Microwave absorbers are materials that can effectively absorb incident microwave energy to make objects invisible to radars. These are therefore commonly used for aircraft cloaking and warship stealth applications. 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 300 MHz to 2 GHz). onventional 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 for use in aircraft cloaking, warship stealth and broadband antennas. 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. Its absorption range covers a broad band from 0.7 to 1.9 GHz below -10 dB, and the total thickness of the absorber is only 7.8 mm, which is one of the thinnest microwave absorbers reported. The unit patterned cell absorbs microwaves and can also be actively controlled by stretching to expand the tunable bandwidth. In a paper published this week in the Journal of Applied Physics, from AIP Publishing, the researchers presented this work.
The usual thickness of conventional radar absorbers is a quarter the wavelength of the incident microwave. In the high frequency regime, take one gigahertz as an example, the thickness of the absorber would be around 7.5 centimeters, which is too thick and heavy to be used in aircrafts or warships. The proposed absorber is almost ten times thinner than conventional ones. Other alternative absorbers, such as metamaterial absorbers made from a resonant metallic structure printed on a dielectric substrate, though significantly thinner than the wavelengths absorbed, have a narrow working bandwidth.
To develop a novel absorber that is both thin and with 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. Besides the lumped impedances of the loaded elements, the researchers discovered that the parameters of the unit patterned cells contribute to the device’s absorption performance as well.
Xu noted that it is the first time that stretching transformation pattern is used in the active frequency-selective surface absorber to expand the bandwidth, which turns out to be an effective technique for producing broadband tunability. The researchers’ next step is to study the polarization and the oblique incidence performance for the proposed active frequency-selective surface absorber.
