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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

Volume 2, Issue 3 May – June 2013 80

Abstract: This paper speaks about Design & Optimization of Helical antenna used at transmitting end of GPS in L-band. 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 normal-mode 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.

Figure 1 Geometry of Helical Antenna

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: ( ) degrees

Effective aperture = square units

Terminal Impedance= (140 ) ohms

Axial Ratio = Bandwidth BW = = (1.2 – (0.8 0.4 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.

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& 3Vignan’s Institute of Technology & Aeronautical Engineering, Deshmukhi, Nalgonda(Dt) Andhra Pradesh, India-508284



Volume 2, Issue 3 May – June 2013 Page 181

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 (ߣ) = 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 = N x S

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 =

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.




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.

1.20 1.30 1.40 1.50 1.60 1.70 1.80Freq [GHz]

-17.50

-15.00

-12.50

-10.00

-7.50

-5.00

dB(S

(1,1

))

HFSSDesign1XY Plot 3 ANSOFT

Curve InfodB(S(1,1))

Setup1 : Sw eepM='-1mm'

dB(S(1,1))Setup1 : Sw eepM='0mm'



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.

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 6.2.1 Radiation pattern at different frequencies

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.




Figure 5 Return Loss

Figure 6 VSWR

Table 1 Gain Measurement

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.

[3] Antenna Engineering Handbook, ed. R. Johnson, McGraw-Hill 1993

[4] “K D Prasad” Antenna and Wave propagation- published 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

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).

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