Browsing by Subject "Radomes."
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Item Open Access Design, fabrication and measurement of hybrid frequency selective surface (FSS) radomes(Bilkent University, 2009) Sağlam, ÖzkanIn modern military platforms such as ships, aircrafts and missiles, frequency selective surfaces (FSS) are widely used for antennas and radar cross section (RCS) reduction. The RCS of complicated objects such as antennas are difficult or impossible to control over a wide frequency range. The most efficient and cost-effective approach in these situations is to shield the scattering object from the threat radars by making use of wide-band radar absorbing material (RAM) coating. If the object is an antenna, then obviously, the system served by this antenna cannot operate when it is stowed. An alternate approach is to cover the antenna with an FSS that is transparent at the antenna operating frequency, yet opaque at the threat radar frequencies. In this thesis, different types of FSS structures comprising slot elements and modified loop elements, namely single polarized loop FSS, have been investigated intensively with their applications to hybrid FSS radomes. Their resonance mechanisms and transmission properties are examined in detail. The main focus of the thesis is to design a hybrid FSS radome based on different unit element types. Complex dielectric constant measurements are conducted as aninput to the FSS radome design. Experimental results based on measuring the transmission curves of fabricated radome prototypes are supported by computer simulations. Transmission properties of the slot FSS structures and the single polarized loop FSS structures have been compared and discussed. In contrast with most of the published work in literature, transmission measurements are supported by the radiation performance measurements. Adaptation of the single polarized loop FSS radome to the slotted waveguide antenna has been achieved without any significant reduction in the radiation performance. The antenna with this metallic radome has the advantage of superior mechanical durability as well as reduced out-of-band RCS.Item Open Access Radiation fields of a complex source in 2-D circular radome with metal gratings(Bilkent University, 1997) Ouardani, SlimIn this thesis, the transmission effect of a two-dimensional circular radome with periodic metal gratings is analyzed. We started with the study of gratings consisting of periodic arrays of thin lossy strips surrounded by vacuum. Then we investigated the behavior of such gratings if a dielectric hiyer is inserted between them. Complex line sources are considered to simulate directed beam fields used in practice. The fields on the interior and exterior sides of the radome are represented by modal cylindrical waves. Taking advantage of theoretical considerations recently published, we propose an approximate method and stress the numerical aspect. Data is obtained for the far field solutions and the directivity, and their dependences on different radome parameters. It appecirs that directivity variations with beam orientation are decreased considerably by a proper insertion of the dielectric layer.Item Open Access Radiation fields of the line source in a cylindrical wire grating(Bilkent University, 1998) Karapınar, HakanIn this thesis, the transmission effect of a grid structure is analyzed. The grid structure is intended to model the metallic support elements of a radome. The total field for the real and complex position line sources surrounded by the grating structure is obtained in both TM and 7'E polarizations in the far-field region. The grid is considered to be a cylindrical array of perfectly conducting cylinders parallel to the z-axis. The radius of each cylinder is small as compared to the wavelength and the length of the cylinders is infinite. We started with the study of a single perfectly conducting cylinder illuminated by a line source and obtained the formula for the electric and magnetic fields in the far-field region. Then, we calculated far-zone total field for a set of cylinders which form the surface of the radome as a cylindrical periodic grating. The equation for the total electric field in the far-field region is found by using superposition and applying the boundary conditions at the conducting cylinders to find scattering coefficients. Complex line sources are considered to simulate directed beam fields used in practice. The power pattern and directivity are computed for different parameters of the grating and cylinders. A set of figures is presented to show the relationships between the power pattern, directivity and different parameters of the structure.