1) The document presents research on modifying the design of a frequency selective surface (FSS) to achieve multi-frequency operation and size reduction. 2) The initial FSS design uses a square patch that resonates at 11.2 GHz. Modifications replace the square patch with an H-shaped patch, then add patches, achieving additional resonant frequencies down to 3.83 GHz and reducing the size. 3) The best design is the third modification, which introduces a hexagonal slot and obtains three resonant frequencies between 3.83-9.2 GHz while minimizing the size. This design demonstrates a compact, multi-frequency FSS.

1. Project on a Frequency Selective
Surface with Multi Frequency
Operation
1 Soumya Saha,2 Supriyo Das,3 Sangita Sarkar,4 P.P. Sarkar
1,2,3,4 D.E.T.S, University Of Kalyani, Kalyani, India
© Soumya Saha 1

2. Introduction
• Frequency Selective Surfaces (abbreviated as FSS) is any planar
surface designed as a ‘filter’ for microwave frequency waves.
• These are formed by a two dimensional array of metallic patches
printed on a dielectric substrate.
• The response of FSS to incident radiation varies with frequency.
• They have found wide use in various applications such as screening a
radar transmitter or receiver from hostile emissions and providing a
reflective surface for beam focussing in a reflector antenna system.
© Soumya Saha 2

3. FSS Characteristics
• Typically narrow band
• Periodic, typically In 2-D
• Element type: dielectric r metallic/circuit
• Depends upon element shape, size
• Depends upon element spacing and orientation
© Soumya Saha 3

4. Evolution of FSS
• Once exposed to the electromagnetic radiation, a FSS acts like a
spatial filter: some frequency bands are transmitted and some are
reflected
• Early 1960s, FSS structures have been the subject of intensive study
for Military Application.
• Marconi and Franklin are believed, to be the early pioneers in this
area for their contribution of a parabolic reflectors.
© Soumya Saha 4

5. Evolution of FSS (Cont.)
• Nippon designed shied film for windows that can shield the desired
frequency i.e. 2.45 GHz for WLAN or 1.9 GHz for PHS (Personal Hand
phone Systems).
© Soumya Saha 5

6. FSS General Mechanism
• 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.
© Soumya Saha 6

7. FSS General Mechanism (Cont.)
• 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
behaviour of the FSS.
© Soumya Saha 7

8. Resonance Characteristics of FSS
Depends upon:
• 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.
© Soumya Saha 8

9. FSS Design Types
• Patch-Type : Capacitive Response, acts as a Band Stop Filter
• Mesh-Type : Array of slots is Inductive, acts as a Band Pass Filter
© Soumya Saha 9

10. Software Used
• ANSOFT Designer® : is a microwave engineering CAD suite that allows
for circuit and full wave Simulation.
© Soumya Saha 10

11. Simulation Setup Methods
The methods used to setup the simulation are outlined. In particular,
the following steps were followed:
• Planar EM Design Setup which includes
• Layout Technology Selection
• Layer Design & Type Selection
• Model Setup
• Excitation Setup
• Analysis Setup
• Plotting Results
© Soumya Saha 11

12. Design Approach of FSS
The FSS structure consists of two dimensional arrays of patches. The
arrays of metallic patches are aligned on top of a dielectric substrate.
The substrate is basically glass-PTFE. Its dielectric constant is 2.4 and
thickness is 1.6 mm in conventional FSS structure.
© Soumya Saha 12

13. • Reference Patch
Dimension of each patch is (20 x 20) mm. Periodicity is taken 24 mm
along both X and Y direction for constructing array of patches.
© Soumya Saha 13

14. Observations
• After Simulation, with ANSOFT Designer® the resonant frequency is
obtained at 11.2 GHz (the deeper one is considered) and the value of
percentage bandwidth is 14.06.
© Soumya Saha 14

15. Observations (Cont.)
• According to the transmission characteristics, this model is not
appropriate in the range between 11.8 GHz & 12.2 GHz, as the curve
does not show considerable behaviour and gets highly congested in
the tolerance level.
• The design is further modified for improvements.
© Soumya Saha 15

16. • Modification 1
In the 1st step of modification, the square reference patch is converted
to H-shaped patch. Within this transformed patch, a circular shaped
slot is cut.
© Soumya Saha 16

17. Observations
• Further after Simulation, with ANSOFT Designer® the resonant
frequency is obtained at 5.67 GHz (1st one) and 6.86 GHz (2nd one),
which shows reduction in resonant frequency (and multiple resonant
frequencies indicate multi frequency operation).Compactness has
been achieved as the resonant frequency shifts towards left side, i.e.
which means size reduction. Also the value of percentage bandwidth
obtained are 29.10 (1st one) and 11.7 (2nd one).
© Soumya Saha 17

18. Observations (Cont.)
© Soumya Saha 18

19. • Modification 2
• In the 2nd step of modification, FSS patch is further modified by
adding metallic patches to the H-shaped patch as shown.
© Soumya Saha 19

20. Observations
• Further after Simulation, with ANSOFT Designer® the resonant
frequency is obtained at 5.53 GHz (1st one) and 9.37 GHz (2nd one),
which shows reduction in 1st resonant frequency (and multiple
resonant frequencies indicate multi frequency
operation).Compactness has been achieved as the resonant
frequency shifts towards left side, i.e. which means size reduction.
Also the value of percentage bandwidth obtained are 31.25 (1st one)
and 20.91 (2nd one). Bandwidth utilization has been increased in this
design segment.
© Soumya Saha 20

21. Observations (Cont.)
© Soumya Saha 21

22. • Modification 3
In the 3rd step of modification, within this transformed patch, a
hexagonal shaped slot is cut.
© Soumya Saha 22

23. Observations
• Further after Simulation, with ANSOFT Designer® the resonant
frequency is obtained at 3.83 GHz (1st one), 6.12 GHz (2nd one) and
9.20 (3rd one). So far, this is the best design as 3 different resonant
frequency is obtained. Also compactness has been achieved as the
resonant frequency shifts towards left side, i.e. which means size
reduction. Further, the value of percentage bandwidth obtained are
23.45 (1st one), 3.96 (2nd one) and 10 (3rd one).
• Being a simple design and having design similarities with the
Modification 1, this can be easily deduced from it.
© Soumya Saha 23

24. Observations (Cont.)
© Soumya Saha 24

25. Why -10 Db Line ?
Because at this level the reflected power is minimum and is equal to
the 0.1 of that of the incident power. In theory -3 dB point is widely
used where half of the maximum value was taken, but for practical
accuracy, we refer to the logarithmic axis and the -10 dB line on the
logarithmic scale.
© Soumya Saha 25

26. Conclusion
• It has been seen that as the reference patch is modified step by step,
resonant frequency decreases in each step of modification. So it can be
said, in each modification step, size reduction is achieved. In step 3
resonant frequency is shifted left most as compared to all other modified
designs and reference patch. So the best size reduction is achieved in this
step.
• Considering number of resonant frequencies obtained in the modified
designs, it can be said that in step 3, three numbers of resonant
frequencies are obtained. Frequency separation between resonant
frequencies is quite good. In step 3 regarding resonant frequency and
number of resonant frequency the best result is obtained. This proposed
FSS can be used both as compact one and can be used in multiple resonant
frequency operation.
© Soumya Saha 26

27. Acknowledgement
Our sincere gratitude to Prof. Partha Pratim Sarkar, Project Head &
H.O.D, Department of Engineering & Technological Studies, University
of Kalyani, for providing us an opportunity to do our project work on
“Frequency Selective Surface with Multi Frequency Operation” as a
part of the Final Year B. Tech Curriculum. We, as a team sincerely thank
you for your guidance and encouragement in carrying out this project
work.
&
Special thanks to our M. Tech senior Ms. Diptargha Baul for her
support and guidance throughout the project.
© Soumya Saha 27

28. Thank You. Signing Off.
Credits:
SOUMYA SAHA
SUPRIYO DAS
SANGITA SARKAR
© Soumya Saha 28