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Pasternack BlogConductors, semiconductors, and insulators are the main building blocks of virtually all RF/microwave electronics. Insulators have dielectric properties that impact the electric fields that pass through them. In order to design an RF component or device, detailed knowledge of the dielectric properties of the insulators used in the construction of the component/device is essential. The two main dielectric parameters to consider are the dielectric constant and loss tangent, or the real and imaginary part of complex dielectric permittivity, respectively.
In more detail, a dielectric is an insulating material that can be polarized while subjected to an electric field, called dielectric polarization. Though the magnetic permeability of space is generally static, except in the presence of ferromagnetic objects; the electrical permittivity of space, however, is heavily influenced by the gases, liquids, and solids in the space filled by the material. As with magnetic permeability, electrical permittivity has frequency dependent effects. Hence, complex dielectric permittivity is measured over frequency.
Dielectric Polarization Mechanisms
• Dipolar Polarization
• Ionic Polarization
• Electronic Polarization
• Atomic Polarization
• Interfacial or Space Charge Polarization
• Dielectric Loss
• Relaxation Time
Moreover, dielectrics require very precise measurements in order to determine the dielectric constant and loss tangent to any degree of accuracy. There are also many physical factors to consider when testing dielectrics, as electrical phenomena are impacted by dielectric performance and electromagnetic waves have physically dependent parameters (i.e. wavelength). This means that some types of microwave measurement methods for dielectrics are better for some types of dielectrics and applications than others. The bases for dielectric characterization and testing is the passing of electric fields or electromagnetic waves through the dielectric with a controlled environment around the dielectric and sensitive measurement equipment that can detect the changes to the fields and waves by the dielectric.
Dielectric Measurement Considerations
Dielectric physical size/dimensions
Can it be measured as a sample?
• Bulk sample
• Thin sample
• Ultra-thin sample
• Liquid
• Gas
Does the test need to be non-destructive, or can it be destructive?
What frequencies need to be measured
• Banded
• Broadband
• Narrowband
• Single frequency (resonant)
What measurement accuracy requirements are there?
Does the dielectric have a high or low dielectric constant?
Does the dielectric have high or low dielectric loss?
Due to the limited bandwidth, geometry, fixturing, dielectric sensing range, and other physical factors of a dielectric measurement technique, many methods have emerged to measure dielectrics. The method used depends on the dielectric under study, as well as the requirements for bandwidth. For full-wave field simulators, it is helpful to have broadband dielectric performance from the lowest to highest frequencies of interest.
Microwave Measurement Methods of Dielectrics & Application Frequency
Whispering Gallery
• Dielectric Resonator
10 GHz to 100 GHz
• Split-Post Resonator
Hundreds of MHz to several GHz
• Split-Cylinder Resonator
Several GHz to tens of GHz
• Re-entrant Cavity
Hundreds of MHz to several GHz
• Waveguide Resonator
Several GHz to tens of GHz
• Transmission Line Resonator
Several MHz to hundreds of GHz
• Multiple Transmission Lines
Several MHz to hundreds of GHz
• Filled Waveguide
Several GHz to tens of GHz
• Filled Transmission Line
Several MHz to hundreds of GHz
• Parallel Plate Capacitor
DC to over 1 GHz
• Open Coax Probe
Near DC to several GHz
• Thin Film Resonator
Hundreds of MHz to several GHz
• Evanescent Probe
Hundreds of MHz to several GHz
• Thin Film Transmission Line
Hundred of MHz to over 100 GHz
• Thin Film Capacitors
kHz to several GHz
Many of these methods require precision transmission lines, such as coaxial assemblies, precision coaxial connectors, waveguides, and planar transmission lines, especially the methods that range beyond several hundred megahertz to over 100 GHz. Some methods, such as microwave or millimeter-wave free space dielectric measurements, require setups complete with highly directional antennas, coaxial or waveguide interconnect, and a vector network analyzer.
Learn more about Pasternack’s expansive line of RF/Microwave test hardware and systems by following these links:
