A dual‐band dual‐polarised antenna for on‐/off‐body wireless body area network (WBAN) applications is presented, operating at Industrial, Scientific, and Medical bands of 2.45 GHz with circular polarisation and 5.8 GHz with vertical polarisation. The TM ₁₁ mode is used for the off‐body mode in the first band because of its broadside radiation pattern, meanwhile, the TM ₀₂ mode is chosen for on‐body mode due to its monopole‐like radiation pattern in the second band. Composed of a circular patch with eight slots, the structure of the proposed antenna is simple, and it is fabricated with textile material. The performance of the antenna is investigated in free space and on the body. Then, the performance influenced by the bending of the antenna is discussed with a different bending radius. The specific absorption rate distribution is simulated on body tissue. And the maximum value at both bands is below the IEEE C95.3 standard of 1.6 W/kg. The acquired performance demonstrates that the proposed antenna can be applied to the applications of WBAN.

A novel self-matched negative group delay (NGD) microwave circuit based on the signal interference technique is proposed to eliminate the performance degradation due to the temperature-dependent resistance variation. The proposed circuit consists of unequal power dividing/combining structures and coupled-line phase shifter, which can be directly synthesized with prescribed NGD time and insertion loss (IL). To verify the proposed method, an NGD circuit with a size of 0.41λ _g × 0.41λ _g is designed and fabricated. From the measured results, NGD time of -2.09 ns at the center frequency of 1.016 GHz is obtained with NGD bandwidth of 14.2% (0.944-1.088 GHz), in which the IL is less than 18.1 dB and the return loss is greater than 33 dB.

Electromagnetically induced transparency (EIT) originates from quantum physics, where a narrow transparent peak appears in the opaque band due to the destructive interference between quantum states of atoms and molecules. Similar phenomena can be realized based on strong-coupling resonators with a similar spectrum of transmission peaks and abrupt dispersion variations. These classical systems, ranging from elastic to optical, are named analogs of EIT. The sharp resonant peaks with high-quality factors in the spectrum exhibit powerful potentials in sensors with ultra-high sensitivity. In order to better understand the development history of EIT-like metamaterials and their specific applications in the field of sensors, this paper makes a brief review of the EIT-like phenomenon in metamaterials. Firstly, we conduct the universal mathematical formulation based on the coupling oscillator model. Then, we classify specific metamaterial designs and practical applications of EIT-like devices in acoustic, electromagnetic, and optical waves, respectively. We also summarize the recent technologies of dynamic modulations of EIT-like metamaterials and discuss future research directions.

A compact self-matched negative group delay (NGD) circuit without the need for an external matching circuit is proposed. The proposed NGD circuit is composed of two-port open-circuited coupled lines, a short-circuited transmission line and two resistors connected by two transmission lines. The coupled lines are utilized to obtain port matching. To verify the design concept, an NGD circuit with the size of $0.39\lambda _{g}\times 0.19\lambda _{g}$ is designed and fabricated. From the measurement results, NGD value of −8.75 ns at the center frequency of 1.570 GHz is obtained with NGD bandwidth of 3.82%, signal attenuation of less than 20.5 dB and return losses of more than 32 dB.

A single-feed dual-band dual-sense circularly polarized (CP) stacked patch antenna with a small frequency ratio is proposed for Chinese 842.5/922.5-MHz radio frequency identification (RFID) reader applications. Two elliptical-ring patches are configured orthogonally to operate at different frequency bands with different senses of circular polarization. A π-type dual-band complex impedance transformer is modified with asymmetric open-ended stubs to easily tune the input matching for both bands. Measured impedance bandwidth is 2.99% (824-849 MHz) for the lower band and 2.72% (908-933 MHz) for the upper band, in which the input return loss is greater than 10 dB. The measured 3-dB axial ratio (AR) bandwidths for the lower and upper bands are 1.07% (838-847 MHz) and 1.19% (918-929 MHz), respectively. The measured frequency ratio is 1.10. The measured gain is more than 4.5 dBic over both bands. Therefore, the proposed antenna can be a good candidate for Chinese RFID readers operating in the bands of 840-845 MHz and 920-925 MHz.

Electromagnetically induced transparency (EIT) is a destructive interference phenomenon which has been widely reported in both quantum and classical wave systems. In an EIT-like system, sharp transmission windows appear within stop bands, indicating potentials in sensor applications. In this work, we design a planar microwave circuit with an EIT-like spectrum for dielectric characterization of liquid mixtures. The sharp EIT-like transmission window relies on the strong coupling between an open-circuited stub and a split ring resonator, which are sensitive to surrounding environment. Binary liquid mixtures of water-ethanol and water-methanol are injected into the container fixed on the surface of the sensitive area. Complex permittivity of the liquid under test affects both the center frequency and the quality factor of the EIT-like transmission window. Thus, complex permittivity of liquid mixtures can be further retrieved from measured S ₂₁ . Measurements together with theoretical and numerical analysis are discussed in detail and results match well with Debye function. We also analyze the achievable high sensitivity (5%) of the proposed EIT-like sensor when measuring solid samples with low permittivity dynamic range and conduct experiments. The proposed design is easy to fabricate and can be used to test various materials.