# An Improved PIN Diode Model Design for a Tunable Frequency Selective Absorber
## 4 Experimental Verification  
To implement the tunable absorber, a suitable bias net microwave anechoic chamber is essential. It must not only provide a stable bias voltage for the PIN diodes but also minimize any impact the network might have on the original structure's response. The sample consists of an AFSS layer, a metal backplate, nylon washers, screws, and two bias wires. The overall dimensions of the structure are 361 mm \(\times\) 357 mm, with the longer side due to the inclusion of the bias network. The AFSS layer comprises a 15 \(\times\) 15 cell array, and the bias network, as shown in Figure 10, has a line width of 0.2 mm. In this configuration, each column of PIN diodes is connected in series, allowing for better control of the bias voltage and minimizing interference from unnecessary bias lines. EPCOS series inductors from TKD, with values of 1.8 nH, 2.7 nH, and 3.9 nH, are connected in series between the edge cells and the bias line to isolate high-frequency-induced currents from entering the bias network. PIN diodes are soldered in the middle of each unit. The air spacer is realized using nylon washers, which are free of electromagnetic interference. The testing environment, as shown in Figure 11, involves a pair of 1-18 GHz horn antennas connected to two ports of a vector network analyzer. One antenna serves as the transmitting antenna, while the other functions as the receiving antenna. Notably, the transmitting and receiving antennas are positioned adjacent to each other in the same  
Figure 9: Surface current distribution and electric field distribution of the tunable FSS absorber. (**a**) Surface current distribution, R = 150 \(\Omega\) and C = 0.1 pF; (**b**) electric field distribution, R = 150 \(\Omega\) and C = 0.1 pF; (**c**) surface current distribution, R = 10 k\(\Omega\); (**d**) electric field distribution, R = 10 k\(\Omega\). vertical plane and at the same height. To minimize coupling interference, an absorbing material is placed between the two antennas. The test sample is positioned within an absorbing wall made of absorbing materials to reduce edge diffraction effects. The distance between the antennas and the test sample satisfies the far-field condition, and the centers of the antennas are aligned with the center of the test sample at the same height. The detailed testing procedure is as follows:  
SOLT Calibration: Perform SOLT (Short, Open, Load, and Through) calibration on both ports of the vector network analyzer. Once calibration is completed, check the Smith chart on the analyzer. Successful calibration is indicated when \(S_{11}\) is located at the origin of the Smith chart, and \(S_{21}\) appears at the far-right endpoint of the real axis. Connect Antennas: Use high-frequency cables to connect the two ports of the vector network analyzer to the transmitting and receiving antennas, respectively. Set the working frequency band, test parameters, measurement steps, and transmission power on the analyzer. Figure 11: Experimental environment in a microwave anechoic chamber. Figure 10: Prototype of the fabricated tunable absorber. Metal Plate Measurement: Place a metal plate with the same dimensions as the test sample in the window of the absorbing wall. Record the reflection coefficient displayed on the vector network analyzer. Test Sample Measurement: Replace the metal plate with the test sample, keeping the environment unchanged, and record the measurement results. The final reflection coefficient of the test sample is calculated by subtracting the reflection coefficient of the same-sized metal plate from that of the test sample. Figure 12 shows the reflection coefficients at various bias voltages for the tunable absorber with embedded PIN diodes. As the voltage increases, the resonance frequency decreases. For example, at 12.1 V, the reflection coefficient drops below \(-15\) dB in the 7.7-9.4 GHz range, resembling the case with \(R=60\)\(\Omega\) and \(C=0.01\) pF shown in Figure 3b.