Dielectric Kit for Materials Characterization at Microwave Frequencies

Dielectric materials have many applications ranging from microwave components and subsystems to industrial and manufacturing processes. Often in these applications, a successful design greatly depends on an accurate knowledge of the complex permittivity of the materials at these frequencies.

The Dielectric Kit for Vials is a new instrument developed by the Microwave Division of the institute ITACA to determine the complex permittivity (dielectric constant and loss factor) of a wide range of liquid, granular or powdered materials around the ISM frequency of 2.45 GHz. It is a complete solution which includes all necessary components to perform the generation, control and analysis of microwave signals, calculation of dielectric properties and display of results.

Two versions of the Dielectric Kit for Vials have been designed depending on the sample volume (with standard vials of 1mL or 8 mL). The measurement probe dimensions have been optimized in each case to provide repeatability and high accuracy in the determination of materials dielectric properties. Unlike other available equipment, calibration and adjustment with reference liquids is not necessary, and measurement of sample dielectric properties is fast and convenient.

OPERATION PRINCIPLE

The equipment comprises a source (PLL Microwave Synthesizer), which generates the microwave stimulus, a set of directional couplers to separate reflected and transmitted signals, a microwave receiver based on the AD-8302 integrated circuit for magnitude and phase detection and a measurement probe, where the material to be measured is placed. A control unit (Microprocessor System) is connected through a USB link to a personal computer which includes all the required processing to determine dielectric properties from the measurements and to transform the outputs into the desired representation.

The dielectric probe is designed as a microwave resonator where the interaction of the signals with the material takes place. Vials containing the material under test are introduced in the microwave resonator though a hole at the top of the probe. After sample insertion, the probe response (resonant frequency and quality factor) is shifted depending on the sample dielectric properties. From the measurement of the new response, the complex permittivity of the sample is calculated by using the electromagnetic model of the structure by a numerical procedure.

The Vials Dielectric Kit is fully controlled by a Labview-based software, which has been developed to perform all the necessary functions with a user-friendly interface.

EXAMPLES OF MEASUREMENTS

This equipment is specially designed for liquid, granular or powdered materials. Solid materials might be measured if machined in rod shape with the inner vial dimensions. The following table shows some measurements of common materials. Comparisons with other measurement methods or published data are also included.

TECHNICAL SPECIFICATIONS

  • Frequency range: 1.5GHz to 2.6GHz (nominal, limited by MUT properties)
  • Dielectric constant: ε′<100
  • Loss factor: 0.001<ε′′<15
  • Accuracy: About 1% in dielectric constant and 2-5 % in the loss factor
  • Repeatability and Linearity 0.2%
  • Material under test assumptions:  (Minimum sample volume required :1mL (vial external diameter 4.1 mm) or 8 mL (vial external diameter 5.1mm), non-magnetic, isotropic, homogeneous (uniform composition). For granular materials (particle diameter < 2 mm) measurement repeatability is dependent on density variation.
  • Calibration: No calibration is needed
  • Microwave output: 0 dBm
  • Communication with PC: USB
  • Probe Materials: Aluminum
  • Dimensions: 190mm x 230mm x 85mm
  • Operating Temperature: 22-50 °C
  • Required OS: Windows XP/Vista/7