This paper describes a low-cost, small size, and high-sensitivity microwave sensor using a Complementary Circular Spiral Resonator (CCSR), which operates at around 2.4 GHz, for identifying liquid samples and determining their dielectric constants. The proposed sensor was fabricated and tested to effectively identify different liquids commonly used in daily life and determine the concentrations of various ethanol–water mixtures at by measuring the resonant frequency of the CCSR. Using acrylic paint, a square channel was drawn at the most sensitive position of the microwave sensor to ensure accuracy of the experiment. To estimate the dielectric constants of the liquids under test, an approximate model was established using a High-Frequency Simulator Structure (HFSS). The results obtained agree very well with the existing data. Two parabolic equations were calculated and fitted to identify unknown liquids and determine the concentrations of ethanol–water mixtures. Thus, our microwave sensor provides a method with high sensitivity and low consumption of material for liquid monitoring and determination, which proves the feasibility and broad prospect of this low-cost system in industrial application.

In this paper, complementary metamaterial sensor is designed for nondestructive evaluation of dielectric substrates. The design concept is based on electromagnetic stored energy in the complementary circular spiral resonator (CCSR), which is concentrated in small volume near the host substrate at resonance. This energy can be employed to detect various electromagnetic properties of materials under test (MUT). Effective electric permittivity and magnetic permeability of the proposed sensor is extracted from scattering parameters. Sensitivity analysis is performed by varying the permittivity of MUT. After sensitivity analysis, a sensor is fabricated using standard PCB fabrication technique, and resonance frequency of the sensor due to interaction with different MUT is measured using vector network analyzer (AV3672series). The transcendental equation is derived for the fabricated sensor to calculate relative permittivity for unknown MUTs. This method is very simple and requires calculating only the resonant frequency, which reduces the cost and computation time.

A high sensitivity complementary multiple split ring resonator (CMSRR) based submersible microwave sensor for edible oil detection and quality analysis is proposed. The main part of the sensor is the CMSRR loaded flared microstrip patch, and the proposed single-port structure is quite attractive for liquid detection. The proposed sensor was fabricated and measured to operate at 8.49 GHz with very high Q factor, which agrees very well with the simulated results obtained by High Frequency Structure Simulator (HFSS). A strong electric field can be observed along the sides of CMSRR at resonance, creating a sensing region sensitive to changes in the nearby dielectric material. By simply immersing the sensor into some familiar oil samples and heated peanut oil, the device's resonance and peak attenuation change greatly. The measured results have proven the improved sensitivity of the proposed sensor on the liquid samples compared with the methods described in the literature. The complex permittivity of the oil samples can be estimated by establishing an approximate model in HFSS. Two empirical formulas were built and fitted to estimate the complex permittivity of unknown several oil samples and heated peanut oil samples. The estimated dielectric constants agree well with the existing data. The estimated error is within 1.4% and the measured sensitivity is up to 7.25%. The proposed sensor has advantages, being compact, inexpensive, reliable, easy to operate, and highly sensitive, and has potential applications in oil detection and quality analysis.

In this paper, an extremely sensitive microwave sensor is designed based on a complementary symmetric S shaped resonator (CSSSR) to evaluate dielectric characteristics of low-permittivity material. CSSSR is an artificial structure with strong and enhanced electromagnetic fields, which provides high sensitivity and a new degree of freedom in sensing. Electromagnetic simulation elucidates the effect of real relative permittivity, real relative permeability, dielectric and magnetic loss tangents of the material under test (MUT) on the resonance frequency and notch depth of the sensor. Experiments are performed at room temperature using low-permittivity materials to verify the concept. The proposed design provides differential sensitivity between 102% to 95% as the relative permittivity of MUT varies from 2.1 to 3. The percentage error between simulated and measured results is less than 0.5%. The transcendental equation has been established by measuring the change in the resonance frequency of the fabricated sensor due to interaction with the MUT.

A photoconductive logarithmic spiral antenna with saw-toothed plasmonic contact electrodes is proposed to provide a higher terahertz radiation compared with the conventional photoconductive antenna (PCA). The use of saw-toothed plasmonic contact electrodes creates a strong electric field between the anode and cathode, which generates a larger photocurrent and thereby effectively increases the terahertz radiation. The proposed PCA was fabricated and measured in response to an 80 fs optical pump from a fiber-based femtosecond laser with a wavelength of 780 nm. When the proposed antenna is loaded with an optical pump power of 20 mW and a bias voltage of 40 V, a broadband pulsed terahertz radiation in the frequency range of 0.1–2 THz was observed. Compared to the conventional PCA, the THz power measured by terahertz time domain spectroscopy (THz-TDS) increased by an average of 10.45 times.

A frequency and radiation pattern reconfigurable antenna based on graphene at terahertz band is presented in this paper. By applying different chemical potentials on the one arm of the proposed antenna while keeping it unchanged of the other arm, the antenna can operate at 1-2 THz, and the obtained maximum impedance bandwidth can be up to 26.68 %. Radiation patterns of the proposed antenna can also be controlled by the applied chemical potentials, which makes it very promising for beam-scanning in terahertz systems or devices.