On the Scattering Reduction of an Aircraft Wing Profile Enclosing an Antenna

Abstract: In this thesis we study the radar cross section reduction and transparency of anantenna radome placed in the front part of an aircraft wing. The antenna, notstudied here, is a linearly polarized antenna operating at 1 GHz with abandwidth of 30%. The thesis is split into two parts: a radar cross sectionreduction part for 2-16 GHz and a low-pass filter design to make the radometransparent at the antenna operating frequency and polarization. In the first partwe design and compare shape optimization with a radar absorber as twomethods for radar cross section reduction. For an optimized Jaumann absorberdesign we achieved a 5 dB reduction of the two-dimensional mono-static radarcross section over 68% of the frequency band as compared with the originalmetallic wing and for the polarization of the electric field is parallel to the wingaxis. It was observed during the design process that Jaumann absorber keeps itsmajor feature when it is applied to circular and elliptical surfaces; however, aresonant frequency shift occurs for elliptical structure due to varying thicknessof the elliptical Jaumann layers. It is shown that the Jaumann absorber has abetter performance for radar cross section reduction than shape optimizedscreen for the above mentioned polarization. Furthermore, by using theJaumann absorber design we leave more space for antenna inside the wing.The second part of the thesis is concerned with the transparency of theradome. It is required that the antenna enclosed by the radome can radiatethrough it. We propose a design where the main blocking element in theJaumann absorber, the metal back sheet, has been replaced with a frequencyselective surface. The frequency selective surface should be almost totallytransparent over the antenna frequency band and conversely, it must be almosttotally reflective over the remaining frequency band for the Jaumann absorberto function efficiently. The proposed FSS is a triangular lattice Gangbusterjoint with a polarizer. This design has a mirror symmetry which can be utilizedfor a cylindrical structure with periodic boundary conditions. An optimizationis done for the FSS backed Jaumann absorber to achieve a desirableperformance. The new FSS back Jaumann absorber design not only provides apartial transparency for the radome but it also shows an improvement in radarcross section compare with ordinary Jaumann absorber. However, the FSS backJaumann absorber suffers from a high transmission loss. The design is modifiedusing circuit analogue absorber to reduce the transmission loss.