# A Miniaturized Design for a Terahertz Tri-Mirror CATR with High QZ Characteristics
## 1 Introduction  
With the rapid development of terahertz technology, the types of terahertz antennas have become increasingly diverse [1, 2, 3, 4, 5]. As an important means of accurately measuring the radiation characteristics of antennas, CATR systems not only generate quasi-planar waves within a confined space to meet measurement requirements but also eliminate the need for long distances inherent in far-field testing [6, 7]. These capabilities have driven increasing demand for CATR systems in evaluating the performance of various terahertz antennas. At present, most CATR systems feature large apertures and significant space requirements, leading to high construction costs. However, when testing small-scale terahertz antennadevices such as those used in 5G/6G high-speed communication systems, vehicle-mounted radar, and low-Earth orbit (LEO) satellite terminals, the available space for the entire measurement system is often very limited [8; 9; 10]. Therefore, to meet the test requirements of high-frequency, small-scale antennas under test (AUTs) while reducing construction costs and the space occupation of the measurement system, there is an urgent need to design and develop a miniaturized, low-cost, high-performance CATR system with integrated and flexible mobility configurations. Currently, based on the number of mirrors, mirror CATRs are mainly divided into three configurations: single-mirror, dual-mirror, and tri-mirror CATRs [11; 12; 13; 14]. Single-mirror CATRs, incapable of beam conversion and shaping, generally exhibit a low QZ usage ratio (typically below 50%) and suffer from higher cross-polarization levels due to inherent offset structural distortions [15]. In dual-mirror CATRs, the introduced mirror, which is comparable in size to the primary mirror, not only greatly increases the edge diffraction effects of the mirrors, leading to significant degradation in system performance, but also increases construction costs [16]. In contrast, tri-mirror CATR systems, incorporating two small shaped mirrors, demonstrate distinct advantages in both performance and cost-effectiveness. These shaped mirrors can redistribute the energy of the electromagnetic beam radiated by the feed source into the desired outgoing field, thereby achieving better QZ characteristics than single- and dual-mirror CATRs. Furthermore, the mirrors' dimensions are substantially smaller than those of the primary mirror, drastically reducing fabrication costs and manufacturing cycles. However, current research and development for integrated, miniaturized CATR systems are predominantly focused on low-frequency, single-mirror configurations [17; 18]. Examples include Keysight's F9650A (Keysight Technologies, Inc., Santa Rosa, CA, USA) miniaturized single-mirror CATR [19], with an overall footprint of 2.8 m \(\times\) 1.6 m \(\times\) 2 m and a primary mirror size of 0.6 m \(\times\) 0.6 m, achieving a QZ usage ratio of 50%. Other notable CATR systems include General Test's RayPact600C (General Test Systems Inc., Shenzhen, China) series [20], C&G Microwave's CATR-30R (C&G Microwave Co., LTD., Daejeon, South Korea) [21], Guoyu Microwave's D0303R (Guoyu Microwave, Beijing, China) [22], and NSI-MI's CRR-CSC-0.3 (NSI-MI Technologies, Suwane, GA, USA) miniaturized single-mirror CATRs [23]. Although these CATR systems can provide a QZ of approximately 0.3 m \(\times\) 0.3 m within a relatively small space, with a QZ usage ratio of 50%, there has been no significant research on miniaturized terahertz tri-mirror CATRs, which are expected to offer superior QZ characteristics, particularly in terms of cross-polarization suppression. Furthermore, although the above-mentioned miniaturized single-mirror CATRs can meet general testing requirements, they still face the following challenges: (1) a relatively low operating frequency and limited frequency bandwidth, (2) a low QZ usage ratio and small QZ size, and (3) high cross-polarization levels.