Delectric materials are used in nearly all RF and microwave structures, outside of those that are strictly made of conductors. Dielectrics are crucial for making transmission lines, dielectric waveguides, and separating conductors. Given that the properties of planar circuits, transmission lines, antennas, and other active/passive RF hardware are dependent on the dielectric constant and loss tangent of dielectric components, understanding of dielectric material properties is essential. However, testing dielectric materials for complex permittivity performance requires some setup and design of a dielectric test structure to yield the most useful results.
Hence, there are a wide variety of methods for measuring the dielectric properties of dielectric materials. It is generally advisable to devise a characterization or measurement strategy that measures a dielectric in a configuration that is as near as possible to the method in which the dielectric will be used. For RF/microwave applications there are four common methods, transmission/reflection method, open ended coaxial probe method, free space method, and resonant methods. Each of these methods makes use of a vector network analyzer (VNA), or optionally simply a network analyzer (NA) for the open-ended coaxial probe and resonant method. With the use of NA instruments, comes the need for high quality metrology-grade interconnect, such as coaxial VNA test cables and potentially high-speed end launch connectors if the dielectric is planar or a circuit board laminate.
Common Dielectric Materials/Use Cases for Dielectric Measurements
- Capacitor materials
- RF substrates
- Radiation absorbing materials (RAMs)
- Sensor materials
- Stealth composites
- Radomes
- Dielectric inserts for transmission lines or waveguides
- Ceramics and composites for semiconductors and IC packaging
- Insulation materials
- Paint, adhesives or environmental coatings
In most RF applications, knowing the dielectric constant, loss tangent, and magnetic permeability of a dielectric are essential criteria for selecting an appropriate dielectric. In the case of dielectric constant, or relative permittivity, the dielectric will have an impact on any electric field passing through the dielectric. How significant this effect is depends on the relative permittivity of the dielectric. In a parallel plate capacitor, the dielectric constant of the material between the parallel conductive plates directly impacts the overall capacitance of the structure. In the same way, the dielectric material in between, within, or coating a transmission line, circuit, or other RF structure affects the characteristics of the structure based on the dielectric constant. This effect also enables the design of structures that can be used to measure transmission lines and other structures and extract the dielectric constant from the measured results. It is important to note that for many dielectrics, the dielectric constant is a function of frequency.
The loss tangent, or dielectric loss, is another crucial dielectric property that dictates the amount of energy lost from an electric field passing through a dielectric. Some dielectric materials absorb more energy from an incident electric field than others, and this results in dielectric loss. For transmission lines and most planar circuits, it is desirable to have a low loss tangent. However, there are applications, such as radiation absorbing materials (RAMs) for anechoic chambers and stealth platforms that prioritizes high loss tangent.
Magnetic permeability is typically an undesirable trait in most RF dielectric applications, though it is important for the permeability of a material to be known to avoid complications or if specifically needed for a specialized application.
Stay tuned for Part 2 and Part 3 of “Highlights of Measuring Dielectrics With RF Equipment”