## 1 Introduction  
In modern satellite communication systems, particularly for X-band and higher frequencies, signal filtering represents a critical component to ensure optimal performance in increasingly congested spectral environments [1].The requirement for extremely narrowband filters in filter banks-- typically characterized by fractional bandwidth (FBW)--commonly implemented as Input Multiplexers (IMUX) [2] becomes essential to maintain signal integrity and selectivity in multi-frequency coexistence scenarios. The quality factor (\(Q\)) of these filters assumes particular importance, as high-\(Q\) implementations enable enhanced near-band selectivity while minimizing insertion loss both critical parameters for receiver chain performance. These characteristics directly impact key system metrics, including Noise Figure (NF) and the gain-to-noise temperature ratio (G/T) [3]. Figure 1 shows a receiver chain with a filter bank, further detailed in Figure 2, where an IMUX consists of two filters representing separate receiver channels for downlink transmission [2; 4]. The IMUX isolates closely spaced bands while pre-filtering signals before High Power Amplifiers (HPAs) to prevent intermodulation near compression regions. Figure 2 presents an IMUX where each filter isolates a dedicated frequency band with low insertion loss and high adjacent-channel rejection. Each branch uses an RF circulator to route signals through the corresponding filter, terminating out-of-band energy in matched loads. The 90-degree hybrid coupler enables the following critical functions: dual-polarized operation (e.g., orthogonal linear polarizations), system redundancy, and band separation. Additionally, it supports sub-band filtering when configured to isolate signals at the Incoming Signal 2 port. This architecture reduces the number of required RF channels, optimizing power efficiency and form factor by minimizing bulky components. Figure 1: Block diagram in communications systems. (**a**) Receiver chain with an IMUX integrated. (**b**) Payload of communications satellite [5]. To meet stringent performance requirements, filters must implement sophisticated responses, as demonstrated in [6], incorporating high-order designs [7], transmission zeros (TZs), and/or equalization zeros (EZs) [8]. Transmission zeros--specific frequencies where the filter achieves deep attenuation through cross-coupling mechanisms [9]--are essential for rejecting interference and enhancing selectivity. Conversely, equalization zeros compensate for phase and amplitude distortions, yielding flatter frequency responses and more consistent group delay (GD) characteristics [6; 10]. These features prove indispensable in multi-band systems where frequency-dependent delays and distortions could otherwise cause signal desynchronization or performance degradation [10]. While microstrip technology initially seems appealing for its integration simplicity, weight advantages, and manufacturing ease, it remains inadequate for high-performance applications [11]. Fundamental constraints--particularly low quality factor (Q) [12], excessive insertion loss, and unreliable TZ/EZ implementation (especially in mass production)--render it incompatible with systems demanding exacting performance criteria [6; 13]. A widely adopted solution for these demanding specifications employs dielectric resonator-based cavity filters (DRCFs) [14; 15]. These filters provide exceptionally high \(Q\)-factors, delivering superior selectivity and stability against temperature variations and mechanical vibrations [16; 17]. Furthermore, high-order DRCF implementations enable steeper band-edge transitions and enhanced bandwidth-critical characteristics for high-precision applications requiring bandwidth expansion. However, DRCFs introduce substantial design and fabrication challenges stemming from their mechanical complexity. Precise input-output probe positioning, strict X/Y/Z-axis resonator alignment, and tight manufacturing tolerances necessitate extensive laboratory characterization.