3.2. Dielectric Matetials Preparation

We chose dielectric films consisting of different proportions of polydimethylsiloxane (PDMS) (εr∼2.8, tanδ∼0.04) and barium titanate (BTO) (εr∼500, tanδ∼0.5) for experimental testing. The BTO powders were uniformly mixed in liquid PDMS with a mixer, then poured into the mold, and finally placed on the heating plate and heated at 90° for 30 min, to form 1 mm thick dielectric films. Figure 5a shows the appearance of films made of PDMS and BTO with different volume fractions. The thin films exhibit good adhesion, attributed to the physical characteristics of PDMS, which can greatly reduce the air gap between the films and the meta-surface, leading to small errors in the testing. The dielectric properties of films are estimated using the following Maxwell–Garnett (MG) mixing rule [17,18], as follows:

where ε1, ε2, and εd are the complex permittivity of PDMS, BTO and the films, respectively, and v is the volume fraction of BTO in PDMS. The theoretical dielectric constants of the films are 2.8, 3.6, 4.2, and 4.7, corresponding to BTO volume fractions 0%, 10%, 15%, 20%, respectively.