4 dBi for configurations I and II, respectively. By employing mechanical rotation of the 15 × 15 antenna array elements [30], the main beam direction can cover the entire azimuth plane, with an angular range of ±60° in the elevation plane. Full-wave simulations show that the beam directed toward (θ = 20°, φ = 0°) achieves a gain of 25.8 dB with a 1 dB bandwidth greater than 28.6%. Experimental measurements confirm similar performance, with a maximum gain of 25.6 dB in the same direction, corresponding to an aperture efficiency of 51.8%. In this work, PIN diode technology is adopted for beam steering, due to its low cost, high efficiency and ease of integration with microstrip antennas.

The present study proposes a new reconfigurable antenna design with the ability not only to increase gain but also to focus radiation in certain directions-eight directional beams and two bidirectional beams-through the switching of four PIN diodes. This enables full 360° beam steering in the XY-plane, achieved by a technique as simple as it is cost-effective, requiring no additional conventional circuitry. By switching four MPP4203 PIN diodes on and off, the four pentagon-shaped radiating elements of the proposed optimal structure are powered by mutual coupling.

The rest of this paper is structured as follows: Section 2 presents the design evolution and operating principle of the proposed antenna, alongside a study of its equivalent electrical circuit model. To validate the design experimentally, a fully functional antenna prototype was fabricated and tested. The simulation, theoretical and measurement results are presented and discussed in Section 3. Section 4 provides the conclusion of this study.

2. Design Principle

2.1. Evolution Design

As shown in Figure 1, the proposed optimal structure consists of four pentagon-shaped radiating elements (orange zone), symmetrically arranged around a central circular patch (green zone), fed by a coaxial cable and acting as a mutual excitation source. The radiating elements are fed by four circular segments (yellow zone) that are mutually coupled. The antenna was etched on a low-loss Rogers RT5880 substrate with a thickness of h = 1.57 mm.

Equations (1) and (2) are used to calculate the dimensions of the rectangular radiating patch in the initial step illustrated in Figure 1a [31].

$$\mathrm{W}={\displaystyle \frac{\mathrm{c}}{2\mathrm{f}}}\sqrt{{\displaystyle \frac{2}{{\mathsf{\epsilon }}_{\mathrm{r}}+1}}}$$

(1)

$$\mathrm{L}={\displaystyle \frac{\mathrm{c}}{2\mathrm{f}\sqrt{{\mathsf{\epsilon }}_{\mathrm{r}\mathrm{e}}}}}-2.∆\mathrm{L}$$

(2)

$$∆\mathrm{L}=0.412\mathrm{h}.{\displaystyle \frac{({\mathsf{\epsilon }}_{\mathrm{r}\mathrm{e}}+0.3)(\frac{\mathrm{W}}{\mathrm{h}}+0.264)}{({\mathsf{\epsilon }}_{\mathrm{r}\mathrm{e}}-0.258)(\frac{\mathrm{W}}{\mathrm{h}}+0.8)}}$$

(3)

$${\mathsf{\epsilon }}_{\mathrm{r}\mathrm{e}}={\displaystyle \frac{{\mathsf{\epsilon }}_{\mathrm{r}}+1}{2}}+{\displaystyle \frac{{\mathsf{\epsilon }}_{\mathrm{r}}-1}{2}}.{\displaystyle \frac{1}{\sqrt{(1+12\mathrm{h}/\mathrm{W})}}}-{\displaystyle \frac{{\mathsf{\epsilon }}_{\mathrm{r}}-1}{2}}.{\displaystyle \frac{{\mathrm{t}}_{\mathrm{m}}/\mathrm{h}}{\sqrt{\mathrm{W}/\mathrm{h}}}}$$

(4)

The parameter $t_m=35$ µm corresponds to the copper layer thickness in the microstrip structure.

All simulation results corresponding to the different evolutionary steps of the proposed structure were considered for the operating state 1 (S1). The proposed design evolved through four stages. Only the shape of the radiating elements was modified, while the shape of the ground plane and substrate remained unchanged, as described in the paragraphs below.

The initial antenna (A1) is omnidirectional, i.e., regardless of the state of the PIN diodes, as all radiating elements are interconnected (Figure 1a). Moreover, as shown in Figure 2, this antenna has a narrow bandwidth and a right-shifted center frequency than the preferred frequency, an unstable gain, and simulated reflection coefficient values for S11 only reached −13 dB. To improve isolation between parasitic patches, enhance antenna gain and reconfigure the directional main lobe (Ant. 2), we modified the shape of the radiating cells, as illustrated in Figure 1b.

As indicated in Figure 2a, antenna (A2) exhibits a relatively narrow operating frequency band and is less shifted toward higher frequencies compared to the target frequency. The antenna’s gain is significantly improved over that of the (A1) antenna, as illustrated in Figure 2b.

To further steer the antenna in a specific direction with enhanced bandwidth and gain, four coupling elements were included to feed the four radiating elements of the antenna (antenna A3), as illustrated in Figure 1c. The bandwidth of antenna A3 ranges from 5.77 to 6.11 GHz (BW = 340 MHz), as given in Figure 2. The antenna exhibits a distinctive gain performance in the operating frequency band, reaching a maximum value of 8.9 dBi.

It was necessary to optimize the bandwidth so that the proposed antenna (A4) would be compatible with Wi-Fi, VANET, V2X, and WiMAX technologies. To achieve this, two slots were added to each radiating element (Figure 1d). As shown in Figure 2a, the antenna (A4) exhibits an operating bandwidth between 5.47 and 6.04 GHz (i.e., 570 MHz). In comparison, the final design (Figure 1d) achieves a significantly broader bandwidth. For instance, under operating state 1 (S1), it reaches approximately 570 MHz. Additionally, the maximum gain has been enhanced (9.29 dBi), as given in Figure 2b.

The effect of primary geometric factors on antenna gains and S11 was examined through a parametric study. This study enabled us to obtain optimal values for all parameters to guarantee the best performance. For some parameters, minimum optimum values were chosen so that the proposed antenna could be easily manufactured using a PCB milling machine. The CST MWS software was used to design, simulate and optimize the proposed antenna. The final optimized dimensions of the proposed antenna are shown in

Table 1. Optimized antenna dimensions.

Parameter (mm)	L	L0	L1	L2	W0	W1
Dimension	16.5	1.6	1.6	4.5	2.1	0.5
Parameter (mm)	W2	W3	D	d	ε1	ε2
Dimension	2.8	6.2	4.1	1.2	0.2	0.4
Parameter (mm)	R1	R2	R3	R4	R5	β
Dimension	2.2	2.4	2.9	3.1	30	108°

.

2.2. Electrical Equivalent Circuit Model

According to the MPP4203 PIN diode data sheet [32], its parameters are defined as follows: ${\mathrm{R}}_{\mathrm{P}\mathrm{D}}=25 \mathrm{k}\mathsf{\Omega }$, ${\mathrm{C}}_{\mathrm{T}}=0.08 \mathrm{p}\mathrm{F}$, ${\mathrm{R}}_{\mathrm{S}\mathrm{D}}=3 \mathsf{\Omega }$, and $\mathrm{L}=0.02 \mathrm{n}\mathrm{H}$. Figure 4a,b shows the RF equivalent circuits of the PIN diode in the ON and OFF states, respectively. According to the literature, a rectangular patch antenna can be modeled by a parallel combination of a resistor (${\mathrm{R}}_{\mathrm{P}}$), a capacitor (${\mathrm{C}}_{\mathrm{P}}$) and an inductor (${\mathrm{L}}_{\mathrm{P}}$). The values of these localized elements are determined by Equations (5)–(7) [33,34].

$${\mathrm{C}}_{\mathrm{P}}={\displaystyle \frac{{\mathsf{\epsilon }}_0{\mathsf{\epsilon }}_{\mathrm{r}\mathrm{e}}\mathrm{L}\mathrm{W}}{2\mathrm{h}}}{\mathrm{c}\mathrm{o}\mathrm{s}}^{-2}\left({\displaystyle \frac{\mathsf{\pi }{\mathrm{Y}}_{\mathrm{f}}}{\mathrm{L}}}\right)$$

(5)

$${\mathrm{L}}_{\mathrm{P}}={\displaystyle \frac{1}{{\mathsf{\omega }}_{\mathrm{P}}^2{\mathrm{C}}_{\mathrm{P}}}}$$

(6)

$${\mathrm{R}}_{\mathrm{P}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{r}}}{\mathsf{\omega }{\mathrm{C}}_{\mathrm{P}}}}$$

(7)

$${\mathrm{Q}}_{\mathrm{r}}={\displaystyle \frac{\mathrm{c}\sqrt{{\mathsf{\epsilon }}_{\mathrm{r}\mathrm{e}}}}{4\mathrm{f}\mathrm{h}}}$$

(8)

where W and L represent the patch width and length, respectively, h designates the thickness of the substrate, and ${\mathrm{Y}}_{\mathrm{f}}$ is the feed point coordinate.

The area around the central circular area is modeled by mutual inductive coupling, noted coupling #1, characterized by a coupling coefficient K with a value of 1 for modes S1, S3, S5, and S7; a value of 0.87 for modes S2, S4, S6, and S8; and a value of 0.76 for modes S9 and S10. The gap separating each coupling element (indicated in yellow, as illustrated in Figure 3a) from the adjacent radiating element is represented by a parasitic capacitance C₁. The two notches of each radiating element can be modeled as a parasitic capacitance, noted C₂. The segment between the two notches can be assimilated to an inductance, noted L₀.

Additionally, the charges gathered between the ground plane and the coupling elements are accounted for by a third parasitic capacitance, C₃. Couplings between adjacent radiating elements can be represented by mutual capacitances, denoted C₁₂, C₂₃, C₃₄ and C₄₁.

The RF equivalent circuit of the proposed antenna was analyzed using the ADS simulator. As demonstrated in

Table 2. Equivalent Circuit Parameters after Optimization.

Parameter	RP	LP	CP	L0	C1	C2
Value	265 Ω	0.795 nH	0.88 pF	0.24 nH	0.1 pF	0.25 pF
Parameter	C3	C12	C23	C34	C41	K
Value	0.15 pF	0.2 pF	0.2 pF	0.2 pF	0.2 pF	1/0.87/0.82

, which is based on Equations (5)–(7) and the results of multiple optimization runs, the values of all lumped elements in the equivalent circuit have been optimized. (see Figure 4).

Figure 3. Design antenna geometry: (a) top view, (b) side view, (c) 3D diagram.

Figure 3. Design antenna geometry: (a) top view, (b) side view, (c) 3D diagram.

Figure 4. Equivalent circuit models: (a) PIN diode in OFF state, (b) PIN diode in ON state, (c) DC biasing circuit, and (d) overall antenna structure.

Figure 4. Equivalent circuit models: (a) PIN diode in OFF state, (b) PIN diode in ON state, (c) DC biasing circuit, and (d) overall antenna structure.

3. Results and Discussion

The design procedure of the proposed antenna was carried out in several stages, starting from theoretical calculations and progressing to experimental validation, as shown in Figure 5. The designed antenna is manufactured using a printed circuit board milling machine (LPKF Protomat E33, Garbsen, Germany), as can be seen in Figure 5 and Figure 6. The uncontrolled increase in the bias voltage can induce a thermal effect that negatively impacts the antenna performance. This heating may lead to a degradation of the radiation efficiency, a variation in the impedance matching, and, in some cases, an instability of the resonant frequency. Therefore, precise control of the bias voltage is essential to ensure the reliability and stability of the antenna performance.

Therefore, a DC (direct current) bias circuit has been added to set the four PIN diodes in the ON-OFF position. Eight inductors are inserted in the circuit to block DC current to the radiating cells and isolate the PIN diode DC bias network from the RF patches circuit. These inductors, which act as RF surge chokes, are divided into two groups: four of 12 nH and four of 22 nH. The PIN diodes are protected by four 100 Ω resistors, designed to limit the current flowing through each diode (see Figure 4c). PIN diode switching is controlled by the application of DC bias voltages: the diode is in a blocked state (off) at DCin = 0 V, and switches to conduction (on) at DCin = 2 V. A programmable electronic board controls the DC bias circuit.

By switching the PIN diodes on and off, the antenna’s main lobe can be oriented in different directions. This configuration resembles an array antenna, where the feeding of the elements is achieved through mutual induction. The configuration of the proposed antenna varies with its operating state, involving one or two excited elements in addition to parasitic ones. The fed elements generate a radiated electromagnetic field, which in turn induces a current in the parasitic elements. The induced current in the parasitic elements is phase-shifted with respect to the current in the excited elements. The superposition of the fields radiated by all the elements constitutes the resultant electromagnetic field produced by the antenna in a specific direction, resulting in improved antenna gain and efficiency.

In states S1, S3, S5 and S7, activation of a single PIN diode, as shown in Figure 7, changes the distribution of surface currents over the antenna structure, diverting the direction of radiation away from the fed element. The activation of two PIN diodes in states S2, S4, S6 and S8 also modifies the surface current distribution, orienting the radiation in the opposite direction to that of the fed elements. For states S9 and S10, two PIN diodes located opposite each other are activated, resulting in a surface current distribution such that the antenna radiates in two opposite directions. A radiating element functions as a reflector when its associated PIN diode is forward biased (ON state); otherwise, it serves as a director.

To validate the simulation results, an R&S^® ZVB20 vector network analyzer was used to measure the antenna reflection coefficient, as shown in Figure 8a. The measured, theoretical, and simulated S11parameters corresponding to the different operating states are presented and compared in Figure 9.

The corresponding bandwidths for states S1, S3, S5 and S7 are 9.90% (from 5.47 to 6.04 GHz) for the simulation, 9.70% (from 5.49 to 6.05 GHz) for the theoretical model, and 9.72% (from 5.48 to 6.04 GHz) for the measurement (Figure 9a). For states S2, S4, S6 and S8, they are 7.47% (from 5.54 to 5.97 GHz) for the simulation, 7.65% (from 5.53 to 5.97 GHz) for the theoretical model, and 7.45% (from 5.55 to 5.98 GHz) for the measurement (Figure 9b). Finally, for states S9 and S10, the bandwidths obtained are 4.42% (from 5.75 to 6.01 GHz) for the simulation, 5.63% (from 5.69 to 6.02 GHz) for the theoretical model, and 4.61% (from 5.71 to 5.98 GHz) for the measurement (Figure 9c). Overall, the simulated, measured and theoretical results show good agreement.

Figure 10 shows the gain and efficiency of the designed antenna, which performs well in both parameters. For the four operating states S1, S3, S5 and S7, the maximum simulated gain reaches 9.29 dBi. For states S2, S4, S6 and S8, it is slightly lower, with a maximum of 9.10 dBi. For states S9 and S10, on the other hand, the maximum simulated gain is lower, reaching 7.33 dBi. The graph also shows that, in the operating frequency bands, the simulated efficiency of the proposed antenna is particularly high, ranging from 88% to 97%.

Table 3. Operational conditions of the proposed antenna.

Operating State	PIN Diode State				BW (GHz)	Max. Gain (dBi)	Beam Direction
	$\mathit{D}1$	$\mathit{D}2$	$\mathit{D}3$	$\mathit{D}4$			XY-Plane (Φ)	XZ-Plane (θ)	YZ-Plane (θ)
State 1 (S1)	OFF	OFF	ON	OFF	5.47–6.04	9.29	45°	20°	20°
State 2 (S2)	OFF	OFF	ON	ON	5.54–5.97	9.10	90°	8°	22°
State 3 (S3)	OFF	OFF	OFF	ON	5.47–6.04	9.29	135°	−20°	20°
State 4 (S4)	ON	OFF	OFF	ON	5.54–5.97	9.10	180°	−22°	8°
State 5 (S5)	ON	OFF	OFF	OFF	5.47–6.04	9.29	225°	−20°	−20°
State 6 (S6)	ON	ON	OFF	OFF	5.54–5.97	9.10	270°	−8°	−22°
State 7 (S7)	OFF	OFF	OFF	ON	5.47–6.04	9.29	315°	20°	−20°
State 8 (S8)	OFF	ON	ON	OFF	5.54–5.97	9.10	360°	22°	−8°
State 9 (S9)	ON	OFF	ON	OFF	5.75–6.01	7.33	45°–225°	−40° & 40°	−40° & 40°
State 10 (S10)	OFF	ON	OFF	ON	5.75–6.01	7.33	135°–315°	−40° & 40°	−40° & 40°

summarizes the polarization parameters corresponding to each antenna operating mode.

As shown in Figure 8b, the Geozondas antenna measurement system is employed for the purpose of acquiring far-field radiation patterns. Simulated and measured radiation patterns of the proposed antenna in the XZ plane (ϕ = 0°) are presented in Figure 11. According to the antenna’s various operating states (states 1 to 10), it is possible to direct the main beam towards 20°, 8°, −20°, −22°, −20°, −8°, 20°, 22°, −40° and 40°, or simultaneously towards 40° and −40°, respectively.

On the other hand, Figure 12 shows that, in the YZ plane (ϕ = 90°), the antenna’s main beam is directed towards +20°, +22°, +20°, +8°, −20°, −22°, −20°, −8°, −20° and −38°, or simultaneously towards +38° and −38°, respectively. Furthermore, measured and simulated results are in good agreement, confirming the good performance of the proposed antenna.

Depending on the different operating states of the antenna (states 1 to 10), the main beam is oriented in the azimuthal plane towards 45°, 90°, 135°, 180°, 225°, 270°, 315°, 360°, 45° and 225°, or simultaneously towards 135° and 315°, respectively (Figure 13).

In order to validate our proposed antenna, a comparative analysis between the developed reconfigurable antenna and other designs reported in the literature is presented in

Table 4. Comparison of the proposed antenna’s performance with recent state-of-the-art designs.

Year/Ref.	No. of Switches	No. of Layers	Size (λ03)	Frequency (GHz)	BW (%)	Max. Gain (dBi)	Efficiency (%)
2021/[35]	2 PIN diodes	1	0.22 × 0.27 × 0.001	1.80	41	2.2	80
2021/[36]	14 PIN diodes	3	0.59 × 0.53 × 0.08	2.4 5.8	2.07 2.6	6.2 6.6	NG
2022/[37]	60 PIN diodes	1	1.80 × 1.80 × 0.027	5.1	11.76	7.3	70
2022/[38]	10 PIN diodes	1	0.68 × 0.55 × 0.09	2.4	2.08	5.3	80
2023/[39]	2 MEMS	1	0.43 × 0.61 × 0.046	35	2.4	4.8	NG
2024/[40]	8 varactors	3	1.41 × 1.0 × 0.036	6.06	1.8	8.2	NG
2024/[26]	4 PIN diodes	1	0.86 × 0.86 × 0.03	5.75 5.83	8.86 7.83	6.68 8.25	95
Prop. Ant.	4 PIN diodes	1	0.81 × 0.81 × 0.03	5.57 5.67 5.90	9.72 7.45 4.61	9.29 9.10 7.33	97

. In addition to its radiation pattern reconfigurability according to ten distinct modes, the proposed antenna offers several notable advantages: an expanded measured bandwidth of 9.72%, a maximum simulated gain of 9.29 dBi, an efficiency of up to 97%, and a reduced footprint of 0.81 × 0.81 × 0.03 $\left({{\lambda }_0}^3\right)$.

4. Conclusions

This study focuses on the development of a new compact and switchable circular array antenna with ten distinct beams-eight directional and two bidirectional-for wireless application systems in the sub-6 GHz band. This antenna is capable of switching the main beam towards well-defined directions in the azimuthal plane (XY plane), with a 45° step, thanks to electrical reconfiguration based on control of the polarization circuits of four PIN diodes. Depending on the state of the PIN diodes, the designed antenna can adopt ten distinct operating states while providing full coverage of the azimuthal plane. The antenna also supports beam steering in the elevation plane, covering an angular range of 80°, between −40° and +40°. In addition, it allows the beam to be oriented in the elevation plane over an angular range of 80°, from −40° to +40°, enhancing its adaptability in three-dimensional space. The proposed design demonstrates a successful synthesis of structural simplicity and functional versatility, with notable measured performance indicators including a high peak gain of 9.29 dBi, a measured wide bandwidth of 9.72% (5.49–6.05 GHz) and excellent radiation efficiency ranging from 88% to 97%. With its compact form factor of 0.81 × 0.81 × 0.03 (λ₀³), this antenna has great potential for seamless integration into various modern wireless platforms such as Wi-Fi, WiMAX, VANET, and V2X, where agile beam control and reliable communication are essential.