octagonal shaped uwb Antenna

1. 2946 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 6, JUNE 2014
Planar Octagonal-Shaped UWB Antenna With
Reduced Radar Cross Section
Cengizhan M. Dikmen, Sibel Çimen, Member, IEEE, and Gonca Çakır, Member, IEEE
Abstract—A novel planar octagonal-shaped antenna with
reduced radar cross section (RCS) for ultra-wideband (UWB)
applications is designed and demonstrated experimentally. The
proposed antenna is modified using the geometrical shaping
method in order to get the RCS reduction. This method is based
on the subtraction of metal areas which have the minimum cur-
rent distributions on the surface of printed antenna. Both of the
reference and modified octagonal-shaped antennas are operating
at 2.5–18 GHz frequency range with %151 fractional bandwidths.
The full-wave electromagnetic simulations and laboratory mea-
surements are performed to characterize their RCSs as a function
of frequency, radiation patterns and scattering characteristics.
The radiation performance of modified antenna is stable and con-
sistent with the reference antenna. Also the results show that the
designed UWB antenna has lower RCS compared to the reference
antenna. About 10 dBsm RCS reduction is obtained in the whole
operation bandwidth. The designed antenna has very large RCS
reduction up to 25 dBsm, especially in the low frequency range.
Index Terms—Microstrip antenna, monopole antenna, reduced
radar cross section (RCS), ultra-wideband (UWB) antenna.
I. INTRODUCTION
ESPECIALLY in stealth and military platforms, radar
cross section (RCS) reduction of sensitive targets in order
to avoid detection by the radar is very important subject [1]–[6].
As a part of these systems, antennas are dominant scatterers
due to its nature of efficient electromagnetic radiation [7].
Unless the antenna system has a low RCS, the whole stealth
performance will be destroyed and its stealth systems will be
worthless. Therefore, the designs of antennas with reduced
RCS have a great importance in low observable platforms. In
literature, there are many techniques about RCS reduction of
antennas. Frequently, the RCS reduction of an antenna have
been obtained by using lumped [8], and distributed loads [9].
But these methods cause a decrease in the gain of the antennas.
Furthermore, controlling bias voltage through a varactor [10],
using tapered resistive sheets in antennas or radomes [11],
[12], passive or active cancellations [13], using substrate-su-
perstrate layer structure [14], [15], ferrite substrate [16] and
Manuscript received September 10, 2013; revised January 15, 2014; accepted
March 19, 2014. Date of publication March 26, 2014; date of current version
May 29, 2014. This work was supported in part by The Scientific and Techno-
logical Research Council of Turkey (TUBITAK) under Project No: 110E265.
The authors are with the Electronics and Communication Engi-
neering Department, Kocaeli University, Kocaeli 41000, Turkey (e-mail:
sibelgunduz@kocaeli.edu.tr).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2014.2313855
frequency selective surfaces (FSS) [17] are the other reported
techniques. Beside these techniques, radar absorbing material
(RAM) [18]–[20] and the shaping of the target geometry [21],
[22] are the other popular and practiced solutions used for
reducing RCS. But in ultra-wide operation band, the coating of
RAM becomes useless because of its narrow operation band.
Those proposed RCS reduction designs are decreasing the
radar signature while they are increasing the cost of the system
and reducing the total antenna performance. Hereby, the RCS
reduction of an antenna without compromising its radiation
characteristics has been an important subject.
In stealth platforms such as ultra-wideband (UWB) radar sys-
tems, the microstrip UWB antenna designs have promising ap-
plication values because of its low profile, easy fabrication and
low production cost [23]–[29]. However, the metallic surfaces
of the microstrip antennas cause a large RCS values. But the
RCS reduction in the ultra-wideband operating range is difficult
[30]. Until now, there are few publications on the RCS reduc-
tion of UWB antennas [31]–[37]. In [32], the RCS reduction
of the printed antenna was based on the subtraction of metallic
areas where the surface currents were small. However, in those
studies the RCSs weren’t reduced in the whole operation band.
The RCSs of those structures were reduced in only high fre-
quency range due to their relatively small geometrical size. But
in the low frequency range, the scale of the antenna’s geomet-
rical size was similar to incident wavelength. Hereby, the RCS
reduction in low frequency range has been insufficient for the
reported studies.
So the design of antenna with low RCS has two critical prob-
lems. One is to reduce the RCS in the whole operation band;
the second is to preserve the radiation property of the reference
antenna. To overcome these problems, we present a novel octag-
onal-shaped UWB antenna that is designed to have a minimum
radar cross section, thus makes it invisible to enemy radar. In
this paper, firstly the UWB antenna is designed as a reference
antenna. The designed UWB antenna radiates at 2.5–18 GHz
with a fractional bandwidth of 151%. Then, the reference an-
tenna is modified in order to reduce the RCS. The metal areas
which have the minimum current distributions on the surface of
the printed antenna were subtracted. Finally the proposed an-
tenna characteristics were discussed both in frequency and time
domain analysis. The radiation patterns, scattering characteris-
tics and the RCS of both the modified and the reference octag-
onal-shaped UWB antennas were simulated and experimentally
verified. About 10 dBsm RCS reduction was obtained in the
whole bandwidth with these geometrical modifications. Unlike
from reported studies, the designed antenna has very large RCS
reduction up to 25 dBsm, in the low frequency range. Therefore
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2. DIKMEN et al.: PLANAR OCTAGONAL-SHAPED UWB ANTENNA WITH REDUCED RADAR CROSS SECTION 2947
Fig. 1. Geometry of the proposed octagonal-shaped UWB antenna.
( , , , ,
, , .).
the designed UWB antenna has the lower RCS in the whole op-
eration bandwidth, especially in the low frequency range, com-
pared to the previously reported RCS reduced UWB antennas.
With these novel features, the proposed antenna can be conve-
niently used as an UWB antenna in the low RCS platforms.
II. ANTENNA DESIGN AND PARAMETRIC STUDY
A. Reference Antenna Structure
In the first step, an octagonal-shaped UWB antenna is de-
signed as a reference antenna. Fig. 1 illustrates the geometry
of the reference UWB antenna. The designed antenna is con-
structed on ArlonDiclad 880 substrate with a relative permit-
tivity ( ) of 2.2, a loss tangent of 0.0009, and a thickness of
0.762 mm. The octagonal-shaped patch is fed by a tapered struc-
ture strip line. The overall dimension of the proposed antenna is
70 60 mm. The simulations of the antenna were performed by
using full-wave EM analysis tool, Computer Simulation Tech-
nology (CST) Microwave Solver.
In order to investigate the effect of some particular parame-
ters on the antenna performance, the parametric study has been
conducted. In this parametric study, each time, only one param-
eter was varied as the others were kept constant.
1) Effect of Radiator Patch Element: Different type of ra-
diator elements can be used in order to obtain UWB perfor-
mance. Fig. 2(a) presents the design stages of the octagonal-
shaped antenna geometry. As seen from figure, maximum an-
tenna performance was obtained with octagonal geometry. On
the other hand, the octagonal-shaped patch was compared to
the circular, elliptical and eye-shaped patch elements. As seen
from Fig. 2(b), the octagonal-shaped radiating patch element
has better performance than other type of patch elements. As
a result, the octagonal-shaped patch element was used for a ra-
diator patch element.
2) Effect of Ground Plane, : In the UWB antenna designs,
the ground plane size is significantly controlling the antenna’s
bandwidth. In order to get the best UWB performance various
simulations were carried out with different dimensions of
Fig. 2. Simulated reflection coefficients with different radiating patch element.
Fig. 3. Simulated reflection coefficient with different .
Fig. 4. Simulated reflection coefficient with different .
(Fig. 3). Therefore it is decided to take as the
optimum.
3) Effect of Feeding : Beside them, one of the other im-
portant parameter is the feeding width; . Fig. 4 presents the
simulated results of the proposed antenna with the width ,
from 0.86 to 1.44 mm. It can be observed from the figure, the op-
timum antenna performance is obtained with .
B. Modified Antenna Structure
At the second step, the RCS reduction of reference UWB an-
tenna is targeted. For this purpose, the surface current distribu-
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3. 2948 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 6, JUNE 2014
Fig. 5. Surface current distributions of the reference UWB antenna. (a) 7 GHz. (b) 13 GHz.
Fig. 6. Top and bottom view of modified octagonal-shaped UWB antenna.
tions on the metallic areas of the reference antenna were ob-
tained. The radiation behavior of an antenna is generally based
on the surface current distributions of the metallic areas. The
metallic areas where the surface currents are small can be sub-
tracted in order to reduce the RCS. Thus the radiation perfor-
mance of the modified antenna will be maintained the same,
as the RCS will be reduced. The simulation results of the sur-
face current distributions for reference antenna are presented
in Fig. 5. The currents are distributed unequally. Some places
have the minimum current amplitude as the others have the max-
imum. According to these current distributions, the metal areas
were subtracted, as shown in Fig. 6.
As seen from Fig. 6, a strip line with the width of is
left on the ground layer to feed the radiator element. According
to the current distribution an elliptic geometry is subtracted from
the ground layer. The origin of the elliptic geometry that was
subtracted is on the middle of the feeding point of the antenna.
The radius of the elliptic geometry at - axis is
and - axis is . Beside them, a circular part with
radius is subtracted from the radiator element.
To better demonstrate the effect of the elliptic geometry of
the ground plane on the reflection coefficient, we simulated an-
tenna with various and values. First, we investigated
the effect of . As seen from the Fig. 7(a), increasing the
values, while keeping constant the other parameters cause a de-
crease in the antenna performance. When , the
reflection coefficient characteristic is very close the reference
antenna. Second, the effect of values on the reflection coef-
ficient is investigated. According to the simulation results, when
increases as the other parameters kept constant, the reflec-
tion coefficient characteristic begins to deteriorate in the lower
Fig. 7. Effect of the elliptic geometry on the reflection coefficient. (a) .
(b) .
frequency band [see Fig. 7(b)]. For this reason, the radiuses of
the elliptic geometry are chosen as , .
Finally, the effect of the removed circular part on the re-
flection coefficient is investigated. Fig. 8 shows the simulated
results of the modified antenna with the increasing length
of , from 4 to 6 mm. As seen from figure, the increasing
values cause the decreases in the antenna performance, espe-
cially within 4–9 GHz frequency range. With these parametric
studies, the optimum antenna dimensions are obtained. The
simulation results of the reference and modified antenna are
shown in Fig. 9. We observed that the modifications didn’t
affect the reflection coefficient characteristics of the antenna.
Besides the frequency domain performances of the reference
and modified antennas, the time domain performances must be
obtained in order to ensure a good UWB characteristic. For this
purpose, the effect of subtraction of metal areas on the time
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4. DIKMEN et al.: PLANAR OCTAGONAL-SHAPED UWB ANTENNA WITH REDUCED RADAR CROSS SECTION 2949
Fig. 8. Effect of on the reflection coefficient.
Fig. 9. Simulated reflection coefficient results for reference and modified
antenna.
Fig. 10. Impulse responses of reference and modified antennas.
domain performance was investigated. Fig. 10 presents the im-
pulse responses of reference and modified antennas. In time do-
main analysis, a Gaussian pulse was used for excitation signal.
As seen from Fig. 10, the subtraction process didn’t affect the
time domain performance. As well as the impulse responses of
the antennas, the group delay is another important parameter in
time domain analysis of UWB antenna designs. The simulated
group delays for reference and modified antennas are shown in
Fig. 11. From the figure, after the modifications the group delay
response still has less variation.
Finally, the reference and modified antennas were fabricated
(Fig. 12). They are measured with Rohde Schwarz ZVB 20
vector network analyzer to obtain the reflection coefficient char-
acteristics. The simulated and measured results of the reflection
coefficient for the reference and modified UWB antennas are
shown in Fig. 13(a) and (b), respectively. It can be seen from
the figures that the measurements and the simulation results of
the reflection coefficient are relatively in a good agreement. The
differences between the measurements and simulation results
Fig. 11. Simulated group delays for reference and modified antenna.
are due to the fabrication of the antennas. To verify this fabri-
cation effect, a sensitivity study for the antenna geometry was
carried out [Fig. 13(c)]. As mentioned before, the parameters
“ ,” “ ,” and “ ” have the critical role on the reflection coef-
ficient. In Fig. 13(c), the simulation was performed with 1 mm
smaller dimension of “ .” As seen from the figure, the simula-
tion result came closer to the measurement result that was the
evidence of the fabrication effect.
III. ANTENNA PERFORMANCE
Before presenting the RCS characteristics of the proposed
two antennas, we first review their radiation properties. The
preservation of the reference antenna radiation pattern as well
as the gain is a crucial issue for the antenna design studies with
low RCS. For this purpose we obtained the radiation patterns
and gain curves to show the effect of the modifications.
The gain curves of the two antennas are shown in Fig. 14.
Generally, the UWB systems need an antenna with identical ra-
diation patterns and stable gain curves in the whole frequency
range. As seen from Fig. 14, the gain of modified antenna is
stable and consistent with the reference antenna.
For a complete study, the radiation patterns of reference
and modified octagonal-shaped UWB antennas have been
performed with CST Microwave Studio. The patterns are
obtained for 3, 6, and 13 GHz at the and planes, which
corresponds to - and - planes, respectively (Fig. 15). As it
is apparent from Fig. 15, the radiation patterns of the reference
and modified antennas have stable characteristics, which show
the modifications didn’t cause any degradation on the total
performance of antenna.
Beside this, the manufactured modified antenna was mea-
sured inside a whole anechoic chamber at 3, 6, and 13 GHz fre-
quencies. The radiation patterns were obtained at the planes.
Fig. 16 shows that simulated and measured radiation patterns
are in a good agreement.
IV. RCS REDUCTION OF ANTENNA
A. Theoretical Antenna Scattering Analysis
The feed terminations of the antennas control the scattering
characteristics. When the antenna is fed by a match load, the
scattering of antenna is structural mode. If antenna fed by other
loads (except match load), the part of the energy would be
reflected by the load and reradiate to the space. This type of
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5. 2950 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 6, JUNE 2014
Fig. 12. Pictures of octagonal-shaped UWB antennas. (a) Reference. (b) Modified.
Fig. 13. Simulated and measured reflection coefficient for the reference, mod-
ified and adapted octagonal-shaped UWB antenna.
scattering called as antenna mode scattering. Therefore, total
of antenna is consisting of RCS of structural mode
( ) and RCS of antenna mode ( ), [38]. It is expressed as
(1)
where is the phase difference between these two modes. Be-
side this, total scattering field of an antenna ( ) can be
divided into scattering fields of structural mode ( ) and
antenna mode ( ). Their relationship can be expressed as
(2) [39].
(2)
Fig. 14. Simulated gain curves of reference and modified antennas.
Fig. 15. Simulated radiation patterns reference and modified antennas in
-plane (left) and -plane (right) at (a) 3 GHz, (b) 6 GHz, and (c) 13 GHz.
(3)
(4)
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6. DIKMEN et al.: PLANAR OCTAGONAL-SHAPED UWB ANTENNA WITH REDUCED RADAR CROSS SECTION 2951
Fig. 16. Simulated radiation patterns reference and modified antennas in
-plane (left) and -plane (right) at (a) 3 GHz, (b) 6 GHz, and (c) 13 GHz.
where is the antenna reflection coefficient, is the load re-
flection coefficient of the receiver, is the port load, is
the antenna characteristic impedance, is the radiating elec-
tric field with unit excitation and is the scattering incepting
matching amplitude. As seen from (2), if the feeding port is
match loaded, then the total scattering field will contain only the
structural scattering fields. In this paper, both the reference and
modified antennas are match-loaded, so the antenna mode scat-
tering field has zero effect. Therefore the total scattering fields
were consisted from only structural scattering fields.
B. Numerical and Experimental Results
In this section, the monostatic RCS values of the reference
and modified antennas are investigated. The results were
achieved both by the full-wave EM simulator and trough the
measurements on the fabricated antennas. The monostatic
RCS values were obtained for two cases. In the first case, the
incident and received electric fields are parallel to the -axis.
The incident wave is impinging from the normal direction of
the antenna, which is the most dangerous direction.
As mentioned before, the antenna mode scattering depends
on the antenna termination load, while the structural mode scat-
tering is independent from termination. In this study, the RCS
curves of the reference and modified antennas are calculated
with the termination of match load (50 ) case due to their
ultra-wide impedance band. The RCS values as a function of
frequency within 2–18 GHz frequency range for the reference
and modified antennas were calculated and compared to demon-
strate the advantage of the proposed antenna in RCS reduction.
The simulated results are shown in Fig. 17. As it is seen from
Fig. 17, the RCS of the proposed antenna is largely reduced
in the whole operation band. Especially in the low frequency
range, the RCS reduction becomes larger up to 25 dBsm.
More in detail, we also investigated the monostatic RCS
values for the second case, which the incident and received
electric fields are parallel to the -axis (Fig. 18). Again, the
simulated RCS values of the modified antenna are reduced to
approximately 10 dBsm compared to the reference antenna in
the whole band.
Fig. 17. Simulation results of monostatic RCS for reference and modified an-
tennas for -polarized incident wave impinging from normal direction.
Fig. 18. Simulation results of monostatic RCS for reference and modified an-
tennas for -polarized incident wave impinging from normal direction.
Fig. 19. Measured RCS results of modified antennas for -polarized incident
wave impinging from normal direction.
The RCS measurements were performed in a whole anechoic
chamber which is covered with absorbers from 300 MHz to 110
GHz. In this measurement set up, in order to measure monostatic
RCS, the two identical horn antennas are located in the same
plane. Also, the horn antennas were located 17.6 m away from
the target to obtain the far-field criteria. The target was measured
with Agilent E8362B vector network analyzer and mounted on
a Styrofoam column as a target support structure that has typical
RCS of 40 dBsm. In this paper, the measured monostatic RCS
values were obtained for the linear polarization; both the inci-
dent and received electric fields are parallel to the -axis. The
results are shown in Fig. 19. As seen from Fig. 19, the measured
and simulated monostatic RCS curves for modified antenna are
in a good agreement.
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7. 2952 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 6, JUNE 2014
Fig. 20. Simulated RCS at oblique incidence for -polarized incident wave.
(a) . (b) .
Fig. 21. Simulated RCS at oblique incidence for -polarized incident wave.
(a) . (b) .
Although the normal-incident wave is the most threatening
direction for the RCS, in this study, we also investigated the
RCS of the antennas for oblique incident waves. The simu-
lated RCS curves for the reference and modified antennas at
different incident angles are presented in Figs. 20 and 21. These
curves were achieved for the -polarized and -polarized in-
cident waves impinging from oblique directions, respectively.
The RCS of reference and modified antennas were simulated
when the impinging wave angles are and
.
As seen from Figs. 20 and 21, also for oblique incident angles
the RCS reduction is maintained in the operation band. This
shows that, the modified antenna geometry provide a good RCS
reduction within the whole frequency band and for normal and
oblique incidence angles.
V. CONCLUSION
In this paper, a novel planar octagonal-shaped UWB antenna
was introduced. The designed UWB antenna bandwidth is
2.5–18 GHz covering the entire band assigned for the UWB
applications. The designed UWB antenna was modified by
geometrical shaping in order to reduce the RCS of antenna.
The bandwidth and the radiation properties of both reference
and modified antennas were validated through EM simulations
and measurement results of manufactured antennas. The results
present that the modified antenna has almost the same radiation
pattern, return loss and gain characteristics according to the
reference antenna. Meanwhile, the backscattering character-
istics of antennas were verified by both EM simulations and
measurements. The designed UWB antenna has lower RCS in
the whole operation bandwidth, especially in the low frequency
range compared to reference antenna. Also the modified an-
tenna has lower RCS value for oblique incident waves, as well.
All these results are evidences for the proposed antenna that it
can be conveniently used as an UWB antenna where the low
RCS is required.
ACKNOWLEDGMENT
The authors wish to acknowledge the assistance and support
of The Scientific and Technological Research Council of Turkey
(TUBITAK) for supporting this work (Project No: 110E265).
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Cengizhan M. Dikmen received the B.S. degree in
electrical and electronics engineering from Hacettepe
University, Ankara, in 1994, and the M.S. degree in
electrical and electronics engineering from Sakarya
University, Sakarya, Turkey, in 1998. He is currently
working toward the Ph.D. degree at Kocaeli Univer-
sity, Kocaeli, Turkey,
Since 1994, he has been with Kocaeli University
as an instructor. He has been working on a research
project on designing ultra wideband microstrip an-
tenna with reduced radar cross-section.
Sibel Çimen (S’08–M’13) was born in Çanakkale,
Turkey, in 1980. She received the B.S, M.S., and
Ph.D. degrees in electronics and communication en-
gineering from Kocaeli University (KOU), Kocaeli,
Turkey, in 2002, 2005, and 2009, respectively.
From 2005 to 2009, she was a Research Assistant
with the Microwave and Antennas Laboratory. Since
2010, she has been an Assistant Professor with the
Electronic and Communication Engineering Depart-
ment, KOU. Her research interests include numerical
methods for planar structures, metamaterials, and de-
sign of UWB antennas and microstrip filters.
Gonca Çakır (S’05–M’09) received the B.S.E.E.,
M.S.E.E., and Ph.D. degrees in electronics and com-
munication engineering from Kocaeli University
(KOU), Kocaeli, Turkey, in 1996, 1999, and 2004,
respectively.
Currently, she is an Associate Professor with
the Engineering Faculty, KOU. Her research inter-
ests include analytical and numerical methods for
planar structures (transmission lines, circuits, and
antennas), radars, RCS prediction techniques, and
metamaterials.
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