Method and apparatus for measuring the permittivity of materials

A method for determining the permittivity of a sample using a coaxial probe having a first end positioned at the sample and a second end, comprising the steps of:

• applying an AC electrical signal in the microwave frequency range to the second end;
• measuring the reflection coefficient of a first, second, third and fourth standard having known complex permittivities;
• determining a system constant from the measured reflection coefficients of the first, second, third and fourth standards and the known complex permittivities of the first, second, third and fourth standards;
• determining an admittance parameter of the combination of the probe and the first standard, and the combination of the probe and the second standard, from their known complex permittivities and the system constant;
• measuring the reflection coefficient of the sample;
• determining the admittance parameter of the sample and probe combination from the determined admittance parameter of the first and second standards and the measured reflection coefficients of the sample and the first, second, third and fourth standards;
• and
• determining the complex permittivity of the sample from the admittance parameter of the sample and the system constant.

An apparatus for determining the permittivity of a sample comprising:

• a coaxial probe having a first end positioned at the sample and a second end;
• means for applying an AC electrical signal in the microwave frequency range to the second end;
• means for measuring the resulting reflection coefficient of the sample;
• means for determining an admittance parameter of the sample and probe combination using the admittance parameter of a pair of standards determined from their known complex permittivities and a system constant, and measured reflection coefficients of the sample and four standards having known complex permittivities, wherein the system constant has been determined from the measured reflection coefficients of the four standards and the known complex permittivities of the four standards;
• and
• means for determining the complex permittivity of the sample from the admittance parameter of the sample and the system constant.

An apparatus for determining the permittivity of a sample comprising:

• a microwave source;
• first and second directional couplers, each of said first and second couplers having an input, a main output and an auxiliary output, wherein said input of said first directional coupler is coupled to said microwave source and said input of said second directional coupler is coupled to said main output of said first directional coupler;
• a coaxial probe having a first end and a second end, said first end positioned near the sample and said second end coupled to said main output of said first directional coupler;
• a local oscillator coupled to a first and second mixer, wherein said first mixer is further coupled to said auxiliary output of said first directional coupler and said second mixer is further coupled to said auxiliary output of said second directional coupler;
• first and second analog to digital converters, wherein said first analog to digital converters is coupled to said first mixer and said second analog to digital converters is coupled to said second mixer;
• microprocessor means coupled to said first and second mixers for receiving reflection coefficient data indicative of the sample and for determining the permittivity of the sample using the reflection coefficient data.