# Accurate Phase Calibration of Multistatic Imaging System for Medical and Industrial Applications
###### Abstract  
Multistatic imaging systems are commonly used in radar systems and microwave imaging. In these systems, many antennas are arranged three-dimensionally and connected to RF switches. The length of each transmitter (Tx) and receiver (Rx) channel differs slightly, causing artifacts in high-resolution image reconstruction. This study presents a novel method for the phase calibration of multistatic systems. This method does not require system reconstruction and can automatically perform phase calibration in a short time. This method is expected to facilitate an accurate phase measurement in multistatic systems. The approach involves phase calibration by analyzing the reflection coefficients of antenna elements in the time domain. Imaging experiments were performed on a multistatic imaging system using this calibration method, and the position and shape of a metal rod with a diameter one-fourth of a wavelength were reconstructed by simple back-projection with an accuracy beyond the diffraction limit. V 2024 14 10671 10.3390/app142210671 10. [PERSON] et al. performed calibration using scattered fields by objects inside the antenna array [13]. Calibration coefficients were calculated using the electric field scattered by a phantom placed inside the antenna array and simulated data. In this study, the incident field radiated from the antenna was also used for the calibration. The calibration with the scattering field gave better image accuracy than the calibration with the incident field. It is noted that such a calibration method using simulation could be used for other systems. [PERSON] et al. calibrated 6 ch ground-penetrating radar by connecting the Tx and Rx ports directly with a cable [17]. In this method, calibration was performed for delay and attenuation in the signal lines as well as direct coupling between the antennas. First, the antennas were disconnected from the cables, and each Tx-Rx channel was connected by a cable. A vector network analyzer (VNA) was then used to measure the response of the cables. And the attenuation and delay in the cables and the RF switches were calibrated. Next, the antennas were connected to the cables, and the free space bistatic responses were measured. This allowed for the calibration of the direct coupling between the antennas and the reflection from objects near the antenna array. [PERSON] et al. performed a calibration of a multistatic system using an electronic calibration (ECal) module and a phantom with known dielectric properties [16]. The calibration process consisted of three steps. First, the reflection coefficient of one port of the antenna array was calibrated. The ECal module was connected between the VNA, and the antenna, and the standard reflection calibration procedure was performed. This procedure moved the calibration plane of the VNA from the VNA port to the connector of the ECal module. Next, a phantom was placed inside the antenna array, and then a calibration of the other ports was performed using the reflection coefficient of the antennas. Finally, the coupling between the antennas was calibrated using the transmission coefficients between each Tx and Rx antenna pair. It should be noted that this calibration method has limitations, such as requiring symmetrical antenna configurations and not using a RF switches. However, because of the low effort required for calibration, it is mentioned that it is possible to perform calibration over a short period. Based on these previous studies, this study proposed a method that enables automatic and quick calibration without system reconfiguration. The phase calibration without system reconfiguration reduces the time and effort required for calibration. In addition, errors are caused by the tightness of connectors and bends in cables. Therefore, the phase calibration can also reduce measurement errors. As a result, it is expected that the facilitation of accurate phase measurements in multistatic systems can be easily performed.