Microwave gas sensors (MGSs) have been studied extensively because of their advantages of room temperature operation, easy integration, and potential for wireless detection. However, there is no research that developed a systematic guideline on designing sensitive circuits and selecting appropriate sensitive materials. In this study, a group of complementary split ring resonator (CSRR)-based MGSs were proposed, simulated, and fabricated. The strong electric field (E-field) and magnetic field (H-field) of the CSRR sensor were confined in two separate sensitive areas by simulation. For the features of NH3-sensitive and different electromagnetic (EM) properties, ZnO and Co3O4 were employed as the sensitive materials. The MGSs have exhibited excellent detection limits as low as 0.2 ppm and outstanding selectivity at room temperature. On the basis of simulation and measured results, a compatibility principle between the sensitive materials and the circuit from the perspective of the field distribution has been proposed and discussed in detail. Finally, this study provides an optimized direction for MGS that designs sensitive circuits and selects appropriate sensitive materials from the point of view of field distribution.

In this letter, a novel unequal power divider (PD) with three standard 50- $\Omega $ ports is presented. It consists of two transmission line (TL) sections, two resistors, and a general isolation topology (GIT). Through analyzing voltages and currents of each branch, the constraint conditions of such a GIT are newly established. Considering realization of GIT, two types of topologies, i.e., asymmetrical coupled line and cascaded TL, are selected for verification. From the design charts and several design examples, the proposed PD could not only provide high power ratio $k^{2}$ with compact size but also independently adjust their fractional bandwidths ( $S_{11}$ or $S_{32})$ with the same power ratio $k^{2}$ . Finally, an unequal PD with the power ratio $k^{2}$ $=$ 8 is fabricated, and the measured results meet the simulated results very well.

In this paper, a novel synthetization approach is proposed for filter-integrated wideband impedance transformers (ITs). The original topology consists of N cascaded coupled line sections (CLSs) with 2N characteristic impedance parameters. By analyzing these characteristic impedances, a Chebyshev response can be derived to consume N + 2 design conditions. To optimize the left N − 2 variable parameters, CLSs were newly substituted by transmission lines (TLs) to consume the remaining variable parameters and simplify the circuit topology. Therefore, there are totally 2N − N − 2 substituting possibilities. To verify the proposed approach, 25 cases are listed under the condition of N = 5, and 7 selected cases are compared and discussed in detail. Finally, a 75–50 Ω IT with 100% fractional bandwidth and 20 dB bandpass return loss (RL) is designed and fabricated. The measured results meet the circuit simulation and the EM simulation accurately.