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ELECTRON
Department of ECE, Amrita Vishwa Vidyapeetham, Coimbatore
Conference Proceedings RTCSP’09 57
Design and Studies on Non-planar Conformal
Patch Antennas for Air-borne Vehicles
Anand Selvin.S, Bharathi Ravi.K, G.S.Darwino, Harishankar.R, Jitha.R and Jaya Kumar.M
Applied Electromagnetic Research Group,
Department of Electronics and Communication Engineering,
Amrita Vishwa Vidyapeetham, Coimbatore-641105, India
Abstract─Antennas deployed in air-borne vehicle
communication require a distinctive design approach from the
rest of the applications. In this paper, planar microstrip
antennas are conformed to non-planar structures suitable for
deployment in air-borne vehicles of various sizes. The design
procedure for these non-planar curved microstrip patch
antennas, derived from the basic rectangular microstrip patch
antenna design, is explained briefly. Antenna parameters such
as radiation pattern, return loss, VSWR, gain, directivity and
radiation efficiency are analyzed for various degrees of
curvature over the patch.
Index Terms
─ Curved patch, conformal antenna.
I. INTRODUCTION
The design of antennas used for air-borne
applications differs from other applications. Special
consideration must be given to the environment to which
the antenna is exposed, interaction of the air borne vehicle
with the antenna performance characteristics, safety of
flight and regulatory requirements. Antennas for these
applications must be designed to withstand severe static
and dynamic stresses. Antennas for high speed aircraft
must be designed aerodynamically, to limit physical stress
on the antenna and limit degradation of the body’s flight
characteristics [1].
The size and shape of the air frame play a major
role in determining the impedance and radiation pattern
characteristics of an antenna. The type of antenna used in
the given application would often depend on the size of the
airframe, relative to the wavelength. For e.g., in the case of
propeller driven aircraft and helicopters, the motion of the
blades may give rise to modulation of the radiated signal.
Thorough careful placement of the antenna on the air-
borne vehicle, optimum electrical performance
characteristics can be achieved.
The structures of air-borne vehicles in practice
are not compatible with planar microstrip antennas. They
are curved cylindrical bodies of various diameters usually
ranging from 2 cm to 300 cm. Flush mounting of planar
patch antennas on Air-borne structures with small
diameters such as missiles, are extremely difficult. Though
mounting of planar antennas are possible, it affects the
aerodynamics of these structures to a larger extent.
Employing curved patch antennas would be a good and
practical solution to this problem. Microstrip patch
antennas are often used because of their thin profile, light
weight and low cost. Furthermore, they can be made to
conform to the structure. When the radius of the curved
structure is large, the antenna can be analyzed as the
planar one. However, for smaller radius it is not the case.
Microstrip patch antennas are employed as
radiating or receiving elements in a wide range of
microwave systems such as radar, navigation,
communication, remote sensing and biomedical
applications. The radiating elements in these applications
are usually considered to be planar but more recent
developments seek to mould antennas into curved
surfaces. Particular examples for this are radar antennas
integrated into the body of an aircraft, mm wave imaging
arrays mounted on unmanned air borne vehicles(UAV’s),
and antennas for medical imaging applications which may
be required to conform to the shape of human body.
The purpose of this investigation is the analysis
of the radiation and impedance characteristics of patch
elements embedded in planar and cylindrically curved
surfaces. The impact of curvature on the performance of a
patch antenna is analyzed, and the effect of tolerances of
dielectric substrate material is examined. The procedure
for designing a rectangular microstrip patch antenna and
curved patch antenna which can be deployed in air borne
vehicles is explained and results obtained are analyzed.
II. ANTENNA DESIGN
A planar microstrip patch antenna is taken to be
our reference antenna and this planar structure is curved to
get a structure conformed to the cylindrical structures.
Fig 1: Curved patch placed on a cylindrical structure.
©
ELECTRON
Department of ECE, Amrita Vishwa Vidyapeetham, Coimbatore
Conference Proceedings RTCSP’09 58
The three essential parameters for the design of a
rectangular microstrip Patch Antenna are [2],
• Frequency of operation (
):
The resonant frequency of the antenna must be
selected appropriately. The Missile telemetry and related
applications uses the frequency range from 5.4 to
6.2MHz(ISM band). Hence the antenna designed must be
able to operate in this frequency range. The resonant
frequency selected for the design is 5.8 GHz.
• Dielectric constant of the substrate (
):
The dielectric material selected for the design is
RTduroid whose dielectric constant is 2.2. A substrate
with a high dielectric constant reduces the dimensions of
the antenna as well as its gain and radiation efficiency.
• Height of dielectric substrate (
h):
For the microstrip patch antenna to be used in air-
borne vehicles, it is essential that the antenna is not bulky.
Hence, the height of the dielectric substrate is selected as
4.7 mm.
The resonant frequency of the rectangular patch
is given by [3],
(1)
where,
is velocity of light and is the effective
length of the halfwave patch which is given by,
(2)
is the length due to fringing field which is
approximated as,
(3)
‘h’ is the height of the dielectric substrate.
The
effective dielectric constant can be calculated as,
(4)
The next important design work involved is to
decide the feeding technique involved. The most important
consideration is the efficient transfer of power between the
radiating structure and the feed structure. A number of
feeding techniques [4] are available such as co-axial probe
feed, microstrip (coplanar) feed, proximity-coupled
microstrip feed, aperture coupled microstrip feed and
coplanar waveguide feed. Different feed networks offer
different mechanisms of coupling energy, and relative
merits.
In this design, simple microstrip probe feed is
deployed, as its design involves no further complications
and second order effects. An SMA Panel jack with a
square Flange mount, exposed TFE and blunt post
terminal is employed for this purpose. This operates at
frequency range of 0-18 GHz.
The design values for a planar microstrip patch,
thus obtained by making use of design principles [4] are,
Width of the patch (20.4 mm), Length of the patch (14.1
mm), Dielectric constant 2.2, Height of the Dielectric
substrate (4.7 mm), Feed point Location (6.72, 0) mm
from the centre of the patch and a Patch thickness of 70
µm. Feed type is co-axial probe as mentioned earlier.
Fig 2: SMA Panel Jack, Square Flange, Exposed TFE
The reference antenna for any desired
specification is designed as per the design procedure
mentioned earlier in this chapter. The patch is just curved
along the width depending upon the dimension of the base
structure employing a simple expression,
,
where W is the width of the patch and r is the
radius of the base cylindrical structure and
decides angle
involved. The value of the angle
is found and the
structure is modeled accordingly. The circum length is
now equal to the width of the patch. Various discrete
values of cylindrical bodies are taken and the angle
required is calculated.
Sl.No
Radii of the base
cylindrical structure
Angle inv
olved in
design
1 3.6 cm 28.7333
2 5.0 cm 21.3886
3 8.9 cm 12.4929
4 10.0 cm 11.1812
5 20.0 cm 5.7208
6 50.0 cm 2.3208
7 100.0 cm 1.1659
Table 1: Base cylinder radii considered for design.
The variation in the value of effective dielectric
constant with respect to h/W ratio is analyzed and it is
©
ELECTRON
Department of ECE, Amrita Vishwa Vidyapeetham, Coimbatore
Conference Proceedings RTCSP’09 59
found that as the ratio increases the effective dielectric
constant decreases resulting in improved radiation
efficiency.
Fig 3: Angle of Curvature vs radius of base cylinder
As shown in fig 3, the desired angle of curvature
decreases gradually with increase in radius and dielectric
constant.
Fig 4: Structure of curved Microstrip patch antenna
The conformed curved microstrip patch antenna
is modelled as shown in fig 4, with the help of Ansoft
HFSS module and its performance is also analysed using
the same.
III. RESULTS AND DISCUSSIONS
The non-planar microstrip patch antennas
conformed to various base radii are modelled and their
Performance is analysed for a VSWR<1.5. All these
antennas resonate at 5.8GHz. The return loss obtained for
various degree of curvature without varying the feed point
location is plotted in fig 5.
Fig 5: Return Loss vs radius of base cylinder
The impedance bandwidth performance of the
patch is also analyzed for various models and the
bandwidth obtained for different degrees of curvature is
displayed in fig 6.
Fig 6: Bandwidth vs radius of base cylinder
The gain of these antennas and its directivity also
varies with the radius of the base cylinder. It is found that
as the patch tends to be more planar by increasing the
radius of curvature, the antenna gain and directivity of the
antenna system increases. The change in directivity with
respect to the planarity is displayed in fig 7.
Fig 7: Bandwidth vs radius of base cylinder
The radiation pattern of the various models
designed is obtained and it is found that most of the
antennas exhibit an omni-directional pattern. The
azimuthal angle component and elevation angle
component of the radiation pattern are plotted for discrete
values of theta and phi respectively.
©
ELECTRON
Department of ECE, Amrita Vishwa Vidyapeetham, Coimbatore
Conference Proceedings RTCSP’09 60
Fig 8: Azimuthal plot for planar Patch
The radiation characteristics of the antenna
system are of special interest for air borne vehicles. The
look angles from the ground station, for the instantaneous
changes of trajectory of airborne vehicles, have a greater
dependence on the radiation pattern of the antenna located
on it. The variations in pitch, roll, and yaw in turn changes
the look angle on the radiation pattern.
Fig 9: Elevation plot for planar Patch
Fig 10: Radiation pattern of 3.6cm base radius patch
The radiation pattern of 3.6 cm base radius patch
shows less of an omni-directional pattern compared to that
of a planar patch. A max gain of 6.101 dBi is obtained
over phi varying from 30
o
to 150
o
and theta centered on
15
o
.
The radiation pattern of 5 cm base radius patch
has an omni-directional pattern as shown in fig 11. The
max gain obtained in this case is 6.503 dBi, over phi
varying from 30
o
to 150
o
and theta centered on 30
o
.
©
ELECTRON
Department of ECE, Amrita Vishwa Vidyapeetham, Coimbatore
Conference Proceedings RTCSP’09 61
Fig 11: Radiation pattern of 5cm base radius patch
The radiation pattern of 8.9 cm base radius patch
also has an omni-directional pattern similar to that of the
previous pattern as shown in fig 12. The maximum
radiation 6.161 dBi, is obtained over the entire azimuthal
angle and theta is centered on 0
o
.
The radiation patterns of 10 cm, 20 cm and 50 cm
base radius patches matched the radiation pattern of 8.9
cm patch in all aspects except for the peak gain being
6.088 dBi, 6.625 dBi and 6.931 dBi respectively. The
radiation pattern of 100 cm base radius patch is also
isotropic azimuthally, but the elevation pattern showed
peak gain at 30
o
. The peak gain for this patch was
determined to be 7.345dBi.
Fig 12: Radiation pattern of 8.9cm base radius patch
Fig 13: Radiation pattern of 100cm base radius patch
The radiation efficiency of these patches is
calculated and is plotted in fig 14. The efficiency of the
curved patches over planar antennas offers a huge
©
ELECTRON
Department of ECE, Amrita Vishwa Vidyapeetham, Coimbatore
Conference Proceedings RTCSP’09 62
advantage in achieving a conformal, effective system for
communication.
Fig 14: Radiation efficiency vs Radius of base cylinder
IV. CONCLUSION
In this work, the planar microstrip antenna is
converted into non-planar structure conformed to base
cylinder of various diameters. Various parameters such as
Return loss, VSWR, Radiation pattern, Peak Gain,
Radiation efficiency are taken for the analysis purpose and
effect of the curvature over the patch is analyzed. In order
to obtain omni-directional radiation pattern and impedance
matching, the patch dimensions and feed position are
optimized. The antenna was modeled and its input
impedance, gain and radiation pattern were measured. The
measurement results presented in this paper, including the
radiation pattern with approximately 6 dB
omnidirectionality, approves the employment of circular
cylindrical patch antennas for air-borne applications.
V. ACKNOWLEDGEMENT
The authors wish to acknowledge Amrita
Vishwa Vidyapeetham, Amrita University for providing
computing facilities. Also we wish to thank Mekaladevi.V,
Ramanathan.R and Sabarish Narayanan.B, faculty for their
help and support during the work.
VI. REFERENCES
[1] James J.R., and P.S.Hall (Eds.),
Handbook of
Microstrip Antennas
, Peter Peregrinus, London,U.K 1989.
[2] Pozar, D.M., and D.H.Schauberr (Eds.),
The Analysis
and Design of Microstrip Antennas and Arrays
, IEEE
Press, New York, 1996.
[3] Constantine A. Balanis,
Antenna Theory: Analysis and
Design
, 2
nd
ed. John Wiley and sons, Inc 1997.
[4] Gharg.R, Bhartia, Bahi, Ittipiboon,
Microstrip Antenna
Design Handbook
, Artech House, Inc, 2001.
[5] N. Herscovici, Z. Sipus, P.-S. Kildal,
The Cylindrical
Omnidirectional Patch Antenna.
Proceedings of IEEE
International Symposium on Antennas and Propagation,
Montreal, 1997.
