Low Side Lobe Omnidirectional Cylindrical Conformal
X-Bnad Microstrip Antenna Design
Tehran Regional Electric Co.
Abstract—An X-Band cylindrical conformal low side lobe level
omnidirectional array is designed in this paper. The radius of the
cylinder is 30 mm. The Dolph-Chebychev distribution method
has been applied to design seven elements subarray to lower the
side lobes. The designed antenna array is composed by circular
array of 16 subarrays to cylindrical shape to obtain smooth
radiation pattern. The simulation results show that the first side
lobe level is -21.8 dB, antenna gain is 10dB, and the relative
bandwidth is 2%.
Keywords-component; conformal microstrip antenna;
cylindrical microstrip array antenna; omnidirectional; low side lobe
Microstrip antenna has many advantages, such as small
size, light weight, low profile, and so that is comfortable to
planar and nonplanar surfaces. The cylindrical geometry is a
sample of nonplanar surfaces which can offer desirable antenna
characteristics that is not provided by planar antenna. The
microstrip conformal antenna is widely used in satellite
communications, radar, weapons and missile telemetry 1-4. A
conformal antenna is designed to adopt some prescribed shape,
example is a flat curving antenna which is mounted on or
embedded on a curved surface. The shape can be a part of
aircraft, missiles, and other high speed vehicle.
The communication and telemetry system of aircraft such
as Unmanned Aerial Vehicle (UAV) require antenna which
have high gain, omnidirectional and light weight.
Omnidirectional and 360° coverage needs in the roll of the
UAV. Full coverage in the roll plane of the UAV is obtained
by using circular array configuration of antenna, according to
this view it is studied by many research groups. A type of
multi-layer microstrip array antenna presented in 5 can
radiate a 360° omnidirectional pattern, and can reach high gain
more than 9 dBi of peak gain. Another microstrip antenna for
use in UAV was presented in 6. A 32 GHz cylindrical
conformal omnidirectional millimeter wave antenna array was
studied in 7.
In some precise applications, antennas are required to have
low side lobes. In this paper, seven elements subarray has been
presented. The cylindrical conformal antenna array includes 16
subarrays to obtain completely smooth radiation pattern. The
simulation results show this X-Band cylindrical conformal
omnidirectional antenna has good performance.
II. LOW SIDE LOBE ANTENNA ARRAY
First, The series-fed taper antenna array shown in Fig.1 is
the one as in 8, which was used as the starting point of the
present work. The array mounted on the top of a grounded
dielectric substrate of thickness h = 1.57 mm and a relative
dielectric 2.33 constant. With end-fed arrays, the elements
nearest the feed couple only a small amount of power and
therefore must be fairly narrow. The feed line must be small
compared to the narrowest patch. In this case, a 50 ? line will
be used. The line width is 0.7 mm that is considerably smaller
than the square patch width. The radiating patches themselves
are resonant so that the input line to a patch is matched. In the
simple transmission line model of a patch, this corresponds to a
length of 0.5?g. Because the amplitude and phase of the
radiated fields at each patch are determined by the cumulative
transmission characteristics of the preceding patches on the
line, the transmission characteristics of the patches must be
determined accurately in order to achieve a desired amplitude
and phase distributions of radiating currents along the array 9.
According to the Dolph-Chebyshev method, power
distributions of the elements are shown in Table 1.Values of
power amplitude have been calculated for -30 dB side lobe
level in theory, so that the maximum value of SLL in reality be
-20 dB. The series-fed taper antenna array is designed and
shown in Fig. 1. The detailed geometrical parameters are listed
in Table 2.
TABLE 1. THE POWER DISTRIBUTIONS OF SEVEN ELEMENTS DOLPH-CHEBYSHEV ARRAY
Element 1 2 3 4 5 6 7
Power Amplitude 0.26 0.56 0.87 1 0.87 0.56 0.26
Fig. 1. Configurations of the single antenna array.
TABLE 2. ARRAY GEOMETRIC PARAMETERS
1 4.17 9.74 0.67 11
2 6.7 9.55 0.67 11
3 8.36 9.41 0.67 11
4 8.98 9.31 0.67 11
5 8.36 9.41 0.67 11
6 6.7 9.55 0.67 11
7 4.17 9.74 0.67
Using CST Microwave Studio, the S-parameters of the
series-fed taper antenna array is calculated as depicted in Fig.
2. As illustrated, series-fed taper antenna array shows
resonance at 9.7 GHz and a bandwidth of 200 MHz.
The radiation pattern of the antenna array is shown in Fig 3.
It shows that maximum gain is 15.5 dB, the first side lobe level
is -21.8 dB and half power beamwidth (HPBW) is 11.4? in E-
plane. Fig.4 shows that the HPBW is 82? in H-plane.
Fig. 2. Full-wave simulated S-parameters of the series-fed taper antenna array.
Fig. 3. E-plane radiation pattern of series-fed taper antenna array.
Fig. 4. H-plane radiation pattern of series-fed taper antenna array.
III. CYLINDERICAL CONFORMAL
OMNIDIRECTIONAL MICROSTRIP ANTENNA ARRAY
A circular antenna array must be designed in order to
achieve an omnidirectional and high gain pattern to meet the
demand of long distance for aerospace communication. Based
on the seven elements Dolph-Chebyshev distribution array
regarded as subarray, a cylindrical conformal antenna is
designed to achieve omnidirectional radiation. Assume that the
carrier is a cylinder, and the cylinder radius is 30 mm. In order
to get smooth radiation pattern, 16 Dolph-Chebyshev
distribution subarrays are united. The configuration of
cylindrical conformal antenna array is shown in Fig.5.
Fig. 5. Configuration of the cylindrical conformal antenna array.
The cylindrical structure of Fig. 5 have been modeled
through CST Microwave Studio. Cylinder with radius 30 mm
have been analyzed. The radiation pattern of the antenna array
is shown in Fig. 6, 7. The result in Fig.6 indicates that proposed
antenna array produced omnidirectional radiation pattern with
gain of 10 dB. In Fig.7 result indicates that the maximum gain
difference between Point 1 and Point 2 is 1.42 dB (Point A is
10.05 and Point B is 8.63 dB), showing complete
omnidirectional radiation and minimum distortion in radiation
Fig. 6. The Polar radiation pattern of the omnidirectional antenna array.
Fig. 7. The Cartesian radiation pattern of the omnidirectional antenna array.
IV. MICROSTRIP POWER DIVIDER FEEDING
There is one feed point in this omnidirectional antenna
array. The power divider feeding network must feed 16
subarrays. The configuration of the 1:16 Wilkinson power
divider feeding network is shown in Fig.8. The Wilkinson
power divider mounted on the top of a grounded dielectric
substrate of thickness h = 0.76 mm and a high relative
dielectric 9.9 constant to reduction dimensions of the power
divider. The return loss of the feeding network is shown in Fig.
9. The simulation result shows that the impedance matching is
proper in frequency band of 9.1 – 10.05 GHz.
Fig 8. The configuration of the 1:16 Wilkinson power divider.
Fig 9. The S11 of the 1:16 Wilkinson power divider.
The Dolph-Chebyshev distribution method can lower the
side lobes of the microstrip series-fed antenna array. The first
side lobe level can reach -21.8 dB, which is much lower than
the ordinary series-fed antenna array. The cylindrical
conformal omnidirectional antenna array includes 16 subarrays
when the cylinder radius is 30 mm. The microstrip Wilkinson
power divider feeding network can be used to feed the antenna
array. The measurement results show this 9.7 GHz cylindrical
conformal omnidirectional antenna has good performance, and
the relative bandwidth and the gain of the conformal antenna
array are 2% and 10 dB, respectively.
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