Radome Diagnostics: utilizing Source Reconstruction based on Surface Integral Representations

University dissertation from Department of Electrical and Information Technology, Lund University

Abstract: In this thesis, an inverse source reconstruction method with great potential in radome diagnostics is presented. A radome is a cover that encloses an antenna in order to protect it from environmental influences. Radome diagnostics are acquired in the design process, the delivery control, and in performance verification of repaired and newly developed radomes. A measured near or far field may indicate deviations, e.g., beam deflection, but the origins of the flaws are not uncovered. In this thesis, radome diagnostics is performed by imaging the tangential electromagnetic fields on radome surfaces, disclosing the radome influence on the electromagnetic fields as well as the positions and influences of defects.

The source reconstruction is based on a surface integral representation together with the extinction theorem. The extinction theorem and its associated surface integral equation ensure that the reconstructed tangential electromagnetic fields have their sources within the radome. The presence of axial symmetry in the measurement set-up enables usage of the fast Fourier transform to reduce the computational complexity. Furthermore, the problem is solved by an in-house body of revolution method of moments (MoM) code utilizing a singular value decomposition (SVD) for regularization. The reconstruction is performed on a fictitious surface in free space, located precisely outside the physical surface of the radome, i.e., no a priori information of the material of the radome is requested. Moreover, both synthetic and measured data are used to verify the method.

In Papers I-III, the measurement set-up is a reflector antenna covered by a monolithic radome, and the near field is measured on a cylindrical surface. The height of the radome corresponds to 29-43 wavelengths in the frequency interval 8.0-12.0 GHz. The amplitude and phase of the tangential electromagnetic fields are reconstructed on the radome surface and the influence of the radome is investigated. Moreover, the alteration of the phase due to the transmission of the radome, the insertion phase delay (IPD), is imaged. Defects in the form of square copper patches, with an edge length corresponding to 1.6-2.4 wavelengths in the considered frequency interval, are attached to the radome wall. These might serve as a model for e.g., a lightning conductor or a Pitot tube. The attached patches alter the near field, and by applying source reconstruction, the disturbances of the patches are focused and detectable.

In Paper IV, the field is measured on a spherical sector in the far-field region at 10.0 GHz. Two set-ups with dielectric defects attached to the radome surface, are investigated. The aim is to investigate if variations in the electrical thickness of the radome wall can be detected. It is concluded that it is possible to discover dielectric patches of various edge sizes (0.5-2.0 wavelengths), and with the smallest thickness corresponding to a phase shift of a couple of degrees.

In Paper V, a frequency selective (FSS) radome corresponding to a height of 51 wavelengths at the frequency 9.35 GHz is investigated. The electrical performance of an FSS radome depends on the periodic structure of the elements in the radome frame. The periodic structure of the investigated radome is disrupted by horizontal defects (vertical displacements of elements) and vertical defects (a column of missing elements). The far-field data is measured on a spherical sector, and the far-field data reveals that the radome changes the radiation properties. The tangential electromagnetic fields on the radome surface are reconstructed for several antenna illuminations to image the cause of these alterations. Furthermore, it is shown that the different components of the electromagnetic fields are affected differently by the defects, implying that both co- and cross-components of the electric and magnetic fields need to be considered. Moreover, the Poynting's vector is employed to visualize how the defects block the field from the antenna.