It deals with two microwave systems destined to the determination of the complex dielectric permittivity at 9.22 GHz and 15.05 GHz, which works within a temperature range comprised between –80º C and 250º C and uses the short circuit method. An electronic system controls the acquisition data process between a microprocessor and a computer, by means of a UART that makes possible the series transmission once the communication protocol has been established.
The method employed is a variation of the Roberts-Von Hippel one, with a rectangular waveguide, applicable both to polar liquids and to granular dielectrics. Although this method presents the disadvantage of operating at fixed frequencies, it allows one to study the thermal evolution of a dielectric and to establish correlations with the calorimetric behaviour obtained by differential thermal and thermogravimetric analyses.
The dielectric TDR characterization system used consists of a step voltage generator (HP 54121 T) with 200 mV amplitude, with f ast rise time ( 35 ps ), and a digitizing oscilloscope ( HP 84120B ) working in a bandwidth from DC to 12.4 GHz. The coaxial line presents a characteristic impedance of 50 W with Amphenol APC-7 connectors.
The step voltage which propagates along the coaxial line suffers a reflection in the air-dielectric interface. The reflection coefficient G (t) can be determined from the incident and reflected voltage signals, V + (t) and V - (t). The knowledge of G (t) allows one to obtain the complex dielectric permittivity e * ( w ) and other useful parameters for the characterization of dielectrics, in the frequency domain.