## 1 Introduction  
Radar systems emit electromagnetic waves into space through an antenna. When this wave hits an object, it is reflected and returned to the antenna. The receiver then captures the reflected wave, which is processed using \"Time Delay\" and \"Doppler Shift\" technology. This processing helps to determine the location and speed of the object. Radar systems play a significant role in various fields, including air traffic control, weather monitoring, and military surveillance [1, 2]. Recently, various studies have been conducted on harmonic nonlinear radar, which detects harmonic signals radiating from a target, unlike a conventional radar method, which detects signals reflected from the target. The reception frequency of nonlinear radar utilizes the harmonic signal of the transmission frequency as the operating band. Nonlinear radar systems are advantageous for detecting targets in more complex environments than conventional radar systems and provide improved identification using high-resolution radar imaging [3, 4, 5]. However, nonlinear signals and noise generated by electronic components within the radar system occur at multiple times the operating frequency, leading to degradation in system performance. Nonlinear radars typically use bistatic radar structures, and the harmonic coupling from the transmitter to adjacent receivers reduces the accuracy and reliability of radar systems. They detect the presence of non-existent targets or increase the noise level of the radar signal, thereby making it difficult to detect distant targets. Additionally, there is a disadvantage in that the system requires more power to reduce the noise caused by harmonic signals. To effectively suppress harmonics, multiple filters can be applied to the transmitter. However, a matching network is required within the system, which increases the weight and price of the system and reduces the system gain due to the insertion loss. Therefore, various studies have been conducted on antennas and frequency-selective surfaces using harmonic suppression filters (HSFs). Representative examples include a filter-integrated antenna [6; 7], research using an impedance transformer [8; 9], and various harmonic canceling techniques. Additionally, research has been conducted to effectively transmit the target frequency band using a frequency-selective surface that implements band-pass filter characteristics [10; 11; 12; 13; 14].  
Frequency Selective Surfaces (FSS) enhance radar system performance by selectively allowing or blocking specific frequency bands. Composed of periodic arrays of metal patches or slots, FSS exhibit band-pass or band-stop filter characteristics, reducing interference and improving the signal-to-noise ratio. Despite these advantages, challenges persist in achieving harmonic suppression alongside wide-angle beam steering, stable broadband performance, and precise fabrication. In this paper, we propose an FSS that combines band-pass filter (BPF) and band-stop filter (BSF) characteristics to suppress harmonics and enable wide-angle beam steering. The proposed FSS, featuring a metal grid structure and a metallic loop structure, was analyzed using CST Studio Suite and Keysight's Advanced Design System. Experimental measurements verified significant harmonic suppression and stable performance during beam steering. Specifically, the measured transmission coefficient (\(S_{21}\)) was \(-0.1\) dB at 3.0 GHz and approximately \(-50\) dB at 6.0 GHz. The FSS, when attached to an actual radar system, showed a maximum gain of 17.12 dB at 3.0 GHz (with FSS) compared to 17.25 dB (without FSS), and at 6.0 GHz, 2.69 dB (with FSS) compared to 12.79 dB (without FSS). During azimuth steering at 3.0 GHz, the gain was 16.68 dB (with FSS) compared to 16.13 dB (without FSS), and during elevation steering, it was 15.90 dB (with FSS) compared to 15.74 dB (without FSS). This study provides a comprehensive FSS design combining harmonic suppression and wide-angle beam steering, validated through simulations and practical radar system implementations.