Radome design wi̇th X-band frequency selecti̇ve surface

Frequency selective surfaces (FSS) are two-dimensional infinite periodic arrays of metal patches or aperture elements that are printed on a dielectric layer. Depending on the frequency of the electromagnetic wave, transmission or reflection characteristics of the FSSs are designed in the desired manner. They operate as electromagnetic filters that pass or stop desired frequencies. The transmission and reflection characteristics of the FSSs vary depending on the permittivity of the dielectric layer used, the thickness of the layer, the geometric properties of the metal patches and apertures, the unit cell size, and the angle of arrival of the electromagnetic wave. In addition, the effect of the angle of incidence on the transmission and reflection characteristics was observed. In microwave frequencies they are involved in many applications such as reflector, radome, absorber and electromagnetic shielding. Frequency selective surfaces are used on radomes in order to reduce the antenna radar cross-section (RCS). Due to the curved surface of nose type radomes, FSS applications on those surfaces have significant difficulties with respect to its planar counterparts. Another specific application of FSSs is radomes. A radome protects an antenna from outer environment with smallest effect on the radiation properties of the antenna within a frequency band of interest. Performance of a radome depends on matching its configuration and materials composition to a particular application and RF frequency range. A radome produced by curved FSSs acts as a bandpass filter that allows transmission in an operating frequency band, and reflection out of the band. It is usually difficult or impossible to control the RCS of antennas over a wide frequency range. Instead of shielding the antenna to protect it from a threat radar, which blocks the antenna operation, it is a wiser approach to cover the antenna with an FSS radome that is transparent at the operating frequency band, yet opaque at the threat radar frequency. Although the usage of FSSs as a protective radome leads to a considerable reduction in the RCS of the enclosed antenna outside the operating band, it might affect the electrical performance of the antenna in terms of polarization purity reduction, sidelobe level increase, transmission losses, degradation in impedance matching properties, tilting of the main beam, and strong undesired electromagnetic interference. Hence, accurate radome design and production using FSSs are important. In this study, a new miniaturized FSS radome which has a band-pass filter characteristic operating in the X band was designed. Conductive structures with circular loop geometry have been placed periodically on the dielectric layer. Resonance frequencies and transmission characteristics have been investigated in detail, and the effect of the metal patch and the dimensions of the aperture to the resonance frequency was observed. The designed FSSs have been placed in the mouth openings of Horn and Vivaldi antennas and the effects on these antennas were investigated. Experimental results based on measuring the transmission curves of fabricated radome prototypes have been supported by computer simulations and the whole structure has been simulated using the CST Studio Suit. 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 have been 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.