A free-space method for measurement of complex permittivity of double-layer dielectric materials at microwave frequencies

Two methods for the calculation of complex permittivity of double-layer dielectric materials measured by a spot-focusing free-space measurement system have been developed. Standard materials have been used to test these methods and further works will be carried out on SiO
2
wafer. The dielectric constants obtained were close to the published values. The two methods developed are Transmission Method and Metal-Backed Method. The S
11
, S
21
, and S
22
are measured for Transmission Method, while only S
11
is measured for Metal-Backed Method. In both methods, the samples are sandwiched between two Teflon plates which are quarter wavelength at mid-band frequency. Results are reported in the frequency range of 18–26GHz.