frquency selective surfaces

A Seminar On Frequency Selective Surfaces

And Its Applications(Dichroic)

Presented By,

Bhaskara.Naik.S(11EC63R01)

Mtech(RF and Microwave)

E&EC Dept.

OutlineIntroductionFSS-General Mechanism.Elements of FSS Design

Element GeometriesElement Dimensions

Types of FSS1.The center connected or N-poles 2.The loop types3.Solid interiors or plate types.4.Combinations of 1,2,3

Applications of FSS.DrawbacksConclusionsReferences

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 DesignElement 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.

--Dx and Dz-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

ll

l

ll

ll

• 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

RadomesMulti-Frequency ReflectorsBeam Control ArraysFSS 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

• www.ansoft.com/adf_uk/Frequency_Selective_Surfaces.pdf

• http://www.eda.kent.ac.uk/research/theme_detail.aspx?gid=1&tid=21

• http://en.wikipedia.org/wiki/Radome

• http://www.antenna-theory.com