Engineering news
Huazhong University of Science and Technology's tunable microwave absorber material
A team of researchers from Huazhong University of Science and Technology in China have developed a high-performance, ultra-thin microwave absorber that could be used to improve cloaking technology commonly used for aircraft and warship stealth.
Microwave absorbers are a kind of material that can effectively absorb incident microwave energy to make objects invisible to radar.
Recently, as radar detection devices have been improved to detect the near-meter microwave length regime, scientists are working on high-performance absorbers that can cloak objects in the equivalent ultra-high frequency regime (from 300MHz to 2GHz). However, conventional absorbers for the ultra-high regime are usually thick, heavy or have narrow absorption bandwidth, making them unsuitable for stealth missions.
To solve this problem, a team of researchers from Huazhong University of Science and Technology in China has developed an ultra-thin, tunable broadband microwave absorber for ultra-high frequency applications. This ultra-thin absorbing surface, called an 'active frequency-selective surface absorber', consists of arrays of patterned conductors loaded with two common types of circuit elements known as resistors and varactors.
The unit patterned cell absorbs microwaves and can also be actively controlled by stretching to expand the tunable bandwidth. This means it can absorb a wide range of frequencies for near-meter microwave application.
Wenhua Xu, the primary researcher in the team led by Jianjun Jiang, a professor of School of Optical and Electronic Information at the Huazhong University of Science and Technology, said: “Its absorption range covers a broad band from 0.7 to 1.9GHz below -10 decibel, and the total thickness of the absorber is only 7.8mm, which is one of the thinnest microwave absorbers reported.”
Usually in the high frequency regime, such as one gigahertz, the thickness of the absorber would be around 7.5cm, which is too thick and heavy to be used in aircrafts or warships. “Our proposed absorber is almost ten times thinner than conventional ones,” Xu said.
To develop a novel absorber that is both thin and has broadband performance, Jiang’s team employed a type of thin, light periodic structure called a frequency-selective surface, which consists of an assembly of patterned conductors arranged in a two-dimensional array, usually backed by a thin dielectric, to reflect incident microwaves according to their frequency.
In the experiment, Jiang’s team fabricated a broadband active frequency-selective surface with a stretching transformation pattern on a printed circuit board, and soldered the resistors and varactors between each of the two unit patterned cells. The fact that the surface could be stretched meant that the parameters of the unit patterned cell can be actively controlled by stretching.
By modeling the absorber using a transmission line, the researchers found that the varactor provides a variable capacitance at varying bias voltage, which produces the device’s tunability, while the lumped resistor with constant resistance reliably produces strong absorption at the resonance frequency.
Xu noted that it is the first time that stretching transformation pattern has been used in the active frequency-selective surface absorber to expand the bandwidth, which turns out to be an effective technique for producing broadband tunability.
Xu added: “Our proposed absorber has achieved broadband tunability and ultra-thin film simultaneously. The total thickness of 7.8 millimeters is around one twenty-ninth wavelength of the central frequency of incident microwaves, and the ultra-thin absorber with broad bandwidth may be widely used in warship stealth, airplane cloaking and tunable, broadband antennae.”
The researchers’ next step is to study the polarization and the oblique incidence performance for the proposed active frequency-selective surface absorber.
The researchers recently presented this research in a paper published in the Journal of Applied Physics, from AIP Publishing.