Frquency selective surfaces
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Frquency selective surfaces






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Frquency selective surfaces Frquency selective surfaces Presentation Transcript

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