Low-Cost Phased Array with Enhanced Gain at the Largest Deflection Angle

Abstract

This paper presents a low-cost 1-bit phased array operating at 17 GHz (Ku band) with an enhanced scanning gain at the largest deflection angle to extend the beam coverage for ground target detection. The phased array is designed using 16 (2 × 8) radiation-phase reconfigurable dipoles and a fixed-phase feeding network, achieving 1-bit beam steering via a direct current (DC) bias voltage of ±5 V. Measurement results demonstrate a peak gain of 9.2 dBi at a deflection angle of ±37°, with a 3 dB beamwidth of 94° across the scanning plane. Compared with conventional phased array radars with equivalent peak gains, the proposed design achieves a 16% increase in the detection range at the largest deflection angle.

1. Introduction

Phased array radars on airborne platforms are widely used for ground target detection. To broaden the detection coverage, enhancing the scanning gain at larger deflection angles is crucial in compensating for the gain loss due to increased distance. However, conventional phased arrays typically experience gain degradation at large deflection angles [1,2]. This leads to an imbalance: the array exhibits an excessive gain in the vertical direction, while gain attenuation at larger deflection angles limits the overall angular coverage of the detection system. This limits the radar’s ability to maintain uniform detection performance across its scanning range. As illustrated in Figure 1, if a phased array radar can be designed to reduce the gain in the vertical direction, avoiding significant gain overflow and simultaneously enhancing the gain at the largest deflection angle (i.e., the deflection angle corresponding to the largest detection distance) to compensate for the increased detection range, it would achieve broader detection coverage compared to conventional phased array radars. In the figure, the extended detection range is manifested as extended ground detection coverage, indicated by the blue ground region, and an extended detection angle range, denoted by $\Delta theta$.

Conventional microstrip patch antennas typically exhibit narrow beamwidth characteristics, which restrict the beam scanning range when employed as array elements in phased array radars [3,4,5]. Furthermore, the requirement for phase shifters to regulate each element’s phase inevitably increases system costs [6,7,8,9].

Researchers have explored various approaches to enhance the beamwidth of antenna arrays. For instance, Refs. [10,11,12] designed wide-beamwidth antennas by leveraging the properties of metasurfaces. In [13,14], vertical element loading and equivalent slot loading techniques were employed to broaden the antenna half-power beamwidth (HPBW). In [15], the authors introduced a novel dual circularly polarized antenna based on the septum circular polarizer, achieving both a wide 3 dB beamwidth and wide axial ratio beamwidth. Moreover, researchers have improved both the E-plane and H-plane beamwidth through surface wave suppression techniques [16]. In addition, numerous studies have reported other methods of enhancing the antenna beamwidth [17,18,19,20,21].

Regarding cost reduction strategies, various approaches aim to achieve this by reducing the number of T/R components and phase shifters. For instance, substituting traditional phase shifters with liquid crystal substrates can lead to a significant cost reduction in phased array systems [22,23,24,25]. The adoption of digital phase-shifting architectures or alternative phase-shifting strategies can effectively reduce the number of phase shifters required in the array system [26,27,28,29]. Furthermore, sparse arrays directly reduce the number of antenna channels, effectively lowering the system cost [30,31].

Recently, the upper mid-band has gained significant interest in the fields of radar detection and 6G communication [32]. This paper presents a low-cost 16-element phased array for ground target detection. Operating at 17 GHz (Ku band), the array integrates a fixed-phase feeding network with 1-bit radiation-phase reconfigurable dipoles to achieve beam scanning in the H plane. This design not only mitigates the gain overflow issue in the vertical direction in conventional phased array radars but also enhances the gain at the largest deflection angle to achieve an extended detection range. By replacing traditional phase shifters, the proposed system significantly reduces the manufacturing costs. Measurement results indicate that the array achieves a peak gain of 9.2 dBi at ±37°, which corresponds to a 2.4 dB improvement over the vertical direction gain of 6.8 dBi. Compared to conventional phased arrays with equivalent peak gains, the proposed design exhibits a 2.5 dB gain improvement at ±37°, corresponding to a 16% increase in the effective detection range, which effectively compensates for the gain attenuation caused by the large-angle ground targets. The measured 3 dB beamwidth was found to be 94°.

2. Design of the Phased Array

This section introduces and analyzes the proposed phased array in three parts. First, a radiation-phase reconfigurable dipole antenna (RPRDA) is designed as the array element. This antenna achieves two operating modes with a 180° phase difference by switching PIN diodes. Second, the overall configuration of the phased array and the structure of the fixed-phase feeding network are illustrated and described. Finally, the array factor of the phased array is analyzed for several representative beam steering directions.

2.1. One-Bit Radiation-Phase Reconfigurable Dipole Antenna

For ground target detection applications, the antenna element’s beamwidth must not be too narrow. We chose the dipole antenna as the array element because it exhibits a wide beamwidth in the H plane, providing an advantage when detecting large deflection angles. The artificial magnetic conductor (AMC) surface can emulate an ideal magnetic wall, enabling the design of a patch dipole antenna [10]. The reflected wave from the AMC surface will superimpose with the antenna’s direct radiation, preserving the wide beamwidth of the dipole. We designed the RPRDA based on the reflective properties of an AMC surface. It exhibits a broader beamwidth than conventional microstrip antennas. Furthermore, the 1-bit phased array constructed with this reconfigurable antenna eliminates the need for conventional continuous phase shifters, thereby achieving cost reductions in fabrication. Compared to more complex digital phase control strategies, the 1-bit digital phase control offers application advantages such as a simpler structure and more compact size.

The proposed antenna operates at 17 GHz and employs the Rogers RT/Duroid 5880LZ substrate (ε_r = 2.0, tanδ = 0.0027). The model of the RPRDA is illustrated in Figure 2. The top layer comprises dipole patches, where two PIN diodes aligned in the same direction connect the three segmented patch sections. The PIN diodes are the commercially available MADP-000907-14020. Each section includes metallic vias for signal feeding: the upper and lower vias are used for DC bias injection, while the central via is used for radio-frequency (RF) signal input. The middle layer of the antenna structure comprises an AMC surface composed of periodic square units, with each AMC unit connected to a grounded metallic via at its center. To enhance the stability of the AMC surface’s reflection properties, the RPRDA is positioned within the inter-unit gaps of the AMC structure.

To ensure that the AMC surface exhibits a reflection phase equal to the incident phase—mimicking an ideal magnetic wall at 17 GHz—several key structural parameters affecting the reflection phase must be optimized: the unit width $w$, gap width $g$, and slot width $d$. Subsequently, the dimensional parameters of the RPRDA, including the width $w_a$, length $l_a$, and PIN diode offset distance from center $d_a$, were optimized to maximize the radiation efficiency and improve the radiation patterns. The antenna structure was modeled and simulated using the CST Studio Suite. The optimized parameters are summarized in

Table 1. Dimensions of the RPRDA and AMC (in mm).

Parameter	Value	Parameter	Value
$w$	3.0	$w_a$	0.6
$g$	0.4	$l_a$	8.3
$d$	0.2	$d_a$	0.7

.

This antenna achieves 1-bit radiation-phase control by toggling the ON/OFF states of two PIN diodes via DC bias voltages. As depicted in Figure 2a, the three antenna components are connected to three metallized vias (top, middle, bottom), with the central via functioning as the RF signal input port, whose DC reference potential is grounded (0 V). When a +5 V relative voltage is applied to both the upper and lower vias, the upper PIN diode switches to the OFF state, while the lower one turns ON. Conversely, when a −5 V relative voltage is applied, the diode states reverse: the lower diode becomes OFF and the upper one turns ON. These two distinct states induce opposite current directions on the dipole, resulting in a 180° phase difference in the radiated field.

2.2. Structure of the Phased Array and Fixed-Phase Feeding Network

The proposed phased array shown in Figure 3 is constructed using the RPRDA elements. The phased array comprises 16 antenna elements arranged in a 2 × 8 configuration. The element spacing should be constrained to ≤0.5λ₀ to control sidelobe degradation at large scan angles. Based on this consideration, the antenna array was configured with inter-column spacing of 3 AMC unit widths and inter-row spacing of 2 AMC unit widths. Since the width of an AMC unit is 3 mm, the row and column antenna spacing is 9 mm (0.51λ₀) and 6 mm (0.34λ₀), respectively. The multilayer structure integrates the antenna array, AMC surface, common ground layers, and feeding network, which includes both the RF fixed-phase feeding network and the DC bias lines. The layers are interconnected throughout the structure by metallic vias. The overall structure has a thickness of less than 2.5 mm.

The DC bias lines (±5 V) in the feeding network layer regulate the PIN diodes’ operating states. For the proposed design, antennas within the same column share a common DC bias line since they are configured to operate in identical states. Consequently, the array requires only 8 DC bias lines to independently control the 8 antenna columns. Fan-shaped structures on the DC lines act as 17 GHz band-stop filters, preventing RF interference. In practical applications, these DC bias lines are externally wired via conductive leads and interconnected to dedicated DC bias control circuit boards.

Compared to phased arrays with continuous phase shifting, 1-bit phased arrays exhibit significant cost advantages. However, their beam scanning performance is often compromised. In this work, we propose an appropriate fixed-phase excitation for each antenna column, enabling the 1-bit phased array to meet the beam scanning requirements for the detection of ground targets at large deflection angles.

The initial fixed phases follow a repeating pattern of 0° and 90°, assigned sequentially across the eight columns: 0°, 90°, 0°, 90°, 0°, 90°, 0°, and 90°. Combined with the two reconfigurable states of the antenna elements (0° and 180°), the selectable phases for each column are summarized in

Table 2. Selectable phases for each antenna column.

Column	1	2	3	4	5	6	7	8
Phase (degree)	0/180	90/270	0/180	90/270	0/180	90/270	0/180	90/270

.

The fixed-phase feeding network delivers specified power and phase distributions to each antenna element through the following process: the RF signal enters the bottom layer and splits into eight equal-power in-phase signals via a power divider. Each signal is further divided by a directional coupler in the feeding network, generating two outputs with a 90° phase difference. This results in 16 outputs. The array maintains uniform received power across all antenna elements, with the even-numbered columns exhibiting a 90° phase lag relative to the odd-numbered columns.

2.3. Array Factor

The array factor is calculated by modeling each antenna element as an isotropic source, with inter-element spacing of 0.34λ₀. To steer the main beam of the array toward the target direction θ₀, the ideal excitation phase ϕ_n for each element is given by [33]

$${\varphi }_n=-k_0\times d\times \mathit{sin}{\theta }_0\times \left(n-1\right),n=1,2,\dots ,N$$

(1)

where k₀ denotes the free space wavenumber, d, equal to 0.34; λ₀, in this case, is the spacing between two adjacent elements; n is the index of each array element; and N is the total number of elements in the array, which is eight for the phased array under consideration.

For the proposed 1-bit phased array with fixed-phase feeding, the beam steering capability is discrete rather than continuous. According to Equation (1), the optimal excitation phase for each element under a given beam direction can be determined, with phase values distributed in the range of 0° to 360°. The state of each column of reconfigurable antennas is then determined based on the ideal excitation phase ${\varphi }_n$, as given by the following equations:

$${{\varphi }_n}^{\prime }=\left\{\begin{array}{cc}0°\hfill & \hfill 0°\le {\varphi }_n<90°, \mathrm{o}\mathrm{r} 270°\le {\varphi }_n<360°\hfill \\ 180°\hfill & \hfill 90°\le {\varphi }_n<270°\hfill \end{array}\right.$$

(2)

$${{\varphi }_n}^{\prime }=\left\{\begin{array}{cc}90°\hfill & \hfill 0°\le {\varphi }_n<180°\hfill \\ 270°\hfill & \hfill 180°\le {\varphi }_n<360°\hfill \end{array}\right.$$

(3)

Equation (2) is applied to odd-numbered columns, while Equation (3) is used for even-numbered columns.

From numerous possible state combinations of the antenna array, 13 practical quantized beam directions were chosen. Among these, we further identified seven more widely distributed beam directions for comparative analysis, specifically 0°, 15°, −15°, 30°, −30°, 45°, and −45°. The normalized array factors for these directions were theoretically calculated and are shown in Figure 4. The array factor reaches its maximum at ±47°, showing a 3 dB enhancement compared to the 0° direction. This indicates that the array achieves a longer detection range at deflection angles of ±47°, which aligns with our intended application requirements.

3. Results

3.1. Simulation Results

We simulated the radiation pattern and reflection coefficient ($\left|S_{11}\right|$) of the RPRDA described in Section 2.1 using CST. For the antenna with a 12 × 20 AMC unit configuration, the results are shown in Figure 5. The simulated antenna exhibits radiation efficiency of 76% and total efficiency of 74%. The antenna achieves a peak gain of 6.1 dBi, with a 3 dB beamwidth of 128° in the H plane (xoz plane). In comparison, the 3 dB beamwidth of a conventional microstrip antenna is less than 90°. Furthermore, CST simulations confirm that the two states of the RPRDA exhibit identical radiation patterns and a 180° phase difference.

As seen in Figure 5b, showing the $\left|S_{11}\right|$ of the RPRDA, the antenna resonates at 17.0 GHz with a reflection coefficient of −28 dB. Since the AMC surface exhibits resonance near 19 GHz, the operational bandwidth is estimated based on the lower-frequency side of the main resonance. The −10 dB bandwidth spans 1.7 GHz on the lower-frequency side of the resonant frequency. By assuming symmetrical bandwidth characteristics, the total operational bandwidth is estimated as 3.4 GHz, corresponding to a relative bandwidth of 20%.

Subsequently, we conducted simulations of the entire phased array. The phased array system is fed with a 17 GHz RF signal and eight independent DC bias signals to achieve beam scanning in the H plane. The radiation patterns for the aforementioned 13 beam directions were simulated in CST, with the H-plane scanning results integrated to produce Figure 6. In the figure, 0° on the horizontal axis corresponds to the array’s +z-axis direction, while 90° represents the array’s +x-axis direction. The simulation results demonstrate that the phased array achieves a peak gain of 11.8 dBi at ±45° and a scanning 3 dB beamwidth of 114°, which corresponds to a 2.2 dB gain enhancement compared to the vertical direction gain of 9.6 dBi. As demonstrated by the array factor, the peak gain is achieved at larger deflection angles rather than in the vertical direction. In the simulation, the feeding network introduced approximately 0.9 dB of insertion loss.

Table 3. Simulated insertion loss budget for the feeding network components.

Component	Insertion Loss (dB)
Power divider	0.56
Directional coupler	0.23
Others	0.11
Total	0.90

details the loss contributions from each component.

In the CST simulation of the array, DC bias feeding was implemented through CST port excitation, rather than physically modeling the practical DC bias control boards. In actual manufacturing, we will appropriately expand the area of the array to accommodate components such as such as SMA and FPC connectors, with corresponding microstrip or strip lines added for proper interconnection. Additionally, the aircraft fuselage’s ground plane was not included in the simulation.

All simulation work involved in this paper was carried out using CST Studio Suite 2022.

3.2. Measurement Results and Analysis

Based on our design, a prototype of the antenna array was fabricated and tested in an anechoic chamber to evaluate its beam scanning performance, as shown in Figure 7. The test details are as follows: the anechoic chamber dimensions were 13 m × 7.3 m × 4.6 m, with a horn antenna positioned on the same horizontal plane as the test array. The receiving antenna was placed 5 m away from the array, satisfying the far-field condition.

The DC bias switching process operates as follows: first, control commands are sent to the board’s microcontroller unit via an Ethernet interface; then, an I2C signal is converted into eight independent high-/low-level outputs using a serial-to-parallel chip. These outputs control analog SPDT switches, selecting either +5 V or −5 V for each bias line.

The measured results are presented in Figure 8. Compared with the simulated patterns, although some performance metrics demonstrate degradation in the measurements, the overall shapes of the radiation patterns show good agreement. The specific measurement results are as follows: the phased array exhibits a peak gain of 9.2 dBi at ±37°, with a scanning 3 dB beamwidth of 94°. Notably, both the simulation and measurement results confirm that the array achieves a significantly higher gain at the largest deflection angles compared to the vertical direction, consistent with the original design objective.

We compared the detection range at 37° between our antenna array and a conventional antenna array. The increase in detection range at ±37° can be calculated using the radar range equation. A comparison with a conventional phased array of equivalent peak gain, as shown by the red curve in Figure 8, shows a 2.5 dB increase in gain at the largest beam deflection angle of ±37°. Based on the functional relationship between detection range $R_{max}$ and antenna gain $G_t$ in the radar equation,

$$R_{max}\propto \sqrt[4]{G_t}$$

(4)

The proposed antenna array achieves a 16% improvement in the effective detection range over conventional phased arrays at the deflection angle of ±37°.

Compared with the simulation, the measured results show a noticeable reduction in the gain and largest deflection angle. In future work, we aim to improve the array gain by reducing the losses caused by soldering and PIN diode switching and to enhance the largest deflection angle by optimizing the radiation patterns of the unit elements and the mutual coupling between antennas. Furthermore, different array factor characteristics of the array can be realized by adjusting the current 90°/0° fixed-phase feed network, changing its feed phase, or expanding to digital phase control, and this is expected to further expand its application scenarios.

4. Conclusions

This work presents a 1-bit, 16-element phased array designed for Ku-band radar for ground detection applications. The novelty of this work lies in the replacement of conventional phase shifters with 1-bit radiation-phase reconfigurable dipoles and a fixed-phase feeding network, which not only significantly reduces the system cost but also mitigates gain overflow in the vertical detection direction and enhances the gain at the maximum deflection angle. This represents a low-cost phased array radar solution to extend the detection ranges of ground targets. Measurement results show that the phased array achieves a 3 dB beamwidth of 94° and a peak gain of 9.2 dBi, with the peak gain occurring at ±37°. Compared to conventional phased arrays with the same peak gain, the proposed design achieves a 16% improvement in the detection range at the largest deflection angle.