Designing a multi-band bandpass filter (BPF) with controllable bandwidths is an alternative process to several technologies suggested by researchers. Hence, this paper presents a tri-band BPF in microstrip technology where T-shaped short-and-open stubs have alternating positions to use the maximally flat theory, based on the overall ABCD parameters of the circuit. The combination of the design Q-factor and operating frequency to mismatch the design is the technique basis. The proposed structure comprises quarter wavelength (λ/4) line section to develop a tri-band BPF frequency. All stubs are symmetrical relative to the center axis, while the prototype has been fabricated on a wafer of 22.42 × 7.62 mm 2 . Using an FR4 HTG-175 with a thickness 1-mm, dielectric constant ε r = 4.4, and loss tangent tan δ = 0.02, the (4.06-4.283) GHz, (5.877-6.408) GHz,) GHz are obtained referring to a 10-dB of the return loss. In contrast, the insertion losses at the center frequencies are 2.107/1.354/4.08 dB and the fractional bandwidths of 2.134%, 5.346%, and 8.645%, respectively. This covers WAS (including RLAN), ISM, and 5G applications. However, the attenuation coefficient is between 1.326 dB and 4.368 dB. The tri-band BPF prototype was validated using the Anritsu MS4642B 20 GHz Vector Network Analyzer. The measured and E-simulated results have been compared with good agreement.

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Based on the simplicity of the design method, this paper presents a new approach for developing matched subbands when splitting two mismatched dual-wideband bandpass filters (BPFs) for the fifth generation (5G), wireless access systems (WAS), wireless fidelity (Wi-Fi), worldwide interoperability for microwave access (WiMAX), wireless local area network (WLAN), radar, and other communication devices. The method’s novelty involves using identical quarter-wavelength resonators terminated by alternated short-and-open stub configurations. Both configurations alternate and describe a perfect symmetry by their location from each other to make the subbands possible in the low-frequency and its harmonic (high-frequency) bandwidths (BW). A stub admittance Y 1 is defined and associated with the mainline section characteristic impedance Z 0 and an operating frequency f 0 . A quality factor Q a p is connected to Y 1 and f 0 to approach the BPF global quality factor Q g initially fixed. The stub characteristic impedance and the mainline one differ, while electric lengths (stub and mainline section) are identical. Using the operating frequency determines physical dimensions, creates harmonic frequencies and the rejected BW, mismatches the main frequency BW, matches the subbands, and creates transmission zero (TZ). Hence, a 28.118-dB stopband that separates the two bandpasses at 9.373 GHz is made. At the same time, the unmatched dual ultrawideband (UWB) covers a large panel of communication systems. The lowest (3.146–5.431) and highest (11.891–14.749) GHz BW exhibit a minimum insertion loss (IL) of 0.656 dB and 3.027 dB. The subbands return losses (RL) are better than 28 dB and 19 dB, respectively, and a flat group delay of 0.205 ns is obtained in the upper band. All subbands adaptation methodology is read from 10 dB of the RL. In that case, the four matched subbands in its lower wideband are 3.327–3.709 GHz and 4.442–5.048 GHz, and in its higher wideband are 11.922–12.486 GHz and 14.281–14.653 GHz. The 2275/2858 MHz is the dual-wideband with a fractional BW 53.282/21.456%. The fabricated prototype has validated the EM-simulations, and Anritsu MS4642B 20 GHz vector network analyzer (VNA) has been used for experimental results by scanning the frequency range 3 GHz–15 GHz. The tested prototype is made with a 1 mm FR4 HTG-175 thickness by considering a dielectric constant of 4.4, and its overall size occupies 22.45 × 5.72   m m 2 ( 0.32   λ 0   ×   0.082   λ 0   mm 2 ).