# A Miniaturized Design for a Terahertz Tri-Mirror CATR with High QZ Characteristics
## 3 Simulation and Measurement Results
First, the simulations employed an idealized feed model with a cross-polarization of less than \(-50\) dB, which contrasts with practical feeds like corrugated horns that show higher cross-polarization, reaching \(-29\) dB during testing. Second, perfect electrical conductor (PEC) boundary conditions were applied to all mirrors and fixtures. However, the actual surface roughness (with an RMS of approximately 12 \(\upmu\)m) and the finite conductivity of aluminum/stainless steel materials were not considered in the model, potentially resulting in underestimated losses and edge diffraction. Third, the electromagnetic (EM) model omitted brackets and fixtures, and while their scattering effects were partially reduced experimentally using absorbers (as shown in Figure 7), this simplification still affects the accuracy. Fourth, at lower frequencies below 200 GHz, the Physical Optics and Theory of Diffraction algorithms may fail to adequately model complex multi-path interactions. Finally, the simulations assumed an ideal alignment, while the prototype assembly introduced positional errors of around 20 \(\upmu\)m and phase center misalignments, further deviating from the idealized simulation conditions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Frequency** & **Co-Polar Amplitude** & **Co-Polar Phase** & **Cross-Polar Amplitude** \\ **(GHz)** & **Ripple (dB)** & **Ripple (\({}^{\circ}\))** & **(dB)** \\ \hline
100 & 1.56 & 9.49 & \(-44.05\) \\
183 & 1.36 & 7.45 & \(-44.52\) \\
275 & 0.95 & 6.39 & \(-44.57\) \\
300 & 0.85 & 6.31 & \(-43.73\) \\
400 & 0.68 & 5.66 & \(-43.78\) \\
500 & 0.62 & 4.14 & \(-44.17\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The simulated QZ characteristics of the developed miniaturized tri-mirror CATR for major cuts (QZ diameter 0.14 m). To further validate the effectiveness of the proposed design methodology, the miniaturized tri-mirror CATR system was fabricated and experimentally tested. The simulation model shown in Figure 3 was positioned in a horizontal orientation, with the three mirrors and feed fixed on a common planar substrate via brackets for stable support. The feed was secured using a replaceable 'cap-style' aluminum alloy module. The physical prototype of the fabricated miniaturized tri-mirror CATR system is shown in Figure 7.  
All fixtures and planar substrates of the CATR prototype are fabricated from stainless steel, while the three mirrors are manufactured from aluminum alloy. Precision high-speed milling and mirror-polishing processes are applied to the aluminum alloy blocks, achieving a root mean square (RMS) surface accuracy of approximately 12 \(\upmu\)m and a surface roughness of 1 \(\upmu\)m for the main mirror. Meanwhile, the relative positional errors of the three mirrors during assembly are controlled within approximately 20 \(\upmu\)m. During system testing, to reduce interference from stray waves reflected by the stainless-steel substrates and fixtures on the system's QZ, a layer of microwave-absorbing material is applied to the surfaces of planar substrates and fixtures to partially absorb stray signals, as shown in Figure 7.  
The feed source for the system employs a laboratory-available sin-squared/horizontal linear dual-profile corrugated horn [33]. This horn feed features a highly symmetric radiation pattern, relatively low side-lobe levels, and a phase center located near the horn's aperture plane at the center operating frequency, facilitating position fixation and alignment during actual testing [34]. Figure 7(a) and Figure 7(b), respectively, show fabricated corrugated horn feeds with center operating frequencies of 183 GHz and 275 GHz; where the feeds taper, both satisfy \(-16\) dB attenuation at 15\({}^{\circ}\). These horns were manufactured using an electroforming method and underwent surface gold-plated treatment.