Browsing by Subject "Frequency selective surfaces."

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Open Access
Design, fabrication and measurement of hybrid frequency selective surface (FSS) radomes
(2009) Sağlam, Özkan
In 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.
Open Access
Frequency selective surfaces for terahertz applications
(2015) Ramzan, Mehrab
This thesis presents Terahertz (THz) Frequency selective surfaces (FSS) that can be realized using standard microfabrication techniques. These FSS structures are designed for frequencies around 0.8 THz, which is a crucial operating frequency in security and medical imaging. Using THz waves for such applications, multilayer frequency selective surfaces are preferred due to their wide at band response, lower dependency to angle of incidence, and low loss. The implementation of such structures requires very thin layers of substrates and membranes in order to improve the performance in THz regime. In order to alleviate the di culty in the implementation of multilayer structures, a fabrication process is proposed where a 100 m-thick glass membrane is formed through HF etching of a 500 m-thick glass wafer. Using this fabrication process, three separate designs consisting of single-layer FSS are investigated using high frequency structural simulator (HFSS). The rst design, consists of a circular ring slot in a square metallic structure on top of a 100 m-thick Pyrex glass membrane with 95% transmission bandwidth of approximately 0.042 THz , which remains nearly constant till 30o angle of incidence. The second design consists of a tripole structure on top of a 100 m-thick Pyrex glass membrane with nearly 95% transmission bandwidth of 0.015 THz, which remains nearly constant till 30o angle of incidence. The third structure consists of a triangular ring slot in a square metal on top of a 100 m-thick Pyrex glass membrane with 95% transmission bandwidth of 0.015 THz, which remains nearly constant upto 20o angle of incidence. These designs show that the re ections from samples can be reduced compared to the conventional sample holders used in THz spectroscopy applications; by using single layer FSS structures manufactured through a relatively simple fabrication process.