# An Improved PIN Diode Model Design for a Tunable Frequency Selective Absorber
## 2 Unit Design Process
### Basic Unit Design  
Most of the PIN diodes are used in tunable frequency selective absorbers only by changing the resistance value, so the initial design of a frequency selective surface absorber is based on variable resistance. The proposed tunable absorber unit, shown in Figure 3a, consists primarily of two symmetrical, round-headed arrow shapes with connected ends. The structure has a sandwich-like design, featuring an upper F4 BM dielectric substrate layer (dielectric constant of 2.2, loss tangent of 0.0007, and thickness, \(t\)). The FSS layer, which incorporates PIN diodes, is etched onto the lower surface of the dielectric substrate. A metallic plane forms the bottom layer, with a uniform-thickness air spacer (\(h\)) positioned between the FSS layer and the metallic plane.  
The tunable frequency selective absorber was simulated and optimized using the High Frequency Structure Simulator (HFSS). Master-slave boundaries and a Floquet excitation port were employed to simulate the unit cell as a periodic structure. In the HFSS, active devices under different bias voltages were modeled using modified lumped RLC boundary conditions. The unit cell consists of a half-ring with an outer radius, \(r\), and a radius difference, \(d\), two rectangular elements (length, \(d\), and width, \(e\)) connected to the semi-ring, and a strip of width, \(w\). The resistance boundary has a length of \(g\). The optimized dimensions are \(m=n=21.4\) mm, \(r=7.1\) mm, \(d=4.3\) mm, \(e=2.1\) mm, \(w=1\) mm, \(g=1.5\) mm, \(t=0.8\) mm, and \(h=7.7\) mm.  
Figure 3b shows the simulation results of the reflection coefficients for different resistance values under normal incident plane wave conditions. At a resistance of \(5\)\(\Omega\), the reflection level stays above \(-3\) dB across the 4-14 GHz range, indicating minimal absorption. When the resistance increases to \(20\)\(\Omega\), the reflection curve shifts downward, with the  
Figure 2: The result of reflectance varying with impedance.  
reflection coefficient dropping to \(-11\) dB at 4.3 GHz, marking a significant improvement in absorption performance. With 60 \(\Omega\), the reflection coefficient decreases further, showing two minimums at 4.7 GHz and 9.2 GHz with values of \(-19\) dB and \(-19.7\) dB, respectively, further enhancing absorption capabilities. At 100 \(\Omega\), the resonance points converge toward the center of the frequency range, producing reflection coefficients below \(-10\) dB, between 4.9 GHz and 10 GHz.  
Given the metallic plane at the base of the absorber, the absorption rate can be calculated as \(1\,-\,|\,S_{11}\,|^{2}\), indicating absorption rates exceeding 90% within this frequency range. However, as the resistance rises to 150 \(\Omega\), the 90% absorption bandwidth narrows to 3 GHz, spanning from 5.2 GHz to 8.2 GHz. These results demonstrate that the reflection coefficient and absorption rate of the absorber/reflector are highly tunable by adjusting the resistance. Lower resistance values result in higher reflection levels, while higher resistance values greatly enhance absorption, though very high resistance reduces the absorption bandwidth.