The main problem in designing a multiband bandpass filter (BPF) is making each passband response highly independent, where each bandwidth of multiband BPF can be controlled and adjusted separately. To overcome this problem, this paper proposes a highly independent multiband BPF based on a multicoupled line stub-SIR with a folding structure. The proposed multiband BPF is constructed as a multicoupled line to generate a highly independent inter passband. Moreover, the multiband performance is produced separately and independently by using three sets of resonators: resonator A1/A2 (RA1 and RA2), resonator B1/B2 (RB1 and RB2), and resonator C1/C2 (RC1 and RC2). The three passband frequencies can be independently arranged and designed. To miniaturize the multiband BPF, a folding structure is also proposed. As a result, the multiband BPF has a compact size that is reduced by over 61.29 % compared to previous structures. The even-odd excitation model and the equivalent circuit model are used to analyze the multiband BPF structure. This BPF is designed for GPS applications at 1.57 GHz, WCDMA (3G) at 1.8 GHz, WLAN (WiFi) at 2.4 GHz, LTE (4G) at 2.6 GHz, and 5G communication at 3.5 GHz. To evaluate and validate the proposed structure of the multiband BPF, the circuits are fabricated and tested. The simulated and measured results of the multiband BPF show good agreement. In conclusion, the proposed multiband BPF structure has a highly independent inter passband response and a compact size.

This study proposes a single-port dual-band independent microwave sensor for detecting solid materials. The sensor structure consists of a pair U-shaped resonators connected by a power divider and a microstrip feed line with an impedance of 50 Ohms with two different sensing hotspots. The $1^{\text {st}}$ and $2^{\text {nd}}$ resonators operate at a frequency of 1.200 GHz and 2.100 GHz, respectively. The type of material under test (MUT) is a standard material with a size of 10 mm $\times10$ mm (RO5880, RO4003C, and FR4). The proposed sensor had a very high accuracy compared with both of using permitivitty from datasheet and calibrated frequency from simulation for the $1^{\text {st}}$ and $2^{\text {nd}}$ resonator, respectively. Moreover, the pair of the U-shaped resonators had independent performances and they did not affect each other when the MUT was loaded. Furthermore, it can also operate with single-port measurement, which is a more efficient resource. Finally, the proposed microwave sensor can be a promising solution for detecting different permittivity of solid materials independently with high accuracy for applications such as the food industry, material quality control, and biomedical.

This paper proposes the integration of microwave sensors (MS) and antenna sensors (AS) for contact and non-contact characterization of solid materials. Integration between MS and AS were determined based on the reflection coefficient (S11) where S11 of ≥ -10 dB was proposed as MS and S ₁₁ of ≤ -10 dB was proposed as AS. The proposed sensor consists of a dual T-Shaped resonator with single port operating at a resonant frequency of f _r1 = 1.81 GHz for MS and f _r2 = 2.34 GHz for AS using FR-4 substrate with ε _r of 4.3, tan δ of 0.0265 and thickness of 1.6 mm. Furthermore, the two resonant frequencies had independent characteristics with different sensing hotspots. MS are proposed for permittivity detection of the Material Under Test (MUT) for contact detection while non-contact detection is proposed by utilizing AS. The procedure for contact detection was carried out by placing the material under test on the surface of the sensor while non-contact detection was carried out by placing the MUT with an air gap between the MUT (d) and the AS was 0.5 - 1.5 mm. The measurement results showed that the average accuracy was 99.30% and 99.31% for the MS and AS for the permittivity range of 1 - 6.15 while the normalized sensitivities of the proposed sensors were 1.15 % and 0.16%, respectively. The results of this research are novel and can be recommended for several purposes related to biomedical and pharmaceutical industries requiring contact and non-contact detection of material characterization.

Design of Triple-Band Bandpass Filter Using Cascade Tri-Section Stepped Impedance Resonators