In this paper, a high-sensitive microwave dielectric sensor targeted for material permittivity characterization is proposed. A reflective voltage controlled oscillator is devised by virtue of the compact microstrip resonant cell (CMRC) and embedded into a fractional-N phase-locked loop frequency synthesizer. The CMRC structure, covered with material under test, changes its resonating characteristics and, hence, the initial oscillating frequency. Due to the presence of the frequency synthesizer, this frequency shift is finally translated into the controlling voltage variation. In this paper, the material permittivity is extracted by measuring the voltage deviation. Several different samples with known permittivity values are tested to verify the effectiveness of the design sensing platform.

A self-sustained dielectric constant (i.e., permittivity) measurement system for liquid is proposed. A tunable substrate integrated waveguide (SIW) resonator is exploited to form a feedback-type voltage controlled oscillator (VCO) that is further embedded into a phase locked loop (PLL)-based frequency synthesizer. The SIW resonator acts as the feedback element of the VCO. When the SIW resonator is exposed to liquid, its frequency response and the corresponding VCO oscillation frequency change accordingly because of electromagnetic field perturbation. Due to the presence of the PLL, the eventual frequency of synthesizer is kept constant, while this potential frequency shift is translated into the output voltage of the PLL loop filter. This voltage is readily measured by a digital multi-meter. Principles and design considerations of the SIW resonator as well as implementations of the VCO and the PLL synthesizer are discussed in detail. In addition, a prototype sensing platform is devised. Correlations between the dielectric constant of liquid MUT and the physical parameters, such as the SIW resonant frequency, the VCO oscillation frequency, and the PLL output voltage are analyzed comprehensively. According to the experimental results of several organic liquids, the proposed system can achieve an accuracy better than 4% for permittivity characterization.

The repetitive computation of the forward problem in distorted Born iterative method (DBIM)-typed inversion method is time-consuming and memory-costly, especially in dealing with three-dimensional (3-D) inverse scattering problem. An improved subspace-regularized DBIM is proposed for the efficient solution of the 3-D inverse problem, which introduces the multilevel Green’s function interpolation method (MLGFIM) to accelerate the computation of the forward problem. In order to stabilize the inversion, an improved subspace-regularized DBIM is used with one new subspace-regularized induced current term in the cost function in virtue of truncated singular value decomposition (TSVD). To further reduce the computation burden, the incident field induced with updated inhomogeneous background medium is used approximately as the total field in the cost function, which saves one forward solver calculation in each iteration. Besides, the resource-intensive multiplication of inhomogeneous Green’s function can be avoided, which further improve the efficiency. Three numerical and two Fresnel experimental examples are given to demonstrate both the efficiency of the forward solver and the accuracy of the inversion algorithm.

A novel uniplanar dual‐band printed compound loop antenna for WLAN/WiMAX applications is proposed. The proposed antenna consists of a small inner rectangular ring, an outer rectangular loop with three slits and a parasitic strip on one side of the substrate. Fed by the inner ring and collaborated with double rings and the parasitic strip, three resonant frequencies can be achieved, where the parasitic metallic strip between the rings can not only induce a new middle resonant frequency but also improve the impedance matching at the low‐frequency band greatly. By magnetic‐electrical coupling, dual‐band performance with the lower‐wide band covering 2.37–3.78 GHz and the upper‐frequency band covering 5.15–5.85 GHz can be obtained. It is experimentally demonstrated that the proposed antenna with the performance of broadband (covering 2.4/5 GHz WLAN including 2.4–2.484 GHz, 5.15–5.825 GHz and 3.5/5 GHz WiMAX including 3.4–3.69 GHz, 5.25–5.85 GHz, respectively), good dipole‐like radiation characteristics, stable gain and high‐radiation efficiency, is an excellent candidate for WLAN/WiMAX applications.

An advanced method of modeling radio-frequency (RF) devices based on a deep learning technique is proposed for accurate prediction of S parameters. The S parameters of RF devices calculated by full-wave electromagnetic solvers along with the metallic geometry of the structure, permittivity and thickness of the dielectric layers of the RF devices are used partly as the training and partly as testing data for the deep learning structure. To implement the training procedure efficiently, a novel selection method of training data considering critical points is introduced. In order to rapidly and accurately map the geometrical parameters of the RF devices to the S parameters, deep neural networks are used to establish the multiple non-linear transforms. The hidden-layers of the neural networks are adaptively chosen based on the frequency response of the RF devices to guarantee the accuracy of generated model. The Adam optimization algorithm is utilized for the acceleration of training. With the established deep learning model of a parameterized device, the S parameters can efficiently be obtained when the device geometrical parameters change. Comparing with the traditional modeling method that uses shallow neural networks, the proposed method can achieve better accuracy, especially when the training data are non-uniform. Three RF devices, including a rectangular inductor, an interdigital capacitor, and two coupled transmission lines, are used for building and verifying the deep neural network. It is shown that the deep neural network has good robustness and excellent generalization ability. Even for very wide frequency band (0–100 GHz), the maximum relative error of the coupled transmission lines using the proposed method is below 3%.

In this communication, an odd and even mode compression method is proposed to extend the bandwidth of conventional thin dipole antennas while maintaining a stable omnidirectional radiation pattern. Wide bandwidth is achieved by simultaneously exciting and compressing 1 ^st -order (odd) and 2 ^nd -order (even) modes, enabling their combination. The offset-fed method is used to simultaneously excite the 1 ^st - and 2 ^nd -order modes of the dipole. The 2 ^nd -order mode is compressed by widening one of the dipole arms, which shifts the 2 ^nd -order mode towards lower frequencies. The 1 ^st -order mode can be compressed by shortening the length of the thin arm of the dipole that is not widened, which shifts the 1 ^st -order mode towards higher frequencies. In addition, to convert the 2 ^nd -order mode splitting radiation pattern into an omnidirectional one, parasitic strips are loaded on both sides of the thin arm of the dipole. This loading also helps to further compress the 2 ^nd -order mode towards lower frequencies. The final antenna design features an asymmetric structure. To validate the concept, a prototype was constructed and tested, demonstrating reasonable agreement between the simulated and measured results. The measured impedance bandwidth is 55.6% (1.62-2.87 GHz) and a stable omnidirectional radiation pattern can be maintained across the entire operating frequency band.