## 2 Experimental Setup and Methodology  
The experimental setup for this project is designed to extend 2 DMMW imaging into range-enhanced data collection using a 105 GHz FMCW signal. At the core of the setup is a custom MMW source (Tx 272 from Virginia Diodes Inc., Charlottesville, VA, USA) that generates a collimated, frequency-modulated signal through a ramp input to a voltage-controlled oscillator (VCO), where the output frequency (101-109 GHz) is proportional to the VCO supply voltage level, creating a chirped waveform suitable for FMCW operation. The frequency sweep of the transmitted signal is designed to be highly linear, ensuring accurate extraction of beat frequencies for range determination. Any non-linearity in the frequency sweep could lead to distortions in the measured range data, affecting the precision of the 3D reconstruction. However, the Tx 272 ensures a stable and linear frequency sweep, thereby minimizing such errors and allowing for reliable depth measurements. To maintain stability, the bandwidth of the FMCW signal is limited based on the operational constraints of the transmitter, ensuring optimal performance within the 101-109 GHz range. A key component of this setup is the Magic Hybrid Tee (from Mi-Wave, St. Petersburg, FL, USA) as shown in Figure 1, which operates within the W-band frequency range (75-110 GHz). This device comprises three orthogonal flanged waveguide sections that allow for effective signal splitting and phase management. When a signal is applied to the H-plane arm, it divides equally between the two main in-line ports, producing equal-power and in-phase outputs. Conversely, applying a signal through the E-plane arm also splits the signal but results in equal-power outputs with opposite phases, thereby facilitating phase-reversed signals necessary for advanced signal processing. To achieve in-phase transmission in both directions, the source output is connected to the H-port through a straight waveguide, minimizing phase distortions. In our setup, the same standard gain horn antenna is used in both arms to ensure uniformity in signal transmission and reception. This Hybrid Tee ensures low insertion loss (approximately 1.0 dB) and maintains excellent isolation (approximately 20 dB) between the E-plane and H-plane arms, which is crucial for minimizing interference in high-frequency applications. The MMW signal is split into two paths using this device: one directed towards the object and the other serving as a reference, both essential for subsequent range determination. In the target path, the MMW beam is collimated using an off-axis parabolic mirror, allowing for precise illumination of the target object. The radiation reflected from the target object is then projected onto a spherical imaging mirror, which focuses it onto the GDD positioned at the image plane. In the reference path, the signal is directed through an additional collimating mirror and projected onto a small flat mirror, known as the reference signal mirror, to establish a stable reference signal for the GDD, enabling precise range detection. The MMW reflections from both the object and reference paths are directed toward the GDD (N523 from International Light Inc., Yorba Linda, CA, USA), which serves as the central detection component. The GDD is sensitive to variations in discharge current when exposed to incident MMW radiation, providing an electrical detection mechanism for the reflected FMCW signals. This detection is achieved through heterodyne mixing in the GDD, where the incident MMW power induces variations in the discharge current. The frequency-modulated signals are thereby translated into measurable current fluctuations,  
Figure 1: Magic Hybrid Tee Configuration: Dual-path signal splitting for main and reference signal alignment. enabling detection. The effectiveness of the GDD in this application stems from its ability to operate without cryogenic cooling, its robustness to high-power MMW exposure, and its cost-effectiveness compared with conventional MMW detectors [11].