## 1 Introduction  
Radar measures the distance, direction, angle, and speed of an object or a target by using reflected electromagnetic waves that return from the object after transmitting them [1]. Radar was developed during World War II for air defense applications to detect German aircraft. Its usage has progressively widened to civilian and commercial applications, such as airports and harbor traffic control, weather forecasting, and Earth remote sensing [2]. Recently, it has been widely used for automotive and industrial applications, such as collision avoidance and adaptive cruise control in advanced cars, for traffic monitoring, level sensing, and motion detection because of low-cost single-chip solutions from semiconductor integration [3]. Radar, in general, is divided into continuous-wave (CW) radar and pulse radar based on the waveform of the transmitted signals [4]. Pulse radar transmits and receives a pulse signal; the shorter the pulse width, the better the distance resolution. Since pulse radar uses pulse waveforms, it has high instantaneous power and requires a wide frequency bandwidth. Therefore, it is challenging to implement in hardware, expensive, and mainly used for military purposes. CW radar is divided into three types: single-tone CW, frequency shift keying (FSK), and frequency-modulated CW (FMCW). In single-tone CW, the object's speed can be detected by the frequency change due to the Doppler from the object's movement, when the object reflects the electromagnetic wave. However, this type of radar cannot measure the distance to the object or the velocity of a stationary object. FSK can measure the speed and distance of a moving object by using two or more discrete transmission frequencies but cannot measure the distance to a stationary object. FMCW uses a voltage-controlled oscillator (VCO) to calculate the shift between the received frequency and the transmitted frequency using the linear frequency modulated transmitted frequency to measure the speed and distance of a moving object as well as the distance to a stationary object. Hence, FMCW is the most accurate and widely used method because of its advantages. Three main operating frequency bands exist for automotive millimeter-wave radar: 24 GHz, 77 GHz, and 79 GHz [5]. The 24 GHz frequency band (24.0 GHz to 24.25 GHz) is allocated to short-ranges up to 30 m and is used in blind-spot detection, rear cross-traffic alerts, and collision avoidance, whereas the 77 GHz frequency band (76 GHz to 77 GHz) is allocated to long ranges up to 250 m and is used in adaptive cruise control and forward collision warning systems. The 79 GHz frequency band (77 GHz to 81 GHz) is allocated to high-resolution short-range applications. The upper millimeter wave band (100 GHz to 300 GHz) or the sub-terahertz band (100 GHz to 1000 GHz) is expected to be used for higher-resolution radar applications shortly [6; 7]. A radar system usually consists of transmitter circuitry producing high-frequency signals, a transmitting antenna sending the signals toward the target, a receiving antenna capturing the signal reflected from the target, and a receiver circuitry extracting the information of interest from the received signal [4]. High-gain directional antennas are required for radar systems. Generally, the antennas used for radar systems can be classified into reflector antennas, lens antennas, and array antennas [8; 9]. Reflector antennas consist of a feed antenna and a parabolic reflector. The feed antenna, placed at the reflector's focal point, is the source of the transmitted waves and is the collection point for the received waves. The reflector reflects the transmitted waves in the direction of its axis to make plane waves and reflects the received waves coming from the axis direction to the focal point. Types of reflector antennas used for radar systems are the parabolic reflector antenna, the Cassegrain reflector antenna with dual reflectors, and the offset feed reflector antenna.