International Journal of Emerging Trends & Technology in Computer Science (IJETTCS)
Web Site: www.ijettcs.org Email: [email protected], [email protected]
Volume 2, Issue 3, May – June 2013
ISSN 2278-6856
A MODERN APPROACH FOR DESIGN AND
OPTIMIZATION OF HELICAL ANTENNA
USED AT TRANSMITTING END OF GPS
B.Venkateshwar Rao1, S.S.G.N.Srinivasarao2, K.Krishna Reddy3
1,2& 3
Vignan’s Institute of Technology & Aeronautical Engineering, Deshmukhi, Nalgonda(Dt)
Andhra Pradesh, India-508284
Abstract: This paper speaks about Design & Optimization
of Helical antenna used at transmitting end of GPS in Lband. The first phase is focused upon deriving a design that is
based on the governing equation of helical antenna and
simulated and functional verification is done using software
tool HFSS (High Frequency Structural Simulator) and made
necessary changes to give rise to a model for design
optimization. In second phase the helical antenna model is
optimized for different parameters like number of turns,
diameter of the ground plate, distance between the bottom of
helix and the ground plate and observed the radiation pattern
of helical antenna at various frequencies. In the last phase,
the model is brought to physical reality.
Key words: HFSS, Simulation, Optimization, Realization
1. INTRODUCTION
Antennas[4] are metallic structures designed for radiating
and receiving electromagnetic energy. An antenna acts as
a transitional structure between the guiding device
(waveguide, transmission line) and the free space.
2. HELICAL ANTENNA
The most popular travelling wave antenna is a helical
antenna (helix)[6] that produces radiation along the axis
of the helix in axial-mode and normal to it in normalmode operation. The advantages of helical antenna are its
wide bandwidth, easy construction, high power handling
capability and circular polarization. The basic geometry
of the helix antenna is shown in Figure 1.
The parameters of the helix antenna are defined below:
D - Diameter of the helix antenna.
C - Circumference of the helix antenna (C=pi*D).
S - Vertical separation between turns for helical antenna.
α - pitch angle, which controls how far the helix antenna
grows in the z-direction per turn, and is given by
α=〖tan〗^ (-1) S/C
N - Number of turns on the helix antenna.
H - Total height of helix antenna, H=NS.
3. CIRCULAR POLARIZATION
Circular polarization [1] occurs when two signals of equal
amplitude but have 90 phase shifted. Circular
polarization can result in Left Hand circularly polarized
(LHCP) where the wave is rotating anticlockwise, or
Right Hand circularly polarized (RHCP) which denotes a
clockwise rotation.
4. DESIGN OF HELICAL ANTENNA
The design of helical antenna is based upon all the above
parameters. In order to find any of the above parameters,
the following set of empirical formulae are
considered:[4],[2]
Directive Gain G = (
)
Gain in dB = 10.8+10 log (
) dB
Half Power Beam Width =
) degrees
Beam Width between First Nulls: (
Effective aperture
=
Terminal Impedance= (140
Axial Ratio
) degrees
square units
)
ohms
=
Bandwidth BW =
= (1.2 – (0.8
Hertz
4.1 Design Parameters
 D= Diameter of Helix
 S=Spacing between turns
 N= Number of Turns
 C= Circumference of Helix= π D
 α= Pitch angle.
0.4
Figure 1 Geometry of Helical Antenna
Volume 2, Issue 3 May – June 2013
Page 180
International Journal of Emerging Trends & Technology in Computer Science (IJETTCS)
Web Site: www.ijettcs.org Email: [email protected], [email protected]
Volume 2, Issue 3, May – June 2013
ISSN 2278-6856
Now the parameters have been evaluated using the
following steps which ultimately lead to the final
theoretical design of helical antenna. Here, the antenna in
consideration is meant to transmit in axial mode with an
operating frequency of 1575.42 MHz.
4.2
Specifications
Helical antenna is to be designed for the following
specifications.
Frequency of operation
: 1575.42 MHz
Polarization
: Circular
Beam width
: 450
Axial Ratio
: 3 dB (max)
VSWR
: 2.5 (max)
Gain
: 10 dBic
4.3 Steps to calculate design parameters
Step I: Take the values from design requirement
Frequency = 1572.42 MHz
In axial mode Circumference C λ (Wavelength)
Pitch Angle 12 < α < 14 degrees
Number of Turns N = 7
Assumption: Pitch angle α =13 degrees
The above conditions are taken to enable the antenna to
transmit in axial mode. The frequency is chosen as L1
Frequency equal to 1575.42 MHz.
Step II: Calculating the parameters
The Wavelength can be determined by lambda ( ) =
Where c= 3x108
190.4
190 mm
Ideal diameter should be around of the order is calculated
by the following so that the transmission in axial mode is
possible. Hence the diameter of the helix is given by the
following
Diameter D = = 0.060606 meters = 60.606 mm.
Helix antennas of at least 3 turns will have close to
circular polarization in the +z direction when the
circumference C is close to a wavelength. As the number
of turns increase the gain of the antenna also increases
Height of Antenna is known by formula
H=NxS
H= Height
N= Number of turns
S= Spacing between two turns
 The ground plate base diameter of the helical
antenna is taken as 3 . It is a function of
wavelength where the antenna resonates.
Therefore,
Ground plate Diameter = ( ) x λ = ( ) x 190
= 150 mm
 The helix antenna functions well for pitch angles
between 12 and 14 degrees. Typically, the pitch
angle is taken as 13 degrees.
Pitch angle =
Volume 2, Issue 3 May – June 2013
 The above can be used to determine spacing
between the two turns or the pitch.
S = π x D x 0.23
Hence,
S = 4.379cms
= 43.79mm.
5. MODELING OF HELICAL ANTENNA
USING HFSS
For the design of helical antenna, software tool HFSS is
used for modeling the structure. HFSS stands for High
Frequency Structure Simulator and is propriety software
of Ansoft Corporation. Based on the available equations,
design parameter of the helical antenna is calculated.
After calculation of helix parameter the model is
generated in HFSS. Radiation box is assigned for plotting
radiation pattern then simulation is started and the results
are obtained for the given parameters of helical antenna.
Finally the optimization is carried out by varying the
number of turns, ground plate diameter and the distance
between the base of the antenna and the start of first turn
of the helix shown in Figure 2.
6. OPTIMIZATION OF THE HELICAL
ANTENNA
For operating the helical antenna in axial mode, the
design parameters are evaluated and modeled. The
simulation study is carried out and results are obtained.
To achieve optimum performance,[5] design parameters
have to be varied one by one. The wire's diameter “d” has
practically very little influence on the antenna's
performance . Based on experimental data and results, the
optimum pitch angle is in a relatively narrow range: 12o
<
<14o. Within the operating frequency band, the
antenna gain varies with frequency. The minimum
number of turns is about N = 4. The size and shape of the
ground plate affect the overall directive gain of the helical
antenna. The minimum diameter of the ground plate has
to be 0.75λ. The gain and return loss of the helical
antenna may be affected by the number of turns and the
distance between the start of the helix and the ground
plate respectively.
Page 181
International Journal of Emerging Trends & Technology in Computer Science (IJETTCS)
Web Site: www.ijettcs.org Email: [email protected], [email protected]
Volume 2, Issue 3, May – June 2013
ISSN 2278-6856
6.1 Parameter variation
From the simulation results it can be observed that the
gain and axial ratio are meeting the design goal. The
return loss has to be improved if possible by optimizing
the parameters like ground plate size, and distance
between ground plate and start point of helix. In the
parametric optimization following three parameters are
varied and simulation is carried out for obtaining desired
return loss.
1. Number of turns
2. Distance between the bottom of helix and the
ground plate
3. Diameter of the ground plate
6.1.1 Varying Number of turns
The number of turns “N” is varied from 5 to 7 turns in
steps of 1. It is observed that the gain increases with no.
of turns as also reported by [7], but in the return loss
value no appreciable improvement is observed.
6.1.2 Distance between the start point of helix and the
ground plate
This distance between helix start point and ground plate
is varied from 2 mm to 5 mm in steps of 1mm to find out
the optimum distance at which return loss is within
acceptable value.
XY Plot 3
HFSSDesign1
-5.00
ANSOFT
Curve Info
dB(S(1,1))
Setup1 : Sw eep
M='-1mm'
dB(S(1,1))
Setup1 : Sw eep
M='0mm'
dB(S(1,1))
Setup1 : Sw eep
M='1mm'
dB(S(1,1))
Setup1 : Sw eep
M='2mm'
-7.50
d B (S (1 ,1 ))
-10.00
In the Figure 4 P is the parameter that represents the
“radius + P” of the ground plate. Return loss is obtained
for different values of P ranging from 0 mm (diameter =
150mm) to 15 mm (diameter =180mm). From the plot, it
can be observed that for P=0 i.e. when ground plate
diameter is 150 mm the return loss is minimum, and
hence acceptable.
6.2 Optimized helical antenna parameters
After examining the optimization plots by varying the
design parameters and selecting them on the basis of
minimum return loss, the optimized parameters are noted
down.
Number of turns (N) = 7
Distance between the bottom of helix and the ground
plate = 5 mm
Diameter of the ground plate = 150 mm
-12.50
6.2.1 Radiation pattern at different frequencies
-15.00
-17.50
1.20
1.30
1.40
1.50
Freq [GHz]
1.60
1.70
1.80
In the Figure 3 the M is the parameter that represents the
distance between starting point of helix and a fixed point
at the height of 3mm from the ground plate. Return loss
is obtained for different values of M ranging from M= 1to M=2 in the steps of 1. Therefore, varying the actual
distance between helix and ground plate is varied from
2mm to 5mm. the optimization graph is obtained and it
can be observed that for M =2 i.e. at the height of 5 mm,
return loss is acceptable throughout the band.
6.1.3 Diameter of the ground plate
In general the diameter of the ground plate is taken as
0.75 λ where λ is the wavelength of centre Frequency.
Therefore, 0.75 x 190 mm is 150 mm. Hence, in the
process of optimization the diameter of ground plate has
to be varied from 150 mm to 180 mm in steps of 10 mm.
Volume 2, Issue 3 May – June 2013
7. RETURN LOSS AND VSWR PLOTS
In the measured plot the return loss and VSWR as shown
in figure 5 & 6 are measured at 1.5 GHz are 9.86 and
1.94 respectively. These figures are acceptable and nearly
matching with the simulation results.
Page 182
International Journal of Emerging Trends & Technology in Computer Science (IJETTCS)
Web Site: www.ijettcs.org Email: [email protected], [email protected]
Volume 2, Issue 3, May – June 2013
ISSN 2278-6856
Figure 5 Return Loss
[3] Antenna Engineering Handbook, ed. R. Johnson,
McGraw-Hill 1993
[4] “K D Prasad” Antenna and Wave propagationpublished by Smt. Sumitra Handa and Satya
Prakashan , New Delhi.
[5] “Antonije R. Djordjević, Alenka G. Zajić, Milan M.
Ilić, and Gordon L. Stüber”- Optimization of Helical
Antennas - IEEE Antennas and Propagation
Magazine, vol. 48, no. 6, pp. 107-116, December
2006.
[6] Walter R. Gulley – Article on Construction Details
for a GPS Helix Antenna.
[7] Hisamatsu Nakano, Yugi Samada and Junji
Yamauchi- IEEE transactions on antennas and
propagation, vol AP-34,No-9, September 1986.
[8] Idine Ghoreishian – The Spiro helical antenna Thesis submitted to the faculty of the Virginia
Polytechnic Institute.
AUTHOR
Figure 6 VSWR
Table 1 Gain Measurement
B.Venkateshwar Rao received B.E degree in
Electronics & Communication Engineering
under Gulbarga Universiry and M.Tech
degree in Digital Systems & Computer
Electronics under JNT University. He is presently
working as Assoc. Prof. in Vignan’s Institute of
Technology & Aeronautical Engineering, Deshmukhi,
Nalgonda(Dt).
Table 2 Design Goal Vs Achieved Results
8. CONCLUSION
By using various bandwidth enhancement techniques, the
bandwidth of helical antenna can be enhanced so that
both L1 and L2 frequencies can be covered by the same
antenna. By shaping the ground plate, gain of the
antenna can be improved to an appreciable extent.
REFERENCES
[1] “John D Kraus , Ronald T Marhefka, Ahmed S
Khan”. Antennas for all applications- The MCGraw
Hill Companies(third edition) 2006.
[2] “Matthew
N
O
Sadiku”
Principles
of
electromagnetics – oxford university press (fourth
edition) 2007.
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