A reflectivity arch is a graphical representation of the reflection coefficient of a material as a function of the incident angle of an electromagnetic wave. This type of measurement is commonly used in radio frequency (RF) material property measurements, where the material under test is typically a thin film or a layered structure.

The reflectivity arch is obtained by measuring the amplitude of the reflected wave as a function of the incident angle of the electromagnetic wave. The incident wave is typically a plane wave, and the measurement is performed using a network analyzer or a similar instrument.

The reflectivity arch typically consists of a series of peaks and valleys, which correspond to the resonances of the material structure. The resonances are caused by the interference of the incident and reflected waves, and they depend on the thickness and composition of the material layers, as well as the dielectric properties of the material.

By analyzing the reflectivity arch, it is possible to extract important material properties such as the refractive index, the thickness of the layers, and the loss tangent. This information is useful for designing and optimizing RF devices and circuits, such as filters, antennas, and transmission lines.