The substrate-integrated waveguide (SIW) is known for its outstanding performance, having numerous advantages such as a low profile, strong compatibility, and high Q factors. Graphene, a two-dimensional carbon nanomaterial, excels at microwave frequencies due to its ability to adjust surface impedance through external voltage adjustments. This allows significant change in its resistivity, supporting the theoretical basis for graphene’s use in tunable microwave components. To meet the future objective of lightweight, thin, broad, and strong microwave absorbers, utilizing graphene’s unique properties in absorption can broaden bandwidths, enhance tunability, and reduce profile thicknesses. Despite graphene’s exceptional electronic properties, its performance at millimeter wave frequencies is influenced by factors such as lattice scattering, necessitating further optimization to meet the demands of microwave devices. Consequently, designing graphene microwave devices represents a challenging endeavor. Current research has yet to fully meet the growing demands of absorbers that offer broadband capabilities, miniaturization, low-profile design, and flexible control. We apply graphene materials to the microwave frequency range and explore ultra-wideband absorbing devices utilizing SIW structures, achieving dynamic control over absorption amplitude and frequency.

We explore the use of graphene materials in millimeter wave devices. Building on the electromagnetic properties of graphene, an innovative SIW structure is incorporated to etch the graphene into periodic patterns. By adjusting the external bias voltage, the chemical potential and conductivity of the graphene can be controlled, thereby influencing its uniform square resistance. To enhance simulation efficiency, graphene is conceptualized as a two-dimensional static resistive film and designed as an impedance boundary condition using the Impedance Boundary operation, with the graphene square resistance as the key simulation parameter. The design process employs HFSS simulation software to model the absorbing structure, utilizing master-slave boundaries as periodic boundary conditions to simulate an infinite planar periodic array. The port excitation is set to Floquet, with the electromagnetic wave incidence perpendicular to the surface of the absorber. The reflective layer employs low-resistance ITO material for a metallic-like reflection effect. A low-profile ultra-wideband absorber with dynamically adjustable frequency and absorption amplitude has been developed, offering excellent switching characteristics and validated through simulation. It achieves an ideal broadband range and variable absorption rate, offering innovative approaches and solutions for the use of graphene materials in microwave device applications.

This device operates in the microwave frequency band. When the square resistance is low, the device exhibits dual frequency absorption. When the square resistance changes to 70 Ω/sq, impedance matching is achieved with free space, resulting in two absorption peaks at 19.5 GHz and 40 GHz. In the frequency range of 15.76?41.73 GHz, the absorption rate exceeds 90%, with a relative bandwidth of 90.35%, achieving ultra-wideband absorption. The overall thickness is only 1.66 mm, and it has ultra-wideband absorption and low profile characteristics [Fig. 5(a)]. When the square resistance is within the range of 40?60 Ω/sq, the real and imaginary parts of the equivalent impedance perfectly match the optimal value near 18 and 40.3 GHz, leading to complete absorption of the incident wave. Similarly, when the square resistance increases to the range of 130?190 Ω/sq, both the real and imaginary parts of the equivalent impedance match the optimal value at 31 GHz, resulting in full absorption characteristics. As the square resistance gradually increases from 140 Ω/sq, the reflected wave increases, the absorption rate gradually decreases, and the absorption mode transitions to the reflection mode. By adjusting the square resistance variation, the device effectively achieves frequency dynamic tuning in the frequency range of 17?40.3 GHz, adapting to different frequency band requirements [Fig. 5(b)]. The designed absorber exhibits wide-angle absorption characteristics, maintaining good absorption stability with changes in polarization angle [Fig. 6(a)]. As the incident angle increases, the absorption performance generally shows a downward trend, but when the incident angle is kept below 60°, the absorber can maintain an ultra-wideband absorption performance of more than 80% in the operating frequency band [Fig. 6(b)]. Further assembly into a 2×2 array (Fig. 8) shows two absorption peaks at 19.5 and 38.7 GHz, with an absorption rate exceeding 90% in the range of 15.77?41.33 GHz, achieving ultra-wideband absorption (Fig. 9). Numerous combinations of bias voltages loaded between graphene and bottom ITO allow arbitrary modulation of the absorption bandwidth.

An SIW absorber featuring ultra-thin and ultra-wideband properties is developed using the HFSS software system and based on the finite element algorithm, marking a significant improvement in relative bandwidth compared to similar absorbers. Simulation results indicate that this design boasts advantages such as low profile, miniaturization, and strong integration capabilities, along with excellent dynamic tunability and strong absorption characteristics. This research offers greater flexibility and convenience in designing dynamically tunable devices using graphene, making it highly applicable across sectors such as smart absorbers, photovoltaic devices, and tunable sensors. In addition, it serves as a foundational exploration for the future applications of graphene in mobile communications and radar technologies.