# Dual-Band Single-Layered Frequency Selective Surface Filter for LTE Band with Angular Stability
###### Abstract  
This study presents an innovative Dual-Band Frequency Selective Surface (FSS) designed for LTE applications, offering an effective solution for minimizing Passive Inter-Modulation (PIM) in contemporary wireless communication systems at the base station. The proposed passband FSS filter is designed to deliver optimal dual-band filtering characteristics with consistent stability over incidence angles up to 80\({}^{\circ}\). Corresponding to antenna systems requirements, the proposed method gives resonant frequencies at 1.9 and 2.1 GHz which operate in the LTE band with bandwidths of 40 and 60 MHz, respectively. Moreover, the proposed design is analyzed to establish the optimal range for each resonant frequency by examining the parametric effects. The suggested FSS-based filter consists of a single-layer structure with the dimension of the unit cell of 0.33\(\lambda_{1}\)\(\times\) 0.33\(\lambda_{1}\) where \(\lambda_{1}\) is the wavelength of low frequency, which delivers desired reflection and transmission coefficients using RT/Duroid 5880 with a thickness of 0.508 mm. The designed filter is validated through measurements of a fabricated prototype, demonstrating its practicality and performance. Simulations carried out with Equivalent Circuit Modeling (ECM) are demonstrated by measurements from a constructed 4 \(\times\) 4 array prototype, showing a robust alignment with experimental findings. This work emphasizes an asymmetric FSS design that improves frequency selectivity and angular stability for the desired LTE dual band and also depicts the future possibilities for tuneable models and broader applications to meet the demands of modern wireless communication. P 2025 20  
remove interference, Orthogonal Frequency Division Multiplexing (OFDM) is adopted as a modulation scheme which increases the peak power of the communication system, but it also adds a PIM issue. The presence of PIM reduces the reliability, capacity, and data rates of wireless systems by diminishing the receiver's sensitivity [5]. This degradation can lead to undesirable outcomes such as dropped calls, reduced system capacity, and lower data throughput. Cancelling PIM is critical for enhancing the performance and efficiency of modern wireless communication systems. An effective approach to tackle this issue involves the implementation of a Frequency Selective Surface (FSS) at the base station. FSS is classified under meta-surfaces [6], which respond to the incident electric field by selectively filtering and controlling electromagnetic (EM) waves, thereby reducing PIM effects and enhancing system performance. It is characterized as a periodic surface, signifying that a unit cell repeats multiple times to form an array [7]. Periodic surfaces have gained substantial research interest due to their exceptional control mechanism of EM waves and ease of fabrication. Throughout the last decade, there has been a marked increase in the application of FSS in diverse fields including spatial filters, electromagnetic shielding applications, radomes, absorbers, and reflectors [8; 9; 10; 11; 12]. This EM performance depends upon the geometry of the structure (square, circle, dipole, etc. ), the element type of the structure (aperture type, loop type), the dielectric constant of the material used in the substrate, and periodicity [13]. Effective EM wave absorbers have high absorption capacity and transmission impedance matching. Impedance matching improves with periodic or multi-layer gradient absorbers. Permittivity and permeability's imaginary function affect absorber EM loss. This can be addressed by introducing EM loss routes to structural design [14]. The existing research has mostly focused on developing various FSS structures to provide a better gain for antenna [15] and EMI shielding [16]. The literature has extensively examined several FSS architectures and their respective uses. In [17], transparent FSS is used as a filter at the 2.6 GHz LTE band, using a glass substrate.