Accurate permittivity measurement to advance radio receivers

Japanese researchers have invented a novel method to measure the permittivity of insulators 100 times more accurately than before. Described in IEEE Transactions on Terahertz Science and Technology, their method is expected to contribute to the efficient development of sensitive radio receivers for radio telescopes, as well as devices for the next generation of communication networks.

Permittivity is a value that indicates how electrons inside an insulator react when a voltage is applied to the insulator, and an important parameter for understanding the behaviour of radio waves as they travel through insulators. In the development of telecommunications equipment, it is necessary to accurately determine the permittivity of materials used for circuit boards and building columns and walls. For radio astronomy, researchers also need to know the permittivity of components used in radio receivers.

By devising a calculation method for electromagnetic wave propagation, researchers and engineers from the National Astronomical Observatory of Japan (NAOJ) and the National Institute of Information and Communications Technology (NICT) have developed an analytical algorithm that derives the permittivity directly rather than by approximation. The team then used the new method to measure lens material for a receiver being developed for the Atacama Large Millimeter/submillimeter Array (ALMA) and confirmed that the results were consistent with other methods, demonstrating its effectiveness in actual device development.

Reducing the error due to approximation by a factor of 100 speeds up the development process. If the permittivity of individual materials is measured inaccurately, the actual fabricated product may not meet the target performance. If the permittivity is known accurately from the design stage, unnecessary trial and error can be reduced and costs can be cut.

Conventionally, there are several methods used for measuring permittivity. One method that can accurately measure permittivity is the ‘resonance method’, but in that case, the material to be measured must be placed in a device called a resonator, which requires precision processing of the material, sometimes less than several hundred micrometres thick. Another drawback is that the permittivity can only be measured at several specific frequencies. Since it is necessary to measure the permittivity of various materials during the development stage of a device, if high-precision processing is required for each measurement, the development process will take a long time. On the other hand, the ‘free-space method’, which has fewer of these drawbacks, is also used, but in this case, an approximation has been used to analyse the measurement results, and the error caused by this makes accurate measurement difficult.

“Compared to other measurement methods, the free-space method has fewer restrictions on the shape of the measurement sample, and it is easy to extend the measurement frequency band,” said Ryo Sakai, an engineer at NAOJ and lead author of the research. The new analysis method is used with the free-space method, which means that permittivity can be accurately measured with fewer constraints.

“The newly developed method is expected to contribute to not only the design of radio telescope components, but also to the development of high-frequency materials and devices for the realisation of next-generation communication networks (Beyond 5G/6G) using the millimetre-wave/terahertz band,” Sakai said.

Image credit: NAOJ.