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A Dual-Band Double Cylindrical Ring 3D Frequency
Selective Surface
Sultan Can, Emrullah Karakaya, Fulya Bağcı, A. Egemen Yılmaz, and Barış Akaoğlu1
This study presents a dual-band 3D frequency
selective surface which provides a large degree of
freedom in frequency and bandwidth adjustability. The
proposed structure is evaluated in terms of its
structural parameters and the prototypes have been
fabricated. The radius of the copper rings and the
height of the cylinders are considered and are shown
through full-wave electromagnetic simulation to have a
significant effect on the frequency characteristics of the
FSS. The measurement results of the fabricated
samples are compared with the simulation results and a
satisfactory agreement has been obtained.
Keywords: Frequency selective surface, dual band filter.
Manuscript received Oct. 18, 2015; revised Aug. 18, 2015; accepted Sept. 19, 2015.
This work was supported by the Scientific Research Projects of Ankara University
(BAP) under Grant no. 13B4343015.
F.B. also acknowledges “The Scientific and Technological Research Council of
Turkey (Tubitak)” through BIDEB-2219 Postdoctoral Research Fellowship.
Sultan Can (corresponding author, sultancan@ankara.edu.tr) and A. Egemen Yilmaz
(aeyilmaz@eng.ankara.edu.tr) are with the Department of Electrical and Electronics
Engineering, Ankara University, Ankara, Turkey.
Emrullah Karakaya (emrullah.fizik@gmail.com), Fulya Bagci (Fulya.Bagci@eng.
ankara.edu.tr), and Baris Akaoglu (akaoglu@eng.ankara.edu.tr) are with the Department
of Engineering Physics, Ankara University, Ankara, Turkey.
RP1510-0922e
I. Introduction
The studies regarding the filtering of electromagnetic waves
are concentrated on engineered surfaces, which are
periodically designed in order to manipulate the
electromagnetic waves. Frequency selective surfaces (FSSs),
single negative and double negative metamaterials and
artificial magnetic conductors have been the demanding
materials for filtering the plane waves at any incident angle
and polarization.
Traditional 2D FSSs can be designed from an array of 2D
unit cells, which are either etched slots of a conductor surface
or printed elements on a dielectric substrate [1]. 3D FSS
structures can be constructed by cascading these 2D layers
with a dielectric in between them or extending these FSS
elements in the perpendicular direction to the geometry. 3D
FSS structures are demonstrated to provide improved filtering
response with enhanced tailoring ability due to their extra
degrees of freedom in tailoring the filtering response [1-4].
This enhanced ability can lead to the introduction of the
desired number of zeros/poles at the desired spectral positions
[4]. Moreover, bandwidth and frequency stability against the
change of the incidence angle and polarization can be
achieved by using angularly symmetric periodic arrays. These
arrays may act as band-pass or band-stop filters for any
incidence angle and polarization. Simply, they may filter the
waves or let them pass depending on the structural parameters
and the periodicity. In this context, circular FSS arrays provide
better angular stability and band separation as well as lower
cross-polarization ability than several other types of FSS
arrays, such as dipole, square loop, tripod, crossed dipole and
Jerusalem cross. Moreover, using concentric ring FSS
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structures, more freedom of adjustability in frequency and
bandwidth can be provided, and multiband filtering at desired
frequencies can be achieved [5-8].
The FSS structures can be designed as active or passive
arrays. There are different techniques for achieving tunability
in
active
FSS
structures,
such
as
placing
micro-electro-mechanical-system switch models and PIN
diodes [9] or varactor diodes [10] between the FSS elements.
Although those elements provide dynamic tunability, their
feasibility is limited because of the requirement to many active
elements which brings complexity in design and increment in
price. On the other hand, the elements are excited by the
incident plane wave in passive FSS arrays [1] and the
tunability is provided by the structural parameters of the unit
cell.
In this paper, a passive 3D cylindrical ring FSS structure is
proposed and investigated. The 3D FSS structures consist of
vertically aligned concentric ring cylinders. The structure and
the simulation method with its boundary condition and
excitation information are given in section II. The simulation
of the structure under the determined condition is conducted
and the effects of the structure parameters are investigated via
CST, which is a Finite Integration Technique (FIT) based
CAD solver. A parametric analysis regarding the diameters of
the inner and outer cylinders and their height is conducted in
Section III. The measurement set up and the fabricated
structures are shown in Section IV as well as the measurement
results with respect to the simulation results are presented. The
simulation and the measurements are conducted under the
assumption of the normal incidence; however, the effects of
the incidence angle and polarization are also investigated.
The proposed 3D FSS structure consists of two 3D
conductor ring cylinders, one at the outer part and the other in
the inner part, as shown in Fig. 1. The outer ring cylinder has a
conductor width of wo and a height of houter while the inner ring
cylinder has a conductor width of wi and a height of hinner.
Outer radius of the outer ring cylinder is denoted as router
and the inner radius of the inner cylinder is denoted as rinner.
Inner and outer diameters of the cylinder are denoted as dinner
and douter, as well. As shown in Fig. 2 the direction of
propagation is denoted with k and the electric field and
magnetic field are perpendicular to the propagation direction
and denoted as E and H, respectively. Since the structure is
rotationally symmetric, the orientation of the E is found to
have negligible effect on the scattering parameter results.
The simulations are conducted by assuming the unit cell of
the double ring cylinder structure as periodically aligned in the
lateral plane with infinite number with a periodicity (p) of
34mm.
II. Geometry of the proposed 3D FSS structure
Fig. 2. 3D FSS structure with p=34mm
III. Simulation results
h
2× rinner
wi
wo
a)
2× router
b)
Fig. 1. Physical parameters of the proposed 3D FSS unit cell structure
a) top view b) perspective view
RP1510-0922e
The S-parameter results are obtained from the simulation
and analyzed by keeping the both height values same (houter =
hinner=h). Moreover, in order to keep the distance between the
cylinders the same, the outer diameters of the inner ring
cylinder and the inner diameter of the outer ring cylinder are
kept constant as 20 mm and 32 mm, respectively (9mm gap
between cylinders).
In the simulation process, several
b) impacts have been
evaluated, such as the outer radius of the outer ring cylinders,
inner radius of the inner ring cylinders and the height of the
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cylinders. The structure having a height value of h=25mm is
evaluated by varying douter for 31mm, 32mm and 33mm and
the results regarding S21 characteristics are presented in Fig. 3.
As there are two structures, one cylinder at the outermost
layer and one cylinder inside of the former, two resonances
occur when S21 is evaluated.
frequency and as a result, the resonance is occurred at lower
frequencies. This observation shows that similar to 2D ring
FSS structures for which the resonance frequency has an
inverse proportion with the radius of the rings [1], the increase
of the douter decreases the lower resonance frequency for 3D
double ring FSS structures.
Changing dinner has negligible effect on both S11 and S21
resonance characteristics since the outer radius is not changed.
This shows that the radius of the outer cylinder (or the width of
the outer cylinder) is a much more important design parameter
in determining the reflection and transmission characteristics.
It must be also kept in mind that in this analysis the distance
between two conductors are kept the same in order to keep the
mutual capacitance the same.
Surface current distributions and electric field distributions
over the cylinders are presented in Fig. 5 and Fig. 6,
respectively.
Fig. 3. S21 characteristics with respect to outer diameter (douter)
(dinner=17 mm, h=25mm, wi=1.5mm and wo=1.5mm )
a)
b)
Fig. 5. Surface current at a) lower S21 resonance frequency and b)
upper S21 resonance frequency
It should be noted that for each resonance one cylinder is
mostly excited. The lower resonance is affected from the outer
cylinder so that the surface current distribution at the outer
cylinder is more intense when compared to the inner cylinder
as shown in Fig. 5(a). Besides, the intensity of the current
distribution is higher at the inner cylinder at the upper
resonance frequency.
Fig. 4. S21 characteristics with respect to inner diameter (dinner)
(douter=32mm, h=25mm, wi=1.5mm and wo=1.5mm )
The outer cylinder radius mainly determines position of the
lower S21 resonance so that the increase of the outer ring
diameter decreases the lower S21 resonance frequency as seen
in Fig. 3. The increment of the outer radius seems to increase
the mutual inductance of the structure regarding the lower S 21
RP1510-0922e
a)
b)
Fig. 6. Electric field concentration at a) lower S21 resonance
frequency and b) upper S21 resonance frequency
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The electric field concentration through the structure is
given in Fig. 6(a) and Fig. 6(b).
The resonance is a function of an inductance and a
capacitance for 2D FSS structures which are assumed to have
zero height or very short in comparison with the unit cell
parameter. The inductance and the capacitance values are
denoted as the equivalent circuit values and similar to the 2D
FSSs, 3D FSS structures have a resonance frequency, which is
a function of Leq and Ceq, where Leq is the equivalent
inductance and Ceq is the equivalent capacitance. It should be
noted that Leq and Ceq are the functions of the radius of the
cylinders, width and the height of the cylinders and the
distance between the cylinders. The 2D ring FSS structures
can be modeled by a simple LC- tank circuit. However, when
the height is relevantly important in comparison to the unit cell
parameter, a series inductance should be included to this
circuit [3]. Therefore, the 3D FSS resonance frequencies are
also affected by this new inductor element and the height
variation affects the resonance characteristics, as shown in Fig.
7.
account that during the analysis douter and dinner are fixed to 32
mm and 17 mm, respectively, so that the effects of these
parameters are excluded from the evaluation of the coupling
between cylinders.
Fig.8. The impact of the width of outer cylinder (wo) (douter=32mm,
dinner=17 mm, h=25mm and wi=1.5mm)
First, wo is varied between 0.5 mm and 2.5mm. Considering
the lower and upper S21 resonance frequencies, when wo
increases the lower and upper S21 resonance frequency values
increase (Fig. 8). In order to better interpret the coupling
mechanism, the surface current distributions at the resonance
frequencies are examined. The surface currents flow in the
same direction for both the cylinders at the two S21 resonance
frequencies. This outcome suggests that the coupling between
the cylinders decreases which causes an increment in the
values of the resonance frequencies.
Fig. 7. Impact of the height of the cylinders (douter=32mm, dinner=17
mm, h=25mm, wi=1.5mm, wo=1.5mm)
The upper resonance frequency is mainly determined by the
height of the cylinder, as seen in Fig. 7. When the height of the
cylinders is increased, the increase of the inductance, which is
due to the serially connected inductor, decreases the frequency
value of the lower and upper resonance.
Up to this point, the space between the concentric ring
cylinders was kept constant at 9mm in each parametric
analysis. In order to investigate the effect of the coupling
between the cylinders to the transmission resonances, the
widths of the outer ring cylinder (wo) and inner ring cylinder
(wi) are varied in this section and the results are presented in
Fig 8 and Fig 9, respectively. It is noteworthy to take into
RP1510-0922e
Fig.9. The impact of the width of inner cylinder (wi) (douter=32mm,
dinner=17 mm, h=25mm and wo=1.5mm)
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This situation is opposite for the lower resonance frequency
when wi is varied between 0.5 mm and 2.5mm as can be seen
in Fig. 9. Considering the lower resonance frequency, the
resonance frequency shifts to lower values as wi is increased.
The induced current in the inner and outer cylinders flows in
opposite directions at the lower resonance frequency, thus
leading to an enhanced inductive interaction between the
cylinders and the decrease of the resonance frequency against
the increase of wi. Besides, the surface currents flow in the
same direction in the two cylinders for the upper resonance
frequency and the resonance frequency increases as wi is
increased for the upper resonance.
The effect of the angle of incidence has also been evaluated
and is demonstrated in Fig..
 The space between the cylinders controls both the lower
and upper S21 resonance frequency. However, wo affects
the lower resonance frequency more than wi.
 The angle of incidence has a negligible effect and the
polarization does not affect the transmission
characteristics.
IV. Measurement set-up and experimental results
Fig.10. The impact of angle of incidence on the proposed structure.
(douter=32mm, dinner=17 mm, h=25mm, wi=1.5mm and wo=1.5mm)
As seen in the figure there is a negligible effect on S21
characteristics between 0° and 40° incidence angles. We have
also investigated the structure under different polarizations.
Since the structure is symmetric, we have obtained the same
transmission characteristics for TE and TM polarizations. In
addition, the wall thickness effect has been evaluated by
changing the dinner, douter but keeping the space between two
cylinders the same or by changing the space between the
cylinders but keeping the dinner and douter.
By considering the simulation results, the evaluations show
that:
 The radius of the outer cylinder controls mainly the lower
S21 resonance frequency.
 The radius of the inner cylinder does not control any of
the S21 resonance bands in the investigated frequency
range (0-8GHz).
 The height of the cylinders controls mainly the upper S21
resonance frequency.
By keeping the capacitance values same (physically the
gaps between the conductor parts):
 If douter increases, the lower S21 resonance frequency
decreases since the mutual inductance increases.
RP1510-0922e
a)
b)
Fig. 11. Fabricated 3D FSS a) the test setup, b) array design
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As aforementioned in the previous sections, the simulation
results are based on an array of infinite unit cells in both x and
y-directions. In order to mimic the array with better accuracy
and measure the S21 characteristics, a measurement set-up has
been arranged. The prototypes have been fabricated and
aligned to have 7×6 unit cells.
The 7×6 array is located between the two horn antennas
(model: Rohde & Schwarz HF907 double-ridged waveguide
horn antennas (800 MHz-18 GHz)) which are 100cm away
from each other. The antennas have a gain value of 9.1 dBi for
3.6GHz and 11.2dBi for 6GHz frequencies. The array is
located at the center of the two horns on top of a foam layer.
Expanded Polystyrene (EPS) Foam is a slab having a
dimension of 42.5 cm ×35 cm ×20cm. The properties of the
foam are quite similar to the air properties so that we consider
it having a permittivity and a permeability value the same as
free space. The surrounding absorber has been arranged to
obtain more accurate results with minimum interference. The
measurement set-up has been calibrated before placing the
array sample. Rohde & Schwarz ZVL13 Vector Network
Analyzer (9 kHz- 13.6 GHz) VNA has been used during the
measurements. The set-up and the array of an h=15mm double
cylindrical FSS structure are demonstrated in Fig. (a) and Fig.
(b), respectively.
Measurement results are plotted on the same graph with the
simulation results in Fig. 12.
of the lower resonance frequency between the simulation and
the measurement. We can claim that the number of the array
size is the most crucial reason. However, the discrepancy
between the measurement and simulation cannot be limited to
this. The manufacturing errors of the cylindrical rings also
influence the measurements. The small difference between the
real and simulated permittivity values of foam may also have
additional effects on the mismatched frequencies.
For analyzing the height of the cylinders, a prototype of
15mm-height cylinders is also considered with an array size of
8×6 and the experiment results in comparison with the
simulation results are presented in Fig. 9.
Fig. 9. Measurement and simulation comparison of S21 characteristic
of 3D FSS (douter=32, dinner=17, h=15, wi=1.5 and wo=1.5mm)
The wall thickness of the prototype outer cylinder is 1 mm
thinner than that in Fig.13. However, since douter does not
affect the upper resonance frequency considerably, we can
claim that the increase of the upper resonance frequency from
5.74 GHz (Fig.13) to 8.07 GHz stem from the decrease of the
height of the cylinders, which decreases the inductance.
V. Conclusion
Fig. 8. Measurement and simulation comparison of S21 characteristic
of 3D FSS (douter=32, dinner=17, h=25mm, wi=1.5mm and
wo=1.5mm)
As seen in Fig. 8, a satisfactory agreement is obtained
although we compare an infinitely aligned unit cell with a
finite array. The most crucial reason that causes the difference
can be the array size. Additionally, some details such as the
alignment of the absorbers, the effect of the foams that hold
the 3D FSS and the fabrication errors may cause the small shift
RP1510-0922e
In conclusion, a dual band double cylindrical 3D FSS
structures have been designed, fabricated and measured which
offer greater flexibility and extra control providing an extra
degree of freedom when compared to a conventional 2D FSS.
Frequency adjustability has been obtained for each frequency
value by tuning the diameter of the inner and outer ring
cylinders. It is shown that the lower S21 resonance is mainly
controlled by the diameter of the outer ring cylinder.
Moreover, the height of the cylinders has a great contribution
to the determination of the upper resonance frequency. The
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width of the ring cylinders influences the coupling mechanism
between the cylinders. The simulation results have been
compared with the fabricated prototypes and a satisfactory
agreement has been obtained.
References
[1] B. A. Munk, Frequency Selective Surface: Theory and
Design, Wiley-Interscience, New York, 2000.
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[10] L. Bao-Qin, Z. Shang-Hong, W. Wei, D. Xin-Yu, Z.
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