Microwave sensing technology has the merit of easy integration, low cost and simple manufacturing process which has been popularized [1]. Microwave sensors have been widely used in many aspects of measurement and characterization. For example, sensors for liquid concentration measurement [2,3], sensors for liquid chemical detection [4], sensors for biomedical applications [5,6], sensors for solid material sensing [7], used to monitor ambient temperature and humidity sensors [8,9], sensors for thickness measurement [10] and mechanical displacement [11,12] and pressure sensors [13].

The characteristics of any material are expressed in its permeability and permittivity of the electrical performance [14], of which permittivity is the most critical factor in understanding how the material responds to the electric field [15]. Metamaterials exhibit significant electric and magnetic field localization, which make them fit for microwave sensing, which permits them to exhibit dielectric properties of solids [16,17]. For materials with specific electromagnetic properties, microwave sensing can effectively identify various materials [[18], [19], [20]]. Many of the sensors in microwave sensors are changes in the resonance frequency [21], reflection coefficient [22,23], transmission coefficient [24,25] or quality factor [26] of the resonant sensitive element caused by the variation to be sensed. The current research on microwave sensor is based on resonant structure, for example, complementary split ring resonator (CSRR) [27,28], single-split ring resonator (SRR) [29,30], rectangular complementary ring resonator (RCRR) [31].

In [32], an ultrahigh sensitivity microwave sensor is proposed, which can realize high sensitivity sensing in measuring the relative dielectric constant of the microfluidics to be measured by adding only one capacitor in the resonator structure. The proposed CSRR microwave sensor in Ref. [33], is presented for measuring dielectric constants of different solids and liquids. A curved slot is applied to accomplish electric field concentration and store large amounts of energy, and the sensing region is determined on the electric field distribution to achieve sensitivity to resonant frequency change. In Ref. [34], a frequency selective surface (FSS) liquid sensor is proposed. Which is fit for the detection of various liquid samples with low detection error and good sensitivity. When samples with different electrical performance are loaded, the capacitance between adjacent FSS elements will change. The vary in capacitance leads to the vary in the resonant frequency, which is able to be used for liquid monitoring. In Ref. [35], a 3D printed sensor based on metamaterial absorber is proposed, which characterizes solid and liquid materials based on their relative dielectric constants. The sensor is designed as a monolithic structure with a cavity on the dielectric substrate. This structure is able to easily fabricated by 3D printing technology. In Ref. [36], a fully 3D printed electromagnetic (EM) microfluidic sensor is proposed for performance of complex dielectric constants of liquids.

In this work, a reflective sensor depend on a five rings (FR) resonator is proposed. The function of this sensor is to sense permittivity of various solid materials. It can detect both the real part and the imaginary part of the permittivity of a solid simultaneously. The working principle of this reflective sensor is that when solid material is in contact with the sensitive element FR, the and reflection coefficient (S11) and resonant frequency will change accordingly. The advantages of the proposed sensor include detection of solid samples, ultrahigh quality factor (Q = 4327), polarization and angle insensitivity. Finally, solid materials PET, PVC and PMMA are used to verify the accuracy of the sensor performance.