# An Improved PIN Diode Model Design for a Tunable Frequency Selective Absorber
## 1 Introduction  
In recent years, frequency selective surfaces (FSS) have gained widespread adoption across both military and civilian sectors due to their remarkable ability to selectively transmit or block electromagnetic waves. These surfaces play a pivotal role in a variety of advanced technologies, including radar systems, antenna radomes, and space electromagnetic filters, and in the modification of absorbing materials [1, 2, 3, 4]. In particular, the reduction in the radar cross-section (RCS) of antenna radome systems has become a critical aspect of stealth applications. This is because radome systems, which are essential for protecting antennas from external environmental factors, often scatter electromagnetic waves in unwanted directions [5]. By utilizing FSS-based radomes, it is possible to reduce RCS by reflecting electromagnetic waves outside their operating frequency band, effectively decreasing monostatic RCS [6, 7, 8, 9]. However, this approach does have its drawbacks. The reflection of waves at certain angles can unintentionally increase bistatic RCS in other directions, creating potential vulnerabilities in stealth systems. In order to mitigate this issue, the development of frequency selective absorbers (FSA) has been proposed. Unlike FSS, these absorbers reduce bistatic RCS by absorbing out-of-band waves rather than reflecting them, making them a highly effective solution for reducing electromagnetic interference and improving overall stealth performance [10]. With the continuous advancement of communication systems, which are becoming increasingly complex and demanding in terms of performance, the role of active frequency selective surfaces (AFSSs) has gained significant importance in recent years. AFSSs are a subset of FSSs that incorporates active components, allowing for dynamic and tunable electromagnetic responses, which makes them more adaptable and versatile [11; 12; 13]. In line with the growing demands of modern technology, various tunable FSS designs have been proposed to address these challenges. For example, in [14], a mechanically tunable absorber was introduced, capable of shifting the absorption frequency by altering the physical shape of the absorber. Similarly, in [15], a novel meta-atom structure based on a split-ring resonator (SRR) embedded in a stretchable polymer was presented. This design allowed for dynamic tuning of electromagnetic properties through mechanical stretching, offering a new way to control the absorber's behavior. Despite their promising potential, mechanical control methods for FSSs have certain limitations. While they do not require a bias network, they suffer from slower response times and more restricted frequency tuning ranges compared to electronic methods. To address these shortcomings, the integration of active components, such as PIN diodes, varactor diodes, or variable inductors, onto FSSs has emerged as a viable solution. By loading these active elements, the impedance of the surface can be adjusted dynamically, enabling the tuning of the absorber's properties. This approach offers several advantages, including fast response times, a compact design, and a broader frequency tuning range, which makes it highly suitable for applications that require rapid and reliable frequency control [16; 17]. For instance, a UHF tunable absorber demonstrated the ability to achieve a reflectivity of less than \(-10\) dB over a frequency range of \(0.68\) GHz to \(2.13\) GHz by varying the voltage applied to varactor diodes [18]. While these tunable absorbers offer significant advantages, they also introduce new challenges. The integration of a bias network, necessary for controlling the active components, can sometimes interfere with the frequency response and reduce the overall efficiency of the system. Additionally, many tunable absorbers currently on the market only achieve around \(90\%\) absorption, which may not be sufficient for applications requiring high-performance absorption, such as stealth technology.