Permittivity measurement in the Millimeter-Wave Band
In recent years, communication technologies and radar applications utilizing the millimeter-wave band (30–300 GHz) have been rapidly advancing. As a result, there is an increasing demand for high-precision evaluation of the electrical properties of materials, particularly dielectric constant (Dk) and dissipation factor (Df). Against this backdrop, the measurement technique known as the resonator method has been receiving growing attention.
Key Terminology
In this article, the real part of the relative complex permittivity (ε') is denoted as Dk, and the loss tangent (tan δ = ε'' / ε') is denoted as Df. Unless otherwise specified, the term "permittivity" refers to the relative complex permittivity throughout this article.
What is the "Resonator Method" ?
The resonator method is a technique for deriving the electromagnetic properties of a material by inserting it into a resonant cavity and analyzing the resulting changes in its resonant characteristics.
- Shift in resonant frequency reflects the material’s Dk.
- changes in the quality factor (Q-factor) are associated with the Df.
By using a resonator with an extremely high Q-factor, even minute changes in loss can lead to clearly observable variations in the measured Q-value. This makes it possible to accurately characterize materials with very low dielectric losses, which is one of the key advantages of this method.
Representative Equipment and Measurement Methods
Several types of equipment based on the principle of the resonator method are available, and each is used depending on the frequency range. Below is an overview of the various resonators we offer and the core technologies behind them.
Cavity Resonator（1-10GHz）
In a cavity resonator, dielectric properties in the 1 GHz to 10 GHz range are measured using TM₀₁₀ and TM₀₂₀ mode resonances generated within the cavity.
In these modes, the maximum electric field occurs at the center of the cylindrical cavity, and by inserting a rod-shaped sample at this location, the dielectric constant is evaluated using the linear electric flux aligned along the sample.
By changing the orientation in which the sample is cut, it is also possible to assess material anisotropy.
Split Cylinder Resonator（10-80GHz）
The method in which the resonator is split at the center to sandwich a flat, plate-shaped sample is known as the split-cylinder resonator method.
Compared to cavity resonators, it offers superior performance at high frequencies and can cover frequencies up to 80 GHz.
In the case of a cavity resonator, measurement errors can arise due to field distortions at the ends of the electric flux lines within the sample. These effects become more pronounced at higher frequencies, where the sample size is smaller.
The split-cylinder method employs TE₀₁₁ mode resonance, in which the electric flux circulates within the plane of the sample. This configuration eliminates the edge-related distortions, enabling accurate measurements even at high frequencies.
Fabry-Perot Resonator（25-330GHz）
The Fabry–Pérot resonator utilizes the TEM mode of linearly polarized waves that arise in an open resonator structure.
Its primary advantage is that it allows for practical-sized fixtures even at very high frequencies such as 330 GHz. (In contrast, extending the split-cylinder resonator method to such frequencies would require fabricating a precision cylindrical resonator with a diameter of less than 1 mm.)
Another key feature is that multiple resonant modes suitable for permittivity measurement appear across a wide frequency range. By effectively leveraging these modes, it becomes possible to evaluate the frequency dependence of permittivity using a single fixture.
Furthermore, since linearly polarized waves are used, material anisotropy can be evaluated by rotating the sample.
Use Cases
The resonator method is particularly well-suited for measuring materials with low Dk and low Df. For this reason, equipment based on the resonator method is widely used in material development aimed at applications that demand such characteristics. Typical examples include the development of substrate materials for 5G/6G, high-frequency packaging materials, and radar absorbers—all of which are essential in a broad range of high-frequency markets.
Conclusion
In this article, we have introduced the resonator method used in several types of permittivity measurement systems operating in the millimeter-wave band. Thanks to its high sensitivity and precision, the resonator method is an exceptionally powerful technique—particularly for evaluating low-loss materials. As applications continue to expand into the millimeter-wave and even higher frequency regions, the importance of permittivity measurement using the resonator method is expected to grow further.
EM labs offer a wide range of resonators that combine high precision, high speed, and ease of use. A variety of measurement examples for each product are available on our website—please take a look to see their capabilities in action. If you are interested in any of our products, feel free to contact us via the inquiry form. One of our engineers will get back to you shortly.