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 the operating frequency to mismatch the design is the technique basis. The proposed structure comprises the 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 22.42x7.62 mm2. Using an FR4 HTG-175 with a thickness of 1-mm, dielectric constant εr=4.4, and loss tangent tanδ=0.02, the (4.06-4.283) GHz, (5.877-6.408) GHz, and (14.281-14.589) 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. These 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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