# A Wideband Termination Based on Laser-Scribed Lossy Microstrip Line Structures
## 2 Structural Design
### Open-End Configuration  
CST Studio Suite (CST) simulations are conducted to demonstrate the radio-frequency performances of the proposed device. The return loss \(\ket{S_{11}}\), which implies the reflected wave at the input port, can be determined as [21]  
\[\ket{S_{11}}=\left.\frac{V_{1}^{-}}{V_{1}^{+}}\right|_{V_{2}^{+}=0} \tag{1}\]  
where \(V_{1}^{+}\) is the amplitude of the voltage wave incident on the input port, \(V_{1}^{-}\) is the amplitude of the voltage wave reflected from the input port, and \(V_{2}^{+}\) is the amplitude of the voltage wave incident on the output port. The insertion loss \(\ket{S_{21}}\), which implies the transmitted wave at the input port, can be determined as [21]  
\[\ket{S_{21}}=\left.\frac{V_{2}^{-}}{V_{1}^{+}}\right|_{V_{2}^{+}=0} \tag{2}\]  
where \(V_{2}^{-}\) is the amplitude of the voltage wave reflected from the output port.  
Figure 2a shows the \(\ket{S_{11}}\) variations in the open-ended configuration (i.e., the RF signal inputs from the one-port while the other one keeps \"open\"). Firstly, the significant drops of \(\ket{S_{11}}\) for the samples with rectangle and tapered cases confirm the efficiency of the attenuation through implantation of resistive films. Secondly, the decreased \(\ket{S_{11}}\) at higher frequencies illustrate the high-pass characteristic of the design. Thirdly, the tapered termination shows a notably lower \(\ket{S_{11}}\) than that in the rectangle case, approximatively 10 dB at above 11.5 GHz, demonstrating that the tapered shape is more preferred for the termination propose.  
Additionally, the length of the resistive film should be considered since the microwave decays exponentially along the propagation direction. In the opened configuration, the microwave signal is attenuated twice due to the reflection at the \"open-end\". Therefore, the signal can be eliminated before the reflected signal reaches the input port. This can be ascertained from Figure 2b, in which the \(\ket{S_{11}}\) comes near to \(-25\) dB at a resistive film length above 20 mm and above 7 GHz.  
The sheet resistance of the rGO film, as shown in Figure 3a, also determines the reflection at the input port. A moderate sheet resistance leads to a typical sub \(-20\) dB \(\ket{S_{11}}\), value at a higher frequency. On the other hand, the over-large sheet resistance of 20 k\(\Omega\)/\(\square\), or small sheet resistance from 2 \(\Omega\)/\(\square\) to 10 \(\Omega\)/\(\square\), leads to a tremendous increase in re  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Width of the strip conductor, \(w_{1}\) & 1.15 mm \\ Width of the rGO resistive film, \(w_{2}\) & 11.70 mm \\ Length of the rGO resistive film, \(l\) & 30 mm \\ Thickness of the dielectric substrate, \(t\) & 0.508 mm \\ Thickness of the rGO film resistor, \(t_{t}\) & 10 \(\mu\)m \\ \hline \hline \end{tabular}
\end{table}
Table 1: The parameters of the termination.  
flection to over \(-7\) dB, due to both insufficient attenuation and impendence mismatch at the discontinuities.  
Note that, for our design with an overlapped resistive film and the strip conductor, the sheet resistance can be varied in two orders of magnitude, ranging from 10 \(\Omega/\square\) to 1.2 k\(\Omega/\square\), with low reflections. However, this range is small for termination without the overlapped strip conductor, as shown in Figure 3b. For the latter case, the sheet resistance can be chosen only in a narrow range of around 10 \(\Omega/\square\). Hence, our design with an overlapped structure does not need a strict fabrication process that produces the resistive film with a certain sheet resistance. This is possible to enhance the tolerance of fabrication errors and reduce fabrication difficulties.