Electrically Reconfigurable Antenna Array with Cross-Polarization Suppression Technique in the Ka-Band Using PIN Diodes

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J. Electromagn. Eng. Sci. 2025;25(6):603-609

Publication date (electronic) : 2025 November 30

doi : https://doi.org/10.26866/jees.2025.6.r.332

Nguyen Van Thang, Jae-Young Chung

Department of Electrical and Information Engineering, Seoul National University of Science and Technology, Seoul, Korea

^*Corresponding Author: Jae-Young Chung (email: jychung@seoultech.ac.kr)

Received 2025 January 10; Revised 2025 March 26; Accepted 2025 April 20.

Abstract

I. INTRODUCTION

The application of reconfigurable antenna arrays (RAAs) in millimeter wave-based communication systems, particularly in the Ka-band, has recently gained significant attention, driven by the increasing demand for high data rates. However, cross-polarization in these antennas often leads to signal degradation and unwanted interference from different polarization modes, impacting the overall performance of communication systems. Therefore, minimizing cross-polarization in dual-linearly-polarized antenna arrays is critical to ensure efficient and reliable millimeter-wave (mmWave) communication.

To efficiently switch between linear polarization modes, various antenna architectures have been investigated over the years, including those based on dual-feed ports [1, 2], micro-electromechanical systems (MEMS), radio frequency (RF) chips, and PIN diodes. However, each of these approaches has its own advantages and disadvantages. For instance, dual-feed configurations are frequently employed in lower frequency ranges, such as the S-band, because of their durability and appropriateness for larger elements within an antenna array [3]. However, it is difficult for them to adapt to the reduced sizes necessary for Ka-band applications with two traditional ports, thus limiting their use in linear antenna arrays [4]. Alternatively, to deal with a planar array, a modern RF chip attached in a package structure along with a complicated multilayer design has been proposed, but this method introduced its own set of complexities [5, 6]. Meanwhile, PIN diodes are widely used to generate linearly polarized radiation across a broad frequency range, starting from low RF frequencies to approximately the X-band [7–9]. Their popularity stems from their compact size, affordability, ease of integration, control of printed circuit boards (PCBs), and high isolation in the OFF state.

Achieving a higher or lower cross-polarization discrimination (XPD) relies on having sufficient isolation in the OFF state of the PIN diode. However, at higher frequencies, particularly in the Ka-band, a single PIN diode may not be able to block RF currents effectively. In such cases, instead of focusing on achieving linear polarization switching, the PIN diode is often used to control the current flow on the antenna patch and to induce phase shifting [10, 11]. Although other switching devices, such as the SPST diode, offer excellent isolation in the OFF state, implementing a dual-linearly-polarized reconfigurable system remains a significant challenge. One of the main difficulties is the small antenna size, which limits the incorporation of an additional DC bias line into the antenna configuration to control the diode.

Drawing on these observations, we propose a cost-effective approach for constructing a compact dual-linearly-polarized antenna array with high XPD in the Ka-band using PIN diodes. To achieve this, we first implemented the image configuration technique to suppress cross-polarization in the dual-linearly-polarized array using two PIN diodes. Notably, the use of a dual-feed structure when implementing this method has been extensively studied in various papers [12–14]. By positioning the PIN diodes at specific positions corresponding to the 0° or 180° port phases, we configured the proposed single-port antenna elements into various 2×2 subarray arrangements to examine cross-polarization degradation. Subsequently, we evaluated the performance of a 4×4 antenna array by conducting simulations and experimentation using an external beamformer, thereby validating the reduction in cross-polarization while ensuring that the effect remains consistent during beam steering.

II. DUAL LINEAR POLARIZATION ELEMENT DESIGN

Fig. 1 depicts the geometrical structure of the proposed antenna element, with its dimensions measuring 6 mm × 6 mm × 1.076 mm. The antenna comprises four metallic layers on two identical Rogers RT/duroid 5880 substrates (with ε_r = 2.2 and tanδ = 0.0009) that are bonded together by an adhesive layer of Copper Clad Laminate DS-7402 (BS #1067, with ε_r = 3.7 and tanδ = 0.02). The patch antenna is located on the top metal layer (L1) and fed by a metalized via (diameter, 500 μm) positioned at the center. For polarization switching, two PIN diodes (MACOM MADP-000907-14020W Flip Chip) are embedded into the patch antenna. They are controlled by a bias line printed on the second metal layer (L2) of the first substrate.

Fig. 1

Geometry of the antenna element: (a) PCB stack-up, (b) top metal layer (L1), and (c) second metal layer (L2).

A bias network consisting of microstrip radial stubs is connected to the patch by metalized vias to ensure high RF signal isolation. Ground connection is established using a short-circuit stub (L2), connected to the ground plane using metalized vias (from L1 to L4). Notably, in addition to its role as the patch antenna ground (L3), the second substrate also serves as the ground for the SMPM connector (L4). Notably, the PIN diode model employed in this study, adopted from previous on-PCB measurements [15] and the MACOM PIN diode datasheet, consists two series circuit (one with R_ON = 4.2 Ω and L_ON = 0.05 nH, and the other with L_OFF = 0.05 nH and C_OFF = 42 fF) in the forward (I_bias = 10 mA) and reverse (V_bias = −1.2 V) states, respectively. All relevant geometrical dimensions pertaining to the proposed antenna element are summarized in Table 1.

Table 1

Dimensions of the unit cell (unit: mm)

The two PIN diodes were configured in opposite states for each polarization mode through a shared bias line, as shown in Fig. 1. When PIN diode D1 is in the ON state, D2 is forced to be in the OFF state, the O-slot is short-circuited at one of its ends, and the patch antenna radiates horizontally polarized outgoing waves. Conversely, for the vertically polarized radiated wave, PIN diode D2 is switched ON. Furthermore, Fig. 2 clearly depicts the current distribution on the edge and main bars, producing corresponding polarizations. The E-field of the two operating modes exhibits stronger radiation on the sides without the activated PIN diodes. Fig. 3 presents the simulated S₁₁ and the co- and cross-polarization performance of the antenna based on two outgoing wave modes at 29 GHz. The proposed antenna achieved a 5 dB realized gain, an XPD of less than 18 dB, and a bandwidth of 1.2 GHz (−10 dB S₁₁) in both polarization modes. Nonetheless, a low XPD could pose many challenges for array implementation at other beam steering angles, particularly in scenarios where achieving high isolation is crucial to mitigating interference from the remaining polarizations in 5G communication systems.

Fig. 2

Single antenna visualization: (a) surface current distribution and (b) electric field distribution.

Fig. 3

Reflection coefficient and realized gain of the antenna element.

III. CROSS-POLARIZATION SUPPRESSION IN ANTENNA ARRAY

1. 2×2 Subarray Configuration

A well-known cross-polarization suppression technique for dual-polarized arrays is the use of dual-feed ports. Notably, this method has been analyzed in many previous studies [12–14]. In this study, based on a 2×2 subarray orientation of the antenna, several configurations were considered—horizontal ports, vertical ports, and both ports mirrored based on the horizontal and vertical planes. For each configuration, co-polarization radiation patterns of the mirrored elements were maintained in phase by applying a 180° phase shift to the mirrored ports, while the cross-polarization radiation patterns were 180° out of phase compared to the reference element inside the subarray.

It is important to note that the current flow in the approach mentioned above differs from that in the proposed antenna with PIN diodes. In antennas comprising two feed ports for generating orthogonal polarization, cross-polarization is primarily generated by the coupling between the H-port and the V-port, resulting in cross-polar fields. In contrast, cross-polarization in the proposed single-port antenna occurs due to structural gap coupling and leakage current through the PIN diode in its OFF state. Nevertheless, the principle of this technique allows the use of PIN diodes to cancel cross-polarization by mirroring the element.

Fig. 4 presents the configurations of the 2×2 subarray based on a single-port structure incorporating PIN diodes. The dots mark the locations of the feed probes, while the PIN diodes, labeled DH and DV, are quadratically placed to enable H- and V-polarizations, respectively. To ensure effective uniform excitation of all elements in the subarray, specific PIN diodes were activated, and the port was fed with either a 0° or 180° phase. The polarization marked “−” denotes 180° phase shift relative to the polarization marked “+.”

Fig. 4

Principle of the 2×2 element subarray configurations of the single port structure using PIN diodes: (a) Configuration A, (b) Configuration B, (c) Configuration C, and (d) Configuration D.

In general, the performance of the subarrays, as depicted in Fig. 5, is relatively consistent with the observations made in previous research on dual feed ports. Fig. 5(a) reveals that the conventional configuration (Configuration A) yields an XPD of merely 13 dB. In contrast, Configurations B and D (developed from Configuration B by reversing the PIN diode position for each element), wherein the elements were mirrored across both horizontal and vertical planes, exhibit excellent XPD surpassing 60 dB. However, as shown in Fig. 5(d), Configuration D has a drawback—broad undesired sidelobes in the vertical plane. This issue arises from the strong inward-radiating field within the antenna element, as observed in Fig. 2(b), which reduces the null depth. Meanwhile, the effectiveness of Configuration C is quite evident in Fig. 5(c), achieving a commendable XPD of 52 dB only in the horizontal plane, where axis mirroring of elements was employed. Overall, the outcomes of the above analysis underscore the superiority of Configuration B in effectively suppressing cross-polarizations in both planes while maintaining minimal sidelobes.

Fig. 5

Simulated radiation patterns of the 2×2 subarray configurations: (a) Configuration A, (b) Configuration B, (c) Configuration C, and (d) Configuration D.

2. 4×4 Antenna Array Configuration

To investigate cross-polar suppression in a larger antenna array as well as the beam steering scenario, we utilized the subarray element discussed previously to construct a 4×4 antenna array operating in the Ka-band. We also investigated phase and magnitude errors concerning cross-polarization suppression in antenna elements. It must be noted that accounting for these factors is imperative, especially since achieving a state of absolute phase and magnitude uniformity in practical prototypes can be extremely challenging. Fig. 6(a) shows the 4×4 antenna array developed from Configuration B of the 2×2 subarray. Notably, we also accounted for the minor discrepancy in magnitude and phase observed for Configuration B, which were within the ranges of (−1.3 dB, 1.3 dB) and (−22.5°, 22.5°), respectively. In comparison to the ideal scenario, the presence of magnitude and phase errors increased cross-polarization by approximately 28 dB, with the sidelobes increasing by approximately 1.5 dB, while maintaining an admirable level of XPD at 33 dB, coupled with a sidelobe level (SLL) of −12 dB.

Fig. 6

The 4×4 antenna array: (a) antenna array configurations and (b) simulated radiation patterns based on ideal (I) and error (E) magnitude and phase.

IV. MEASUREMENT RESULTS AND DISCUSSION

Fig. 7 depicts the antenna array under test in an anechoic chamber. For this analysis, we employed a beamformer, with its insertion loss within the operating frequency range effectively corrected to approximately 0 dB. The measured phase and magnitude of the 16 output ports exhibited variances of 35° and 2.5 dB, respectively, relative to each other. Furthermore, the antenna array was equipped with two main bias lines (DC1 and DC2) to control the PIN diode states for each individual element within the array. It was verified that the proposed antenna is capable of working in two linear polarizations, while also offering beam scanning capability.

Fig. 7

Antenna array prototype: (a) top view, (b) bottom view, and (c) the measurement setup.

Due to the limited sample size of the 4×4 antenna array, we focused on measuring the high directive beam from −30° to +30°, as shown in Fig. 8. The scanning ability of the array was restricted by fluctuating sidelobes and gain reduction. Nonetheless, good agreement was observed between the measured and simulated patterns for co-polarization. For the linear H- and V-polarizations, the peak gain reached 15.2 dB, and the XPD was about 25 dB. Although the observed XPD exceeded 20 dB, the cross-polarization deteriorated by roughly 10 dB compared to the simulated pattern. Notably, this degradation occurred within the operating frequency range, as shown in Fig. 9.

Fig. 8

Measured and simulated radiation patterns: (a) Co-pol of V-polarization, (b) Cross-pol of V-polarization, (c) Co-pol of H-polarization, and (d) Cross-pol of H-polarization.

Fig. 9

Measured and simulated broadside realized gains: (a) H-polarization and (b) V-polarization.

Fig. 6 confirms that cross-polarization suppression is highly sensitive to unidentical elements. This implies that although random magnitude and phase errors were accounted for in the simulation, the main factors contributing to performance degradation are manufacturing inaccuracies and imperfections, which occur particularly during the manual soldering of connectors and small PIN diodes (0.3 mm × 0.7 mm in size). At mmWave frequencies, these sensitivities become more noticeable, significantly affecting the effectiveness of the image configuration technique. As shown in Fig. 8, the scan loss at 30° was found to be approximately 3.5 dB in the orthogonal plane of active polarization and about 2.7 dB in the active polarization plane. Notably, this phenomenon, which resulted from the cancellation of mirroring elements, was predicted in the simulation. The SLL also experienced an imbalance when the beam steered to the orthogonally activated polarization plane, declining to −7 dB from −8.5 dB at ±30° in the activated polarization plane. The XPD was estimated to be more than 20 dB in the operating frequency range, along with a radiated efficiency of 72%, which is a good result compared to the other works reported in Table 2 [16–20].

Table 2

Comparison of the proposed antenna array with previous works

V. CONCLUSION

In this study, we examined a dual-linearly-polarized RAA in the Ka-band by employing the cross-polarization suppression technique. While the individual antenna elements exhibited a low XPD, the overall cross-polarization of the antenna array was effectively suppressed by mirroring the antenna elements with respect to the H- and V-planes. Furthermore, a 4×4 prototype was fabricated and subjected to experimentation in an anechoic chamber using a beamformer. The compact design of the antenna array and its efficacy in cross-polarization suppression underscore its potential for integration with contemporary Ka-band phase shifters. One such example is the use of a transmitarray with 1-bit phase quantization (0°/180°) that employs an O-slot structure with PIN diodes or an RF chip featuring a single port for each element, as opposed to a conventional dual-port setup.

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