Frquency selective surfaces


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Frquency selective surfaces

  1. 1. A Seminar On Frequency Selective Surfaces And Its Applications (Dichroic) Presented By, Bhaskara.Naik.S(11EC63R01) Mtech(RF and Microwave) E&EC Dept.
  2. 2. Outline <ul><li>Introduction </li></ul><ul><li>FSS-General Mechanism. </li></ul><ul><li>Elements of FSS Design </li></ul><ul><ul><li>Element Geometries </li></ul></ul><ul><ul><li>Element Dimensions </li></ul></ul><ul><li>Types of FSS </li></ul><ul><ul><li>1.The center connected or N-poles </li></ul></ul><ul><ul><li>2.The loop types </li></ul></ul><ul><ul><li>3.Solid interiors or plate types. </li></ul></ul><ul><ul><li>4.Combinations of 1,2,3 </li></ul></ul><ul><li>Applications of FSS. </li></ul><ul><li>Drawbacks </li></ul><ul><li>Conclusions </li></ul><ul><li>References </li></ul>
  3. 3. <ul><li>Introduction </li></ul><ul><li>A Frequency Selective Surface (FSS)- is an any </li></ul><ul><li>planar surface designed as a ‘filter’ for microwave frequency waves. </li></ul><ul><li>Evolution from Radar Cross Section (RCS) </li></ul><ul><ul><li>Angular/frequency dependence </li></ul></ul><ul><ul><li>Band Pass/Band Stop behavior . </li></ul></ul><ul><li>FSS Characteristics </li></ul><ul><ul><li>Typically narrow band </li></ul></ul><ul><ul><li>Periodic, typically in two dimensions </li></ul></ul><ul><ul><li>Element type: dielectric or metallic/circuit </li></ul></ul><ul><ul><li>Depends on Element shape, size. </li></ul></ul><ul><ul><li>Depends on Element spacing and </li></ul></ul><ul><ul><li>orientation </li></ul></ul>What is an FSS?
  4. 4. <ul><li>Once exposed to the electromagnetic radiation, a FSS acts like a spatial filter; some frequency bands are transmitted and some are reflected. </li></ul><ul><li>Early 1960, FSS structures have been the subject of intensive study for Military Application. </li></ul><ul><li>Marconi and Franklin are believed, [1] to be the early pioneers in this area for their contribution of a parabolic reflectors. </li></ul>
  5. 5. <ul><li>Nippon designed shied film for windows that can shield the desired frequency. </li></ul><ul><li>i.e 2.45Ghz for WLAN or 1.9GHz for </li></ul><ul><li>PHS (Personal hand-phone systems). </li></ul>
  6. 6. <ul><li>FSS-is based on Resonance. </li></ul><ul><li>EM-wave illuminates an array of metallic elements, thus exciting electric current on the elements. </li></ul><ul><li>The amplitude of the generated current depends on the strength of the coupling of energy between the wave and the elements. </li></ul><ul><li>The coupling reaches its highest level at resonant frequency, when the length of elements is a λ / 2. </li></ul><ul><li>Hence elements are shaped so that they are resonant near the frequency of operation. </li></ul><ul><li>FSS-General Mechanism . </li></ul>
  7. 7. <ul><li>Current which acts as an EM source which gives a scattered field. </li></ul><ul><li>Each phase front has its on delay. These scattered radiation add up. Which makes transmission of that signal. </li></ul><ul><li>Scattered field+ incident field which results total field in the space surrounding the FSS. By controlling the scattered field, we can able to design required filter response. </li></ul><ul><li>Distribution of the current on the elements determines the frequency behavior of the FSS. </li></ul>
  8. 8. <ul><li>Resonance characteristics of FSS are depends, </li></ul><ul><ul><li>On the way the surface is exposed to the electromagnetic wave. </li></ul></ul><ul><ul><li>Incidence angle of the wave. </li></ul></ul><ul><ul><li>Effective aperture size of the FSS </li></ul></ul><ul><ul><li>Diffraction gratings. </li></ul></ul><ul><ul><li>Periodicity of cells. </li></ul></ul><ul><ul><li>Substrate that supporting the FSS element </li></ul></ul><ul><ul><li>Inter element Spacing. </li></ul></ul><ul><ul><li>Arrangement of Elements. </li></ul></ul>
  9. 9. <ul><li>Elements of FSS Design </li></ul><ul><li>Element Geometries: </li></ul><ul><li>2 Groups :- </li></ul><ul><ul><ul><li>Patch-type elements-capacitive effect. </li></ul></ul></ul><ul><ul><ul><li>Aperture-type element-Inductance Effect. </li></ul></ul></ul>
  10. 10. The patch-array produces a capacitive response, Low Pass Filter <ul><li>Patch-type elements </li></ul>
  11. 11. Array of slots is inductive- High Pass filter <ul><li>Aperture-type element. </li></ul>
  12. 12. <ul><li>Element Dimensions </li></ul><ul><ul><ul><li>For resonance, length of the element should satisfy the condition λ /2. </li></ul></ul></ul><ul><ul><ul><li>In Square loop total length of the side is λ /2 </li></ul></ul></ul><ul><ul><ul><li>Planar array of strip dipoles, produces a frequency frequency response when the length of the dipoles is a multiple of λ /2 </li></ul></ul></ul>
  13. 13. <ul><li>Types Of FSS </li></ul>1.The center connected or N-poles 2.The loop types 3.Solid interiors or plate types 4.Combinations FSS (Based on shapes by MONK,[1])– 4 Classes. 1.The center connected or N-poles 2.The loop types 3.Solid interiors or plate types 4.Combinations 3.Solid interiors or plate types 2.The loop types 4.Combinations 3.Solid interiors or plate types 1.The center connected or N-poles 2.The loop types 4.Combinations of 1,2,3 3.Solid interiors or plate types
  14. 14. <ul><li>Class I: Center Connected or N-Pole Elements </li></ul><ul><ul><li>Tripole Array </li></ul></ul><ul><ul><ul><li>Three concentric, thin monopoles which share a common point (the center). </li></ul></ul></ul>**(All Results from book by Munk on FSS, [1])
  15. 15. Fig shows the calculated reflectivity for closely spaced tripoles in an infinite array on x / z plane at design frequency 10 GHz. --D x and D z -Array periodicity along X and Z direction
  16. 16. <ul><li>Jerusalem Cross: </li></ul><ul><ul><li>Two crossing dipoles which are loaded with small, orthogonal sections at their ends. </li></ul></ul><ul><li>Jerusalem cross has got better stability than Tripole array </li></ul>
  17. 17. <ul><li>Class II: Loop Type Elements. </li></ul><ul><ul><li>Four-Legged Element-This element is made up of two meandered, half-wave dipoles. </li></ul></ul><ul><ul><li>The operation mechanism of this element can be explained using the transmission line theory. </li></ul></ul><ul><ul><li>Toatal goemetry of this element should satisfy </li></ul></ul><ul><ul><li>λ /2. </li></ul></ul><ul><ul><li>To Satisfy this l= λ /8. </li></ul></ul><ul><ul><li>Four-Legged Element </li></ul></ul>l l l l l l l l
  18. 18. <ul><li>Basic Idea of designing Four-Legged Element. </li></ul><ul><li>Overall reactive impedance of the dipoles which has to be zero at the resonance. </li></ul><ul><li>At the resonance frequency of the half-wave dipole, shorter dipole behaves as capacitive.[3] </li></ul><ul><li>We need to introduce new load ZL( must be inductive)so that the total impedance of the dipole becomes zero. </li></ul>
  19. 19. <ul><li>Shorting of a small piece of transmission line which creates inductance. [3]. </li></ul><ul><li>Length of the meandered dipole along the two directions, l , must be equal. l = λ /8 </li></ul>I C. Inductance formed( ZL )
  20. 20. <ul><li>Development of four-legged element </li></ul>(a) Fig Shows a simple half-wave dipole with impedance Zd which is shortened into a quarter-wave dipole with impedance Zd-jXd
  21. 21. (b) That is loaded with an inductance formed (c) using a short transmission line.
  22. 22. Two such dipoles are placed side by side (d) and finally are connected at the end points (e). [1]
  23. 23. <ul><li>Class III: Plate Type Elements- </li></ul><ul><ul><li>Plate type FSS .-2 forms . </li></ul></ul><ul><li>1. Array of metallic patches in the shape of a circular disk, square, rectangular, etc. </li></ul><ul><ul><ul><li>These are usually used as reflecting arrays . </li></ul></ul></ul>
  24. 24. <ul><li>2) Array of slots on a metallic plate in the form of circular, square shape, etc. </li></ul><ul><ul><li>This type is often transparent. </li></ul></ul>
  25. 25. <ul><li>Type 1 is highly depends on grating lobe where as type 2 is angle dependent as its patches (holes) dimensions must be the order of λ /2. </li></ul><ul><li>Hence these are not highly used in Filter design. </li></ul>
  26. 26. <ul><li>Class IV: Combination Elements </li></ul><ul><ul><li>Combination of these groups, which gives a new FSS element.. </li></ul></ul>This is the combination of four-legged element and Jerusalem cross
  27. 27. <ul><li>Applications of FSS </li></ul><ul><li>Radomes </li></ul><ul><li>Multi-Frequency Reflectors </li></ul><ul><li>Beam Control Arrays </li></ul><ul><li>FSS as reactive impedance surfaces. </li></ul><ul><li>FSS are used in design of Artificial Magnetic Conductors (AMC) and EM Band Gap (EBG) materials etc. </li></ul>
  28. 28. Application of frequency-selective surfaces as radome covers in the aircraft technology for reducing the antenna RCS [1] <ul><li>RADOME </li></ul><ul><li>RADOME-Operates in </li></ul><ul><li>2 modes. </li></ul>1.Trasnaparent mode 2 .Reflecting mode
  29. 29. <ul><li>Multi-Frequency Reflectors </li></ul><ul><li>FSS as a sub reflector along with Main dish. </li></ul><ul><li>Reflective at a particular frequency band and transparent at another frequency band. </li></ul>
  30. 30. <ul><li>Drawbacks </li></ul><ul><li>Since resonance depends on length, traditional FSS has got high harmonic content. </li></ul><ul><li>The issue of harmonics not only affects the frequency characteristics of the FSS but also degrades its performance. </li></ul><ul><li>FSS needs large no of cells, which increases the size of the system. </li></ul><ul><li>Since it depends on resonant condition of material, it is difficult to design miniaturized circuit. </li></ul>
  31. 31. <ul><li>Conclusion </li></ul><ul><li>FSS can be used as filter in Microwave frequency Region. </li></ul><ul><li>FSS plays a major role in technical as well as commercial aspects. </li></ul><ul><li>Even though FSS are depends on frequency, research are going on in miniaturizing these circuits. </li></ul>
  32. 32. <ul><li>References </li></ul><ul><li>[1]. D. Rittenhouse, &quot;An optical problem, proposed by Mr. Hopkinson, and solved by Mr. Rittenhouse,&quot; Trans. Amer. Phil. SOC., vol. 2, pp. 201-206, 1786. </li></ul><ul><li>[2] T. K. Wu, Frequency-selective surface and grid array,&quot; Wiley, New York, 1995. </li></ul><ul><li>[3] J. A. Kong, Electromagnetic wave theory,&quot; Cambridge, MA: EMW Publishing, 2000,pp. 180-274. </li></ul>
  33. 33. <ul><li>[4].Metamaterial-Inspired Frequency-Selective Surfaces by Farhad Bayatpur. </li></ul><ul><li>Web sites </li></ul><ul><li> Frequency _ Selective _ Surfaces .pdf </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul>