## 1 Introduction  
Temperature monitoring in harsh environments, such as in the automotive, aerospace, energy, and industrial sectors, is essential for detecting structural degradation and preventing functional failures in critical components like bearings with limited operating temperatures [1]. Conventional wired temperature sensors are impractical in such applications due to limitations in physical connections and power supply constraints. Wireless passive temperature sensors overcome these challenges by eliminating the need for local power sources. Several wireless sensing techniques have been explored, including surface acoustic wave (SAW) sensors, LC resonators, and antenna backscattering methods. SAW sensors often produce weak signals, necessitating amplification or advanced signal processing techniques such as filtering and noise reduction to ensure accurate readings [2]. Additionally, their intricate design requires precise engineering, leading to increased manufacturing complexity, calibration challenges, and higher deployment costs [3]. Similarly, LC resonator sensors involve complex designs due to the need for precise alignment of inductive coupling components [4]. Their weak output signals, which are prone to noise interference, require sophisticated signal processing to enhance their reliability [5]. Furthermore, LC sensor performance deteriorates with increased distance from the reader due to variations in coupling efficiency, resulting in inconsistent measurements, particularly in large setups or obstructed environments [6]. In contrast, the antenna backscattering method offers a simpler, low-cost, and highly efficient design, featuring well-defined resonant frequency characteristics and ease of implementation. This method enables high-frequency operation (2.4-15.5 GHz) and supports communication distances ranging from 20 to 500 mm [7; 8; 9; 10; 11]. [PERSON] et al. demonstrated a 500 mm communication distance using a high-gain horn antenna as the interrogator and a rectangular patch antenna as the sensor, achieving a sensitivity of 2.2 MHz/\({}^{\circ}\)C [8]. The antenna backscattering method relies on wireless interaction between the interrogator antenna and the sensor antenna, where the sensor is wirelessly powered by the coupling energy transmitted from the interrogator antenna [7]. While this method offers advantages in wireless sensing, its performance varies depending on whether it operates in the near-field or far-field region. In near-field operation, strong energy coupling enables impedance mismatch techniques, which enhance backscattered signal strength [12]. In far-field operation, however, energy transfer efficiency decreases with distance, making impedance matching essential for reliable sensing. Since impedance mismatch is ineffective in far-field applications, antenna optimization is crucial for maintaining backscattering efficiency and signal stability [13]. Among various antenna configurations for wireless passive temperature sensing, rectangular microstrip patch antennas (MPAs) have been extensively investigated due to their well-established performance. However, alternative geometries, such as circular and equilateral triangular MPAs, possess inherent advantages in compactness, radiation efficiency, and mode diversity [14; 15], which could potentially enhance backscattering signal strength and stability. Despite these promising attributes, no prior study has explored the feasibility of circular and equilateral triangular MPAs as sensor antennas for passive wireless sensing applications. This study investigates and compares the performance of rectangular, circular, and equilateral triangular MPAs for wireless passive temperature sensing, focusing on their impact on backscattered signal strength and stability. These antennas are designed to operate at 2.4 GHz in the fundamental transverse magnetic mode, and the communication established in the far-field region. Sensor antennas are fabricated on alumina substrates, known for their high thermal stability and durability.