Design of a Dual Band Frequency Selective Surface
Cassegrain Antenna System
Juliano F. Mologni and Jefferson C. Ribas
Arismar Cerqueira S. Jr and Igor F. da Costa
Electronic Design Automation Department
ESSS – Engineering Simulation & Scientific Software
São Paulo, Brazil
[email protected]
Lab. WOCA - Wireless and Optical Convergent Access
INATEL - National Institute of Telecommunications
Santa Rita do Sapucai, Brazil
[email protected]
Marco Antonio Robert Alves
DSIF – Department of Semiconductors, Instruments and Photonics
UNICAMP –University of Campinas
Campinas, Brazil
Abstract—A dual band Cassegrain antenna system using a
subreflector based on frequency selective surfaces is proposed.
The main goal is to have two similar radiation patterns for 5 and
9GHz (C and X band, respectively). In order to achieve far field
patterns radiating towards the same direction a system
comprised of two horn antennas, a main solid reflector and a
frequency selective surface subreflector is designed. A planar
infinite array of periodic elements was developed to create a stop
band at 9GHz using a single unit cell simulation. The periodic
elements were then projected on the parabolic surface made of
polyethylene to create the subreflector of the Cassegrain system.
A complete hybrid full-wave simulation is performed to calculate
the performance of the system. The proposed design results in a
system with approximately 32dBi of gain for both 5 and 9GHz
successfully demonstrating the application of a frequency
selective surface as a subreflector for a dual band Cassegrain
antenna systems.
array using Floquet theory [10]. On the other hand, the design
of curved FSS is very challenging because the fundamental
properties of planar FSS (the periodicity and the infinite
extent of the surface) are no longer valid [11]. The
nonexistence of this infinite periodicity means that the
simplification of the unit cell analysis may not be applicable,
thus the numerical analysis must incorporate the complete
FSS subreflector structure.
Figure 1 shows the proposed model of the Cassegrain dual
band antenna system. At the C band the FSS subreflector is
transparent, consequently, electromagnetic waves transmitted
by the 5GHz horn antenna are reflected only in the main
reflector. At the X band, the electromagnetic fields are
transmitted by the 9GHz horn antenna and reflected in the FSS
subreflector. Afterwards, the electromagnetic waves are
I. INTRODUCTION
Cassegrain antenna is a parabolic antenna system in which
the feed antenna (in our case a horn antenna) is mounted at the
surface of the concave primary main parabolic reflector dish
and is aimed at a smaller convex secondary subreflector
placed in front of the primary reflector. The beam of
electromagnetic waves from this horn antenna illuminates the
secondary subreflector, which reflects it back to the main
reflector dish, which in turn reflects it forward again to form
the desired beam [1]. In this work the secondary subreflector
is made of frequency selective surfaces (FSS) that is designed
to reflect electromagnetic waves at 9GHz only.
FSS technology can be applied to several applications,
including ultra-wideband (UWB) antennas [2], radomes [3, 4],
electromagnetic shieldening [5], reflectors [6], resonators [7]
and waveguide frequency splitters [8]. Numerical methods for
planar frequency selective surfaces have matured and today
they are available in several commercial electromagnetic
solvers, such as ANSYS HFSS [9]. The method simulates
only one unit cell and extrapolates the result for an infinite
Fig. 1. Structure of the Cassegrain dual band antenna system.
Fig. 2. Crossed dipole cell model unit cell.
reflected back to the main reflector forming the desired
radiation pattern.
Section II highlights the methodology to design the cross
dipole FSS structure. The simulation of the complete
Cassegrain dual band antenna system is shown in session III
and final remarks and conclusions are addressed in Section IV.
II. CROSSED DIPOLE UNIT CELL
Figure 2 shows the crossed dipole single element made of
copper ( r=1 and thickness of 0.035mm) placed on a
polyethylene layer ( r=2.25 and thickness of 1mm). The
design goal is to achieve a maximum reflection coefficient at
9GHz. A genetic algorithm (GA) was used for this purpose
using the following geometric variables: distance between
cells (l), width (b) and total length (a). The best configuration
obtained by the GA yielded the following dimensions: l=25m,
B=5mm and t=14.58mm.
The transmission (S21) and reflection (S11) coefficients are
shown in figure 3. The resonance is found at the desired
frequency of 9GHz with S11= -0.05dB and S21= -57dB. The
insets from Fig. 3 shows the far field patterns for this infinite Fig. 4. Current density distribution [a.u.] at the surface of the crossed dipole
array for 5 and 9 GHz by having a plane wave travelling from FSS for the following frequencies: a) 9GHz and b) 5GHz.
top to bottom as excitation. For C band the infinite planar FSS
crossed dipole array is transparent with a reflection of roughly -11dB. For the X band there is a very strong reflection as
desired for our subreflector structure that acts as bandrejection filter.
The current density plot on the surface of the circular
element is presented in Fig. 4 in arbitrary units and in the
same scale for both frequencies. At 9GHz, the electromagnetic
wave resonates with the crossed dipole elements, inducing a
current on them and thus reflecting the wave back. On the
other hand, for 5GHz there is no significant induced current,
which makes the FSS electromagnetic transparent at this
frequency.
Fig. 3. Transmission and reflection coefficients. Inset shows the far field of the
infinite array at 5 and 9 Ghz.
III. DUAL BAND CASSEGRAIN ANTENNA SYSTEM
The complete model is shown in figure 5 and includes the
two horn antennas, FSS subreflector, main reflector and the
mechanical support for all structures. The main reflector has a
diameter of 2 meters (equivalent to approximately 60 at
9GHz) while the subreflector has a dimeter of 33cm
a)
b)
Fig. 5. Full dual band Cassegrain system model in ANSYS HFSS.
(equivalent to approximately 10 at 9GHz). 120 crossed
dipole elements were projected and placed on the subreflector.
In order to solve this complete model using a full wave solver
a hybrid Method of Moments (MoM) and Finite Element
Method (FEM) technique was used. The finite element
boundary technique (FEBI) enables a rigorous full-wave
solution by solving the horn antennas and the FSS subreflector
with the finite element method (FEM) and the support with
the Method of Moments (MoM). This hybridization scheme is
required because FEM is suitable to solve complex solids
made of dielectric material (like the horn antennas and the
FSS subreflector) and MoM can solve electrically large
conductors. FEM and MoM domains can be in direct contact
since the coupling is made through near fields and also
conduction of electrical current. This is an important feature
since the mechanical support of the structures, which
interferes with the resulted radiation pattern creating side
lobes, crosses FEM and MoM domains. The simulation with
FEBI at 9GHz took 7 hours and 20 minutes using 75GB of
RAM in a small cluster of 2 CPUs of 12 cores each.
Fig. 7. Electric field plot in a cross section at a) 5GHz and b) 9GHz.
The gain far field pattern be observed in figure 6. The gain
for 5GHz is 31.2dBi and for 9GHz is 32d.6Bi. Although the
gain for both frequencies are nearly 32dBi a big difference can
be noticed on the side lobes: for 5Ghz and 9GHz the first side
lobe is 21dB and 16dB below the main lobe, respectively. The
electric field in a cross section can be observed in figure 7. In
figure 7a only the top 5GHz horn is illuminating the system. It
is possible to observe that the FSS subreflector is transparent
allowing the electromagnetic wave to pass through at 5GHz.
In figure 7b only the bottom 9GHz horn antenna is
illuminating the system. The fields are reflected on the FSS
subreflector and reflected again in the main reflector.
The surface induced current (in arbitrary units) as well as
the 3D far field pattern of the complete system are shown in
figure 8.
Fig. 6. Radiation pattern at 5 and 9GHz.
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Monopole
Antenna”,
IEEE
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[9] (2013)
ANSYS
HFSS
website.
[Online].
Available:
http://www.ansys.com/Products/Simulation+Technology/Electromagne
tics/High-Performance+Electronic+Design/ANSYS+HFSS
[10] G. Floquet, "Sur les équations différentielles linéaires à coefficients
périodiques," Annales Scientifiques de l'É.N.S., 12, 47-88 (1883).
[11] J. Su and X. Xu, “MOM analysis of the planar and curved FSS based
on dipole elements”, International Conference on Microwave
Technology and Computational Electromagnetics (ICMTCE), 127130,(2009)
Fig. 8. Induces current on the surface of the structure and far field pattern for a)
5GHz and b) 9Ghz.
IV. CONCLUSIONS
A dual band Cassegrain antenna system using s FSS
subreflector was proposed and its performance was
investigated using numerical simulation. The final system
provides an approximate gain of 32dBi for 5 and 9GHz.
Higher side lobes were observed on the gain far field pattern
at 9GHz. To the best of our knowledge, this is the first report
in the literature regarding a numerical full wave analysis of a
dual band Cassegrain antenna system using a FSS subreflector
considering all possible interferences such as the mechanical
support of the structures.
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