Stealth technologies based on thin microwave FSS absorbers

Abstract

The objective of this work was to study and develop new solutions for electromagnetic scattering problems in order to improve the radio frequency performance of modern wireless systems. This is achieved by engineering low profile stealthy structures based on metal backed resistively loaded Frequency Selective Surfaces (FSS).

The deployment of ultra-thin (≤ λ/17) microwave FSS absorbers is proposed as a means to reduce the scattering of electromagnetic energy from the surface of satellite platforms which are covered with thermal blankets. This is achieved by exploiting the similarity of the physical construction of this class of absorber and the dielectric clad foil backed outermost layer of space blankets. Simulated reflectivity results are presented for five close packed hexagonal patch FSS based absorber designs, ranging in thickness 140 µm − 112 µm (λ/213 − λ/25 at 10 GHz). These are suitable for Mechanical integration into the top surface of a multi-layer insulator (MLI). A desktop inkjet printer was used to pattern the array elements and the required surface resistance (Rs) which ranges from 50 mΩ/sq−40 Ω/sq, was obtained by employing a suitable mixture of nanosilver particle ink mixed with an aqueous solution in conjunction with controlling the print dot density. The same manufacturing technique was used for all of the experimental test pieces described in this thesis. A single sheet (140 µm, Rs = 50 mΩ/sq) of Polyethylene Terephthalate (PET) substrate was used to create the thinnest absorber and eight sheets were stacked together to create the 1120 µm (Rs = 20 Ω/sq) ultra-thin absorber. Reflectivity and radiation pattern experiments were performed in anechoic chambers, and the results are shown to compare favourably with the numerical predictions. To demonstrate the effectiveness of this concept, a dipole antenna was designed to work at 10 GHz and placed above the metal surface of a 10x10x10 cm3 mock-up of a CubeSat. The installed radiation patterns of the CubeSat with and without the FSS absorber are compared to experimentally confirm that a major increase in the gain and polarisation purity is obtained by suppressing the backscatter from the top surface of the platform. In addition, using the ultra-thin absorber to electromagnetically decouple the antennas from the host vehicle removes the boresight null which occurs when the antenna is placed λ/2 above the CubeSat, and the forward hemisphere radiation pattern is shown to be very similar to the antenna in free-space.

This work also reports the use of resistively loaded FSS superstrate and substrate absorber arrangements as a means to reduce the radar cross-section (RCS) and hence create low-observable (‘stealthy’) metal backed antennas. The design methodology for the superstrate absorber is demonstrated by creating a 3 mm thick absorber, patterned with two nested loops and a centre patch (Rs = 40Ω/sq), placed above a slot array which exhibits a transmission window covering the working frequency band (10−10.2 GHz) of a 4x4 microstrip patch array. It is shown that although the FSS based superstrate absorber reduces the antenna gain by less than 2 dB (using this as the benchmark obtained in the open literature), it has minimal impact on the shape of the beams which are directed at 0°, 22.5° and 45°. Moreover 90% radar backscatter suppression is achieved over 92% of the frequency range 7 − 24 GHz. For the substrate solution, an alternative 3 mm thick broadband metal backed absorber design was created by patterning four loops and a centre patch (Rs = 18Ω/sq) on the PET sheet. The structure was designed to exhibit −10 dB reflectivity over the frequency range 7.04−27.58 GHz, resulting in a fractional bandwidth of 118.65%. It is shown that by carefully removing 24 unit-cells located immediately behind a 7.5 GHz dipole placed λ/4 above the FSS surface, the gain is only reduced by 0.17 dB compared to a conventional metal backed antenna, but in this case the RCS is significantly lower. The experimental results obtained for the two absorber arrangements and integrated antenna designs, are shown to be in close agreement with the computed reflectivity and far field patterns.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	CAPES Foundation, Ministry of Education
Supervisor	Robert Cahill (Supervisor), Vincent Fusco (Supervisor) & Gareth Conway (Supervisor)