Tappered Spiral Helix Antenna

Transcription

1 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) Tappered Spiral Helix Antenna Sneha Talari 1, R.Naraiah 2, Dr.Ramakrishna 3 1 Student & Osmania University, Hyd. 2 Assoc.Prof & GNITC, IBP. 3 SC-G & DLRL, Hyd. Abstract This paper presents the aspects related to the design and development of a tapered spiral helix antenna covering the frequency range GHz. It is a compact stateof-the-art circularly polarized antenna, which works over multi-octave frequency bandwidth ( GHz).The antenna size has been reduced by 5 percent with respect to conventional spiral antenna used for same frequency band. Spiral helix antenna consists of a spiral antenna and a helical antenna where the outer ends of spiral antenna are terminated with a helix. The helix is placed with its axis at 9 degrees to the spiral lies behind it and is designed to produce circularly polarized radiation over a range form GHz. Simulation software used for development of tapered spiral helix antenna is CST studio suite software. Once the antenna design is completed, the various parts will be fabricated and assembled with precision. The assembled antenna will be tested for various characteristics and the achieved specifications will be compared with projected specifications. Keywords circular polarization, computer system tools (CST), direction finding systems, helical antenna, spiral antenna. I. INTRODUCTION Electronic warfare systems require antenna having wide bandwidth capable of receiving signals coming from any direction with any polarization in the absence of prior information about the threat signal. The requirements become more stringent for airborne applications where weight, size and volume are at premium. The bandwidth of conventional antennas is limited because the electrical aperture dimensions change with change of operating frequency. The spiral helix antenna is extensively used as a receiving element in DF systems employing amplitude and phase comparison techniques. Circular polarization is an important parameter for DF systems because the maneuvering aircraft must be able to respond to any particular orientation of threat signals. The threat signals are generally linearly polarized because it is easier to make an efficient linearly polarized radiator than to make one with any other polarization. Spiral helix antennas maintain consistent gain and input impedance over wide bandwidths with circular polarization and hence a wide range of applications exist ranging from military surveillance, ECM and ECCM to numerous and commercial and private uses including consolidation of multiple low gain communication antennas on moving vehicles. 318 II. THEORETICAL BACKGROUND OF SPIRAL AND HELIX ANTENNAS Spiral antenna: E.M Turner has introduced the spiral antennas in 1954.The spiral antenna is class of frequency independent antenna under angular concept. The spiral is a planar structure that is fabricated by photo etching a two-arm spiral on copper clad substrate. When the spiral arms are fed in anti phase at the centre, the spiral radiates circularly polarized energy in bidirectional beam perpendicular to the plane. Only the physical dimensions of the spiral limit the frequency band of radiation. To obtain unidirectional beam, the spiral is mounted at the open end of a closed back metallic cavity,which, when in the region of /4 deep, redirects this half of the energy constructively to for a single beam.however the energy within the cavity is absorbed to achieve a broadband radiation. The spiral radiator, being a balanced device, needs to be fed from a balanced transmission line. This necessitates the incorporation of a balun transformer. Thus the spiral antenna consists of three main components 1) The spiral radiator 2) The backing cavity 3) The balun transformer Helical antenna Helix antennas (also commonly called helical antennas) have a very distinctive shape. The most popular helical antenna (helix) is a travelling wave antenna in the shape of a corkscrew that produces radiation along the axis of the helix antenna. These helix antennas are referred to as axialmode helical antennas. The benefits of this helix antenna is it has a wide bandwidth, is easily constructed, has a real input impedance, and can produce circularly polarized fields. Helical antennas have long been popular in applications from VHF to microwaves requiring circular polarization, since they have the unique property of naturally providing circularly polarized radiation. The gain of the helical antenna is also proportional to the number of turns. Where more gain is required than can be provided by a helical antenna alone, a helical can also be used as a feed for higher gains.

2 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) III. RADIATION FROM A SPIRAL AND HELICAL ANTENNAS The radiation from Archimedean spiral can be explained from the band theory of Burdine. The theory is in good accord with experimental observation. Dual arm spiral can be considered as a two-wire transmission line transformed into a radiating structure. Two radiation bands exist, one which produces a single lobe radiation pattern with a maximum along axis of the spiral, while the second produces a split beam that produces a split beam pattern with a null on- axis. These are known as fundamental or normal and split beam modes respectively. Fig(i):Spiral antenna with helices connecting the spiral to the ground plane Helical Antenna radiates when the circumference of the helix is of the order of one wavelength and radiation along the axis of the helix is found to be the strongest. This antenna is mainly directional. The radiation from a helical antenna is circularly polarized, that is to say that the Electromagnetic field rotates about the axis of the helix in the direction of the helix turn. Therefore, the radiation is either circularly polarized clockwise or counter-clockwise. When using Helical Antennas it is very important to make sure that both antennas have the same thread orientation (i.e. both clockwise) otherwise the received signal will be significantly decreased. IV. GEOMETRY OF RADIATORS The two radiators which are used in this paper is to design and fabricate the spiral helix antenna is 1. Archimedean Spiral Antenna 2. Helical Antenna Archimedean Spiral The Archimedean spiral antenna is a popular of frequency independent antenna. Previous wide band array designs with variable element sizes (WAVES) have used the Archimedean spiral antenna as the radiating element. The Archimedean spiral is typically backed by a lossy cavity to achieve frequency bandwidths of 9:1 or greater. Geometry of Archimedean round is defined by angle and. Fig(ii): Archimedean Spiral Antenna structure The equation for Archimedean Round Spiral is R=a +b Where b is the initial radius and a is the growth rate. The pitch angle is related to growth rate a by Tan = r a Where r is the radius of the antenna. The scaling factor eq (r) =e- 2 tan =e-2π The sides of the strip may be defined in terms of rotation angle and r= a 2 The radial width of the strip is constant independent of W r = a The spiral arm width has to be increased gradually as it expands to get frequency characteristics. However, to accommodate more number of turns in low frequency region, the width of the spiral is decreased to improve axial ratio at lower end of the frequency band. Hence, the actual width of the strip varies slightly with r and is given by W=r The spacing between centre lines of adjacent terms for one arm is given by Actual spacing is given by S r =2 a S=S r sin =2 sin The two arm spiral is self-complementary when = /2 or w/s=1/4.the second arm of the Archimedean spiral structure is obtained by rotating the first arm by 18 around the origin O. a r 319

3 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) In Archimedean round spirals depth of cavity is / π at lowest operating frequency, for getting optimum performance. Cavities of antennas covering such wide bands are usually filled with radiation absorbing materials to prevent reflected energy adding destructively with the forward beam. The nature and position of the absorber is usually arrived after experimentation. Helical Antenna Helical antennas have been widely used as simple and practical radiators over the last five decades due to their remarkable and unique properties. The rigorous analysis of a helix is extremely complicated. Therefore, a radiation property of the helix is extremely complicated. In most cases the helix is used with a ground plane. Typically the diameter of the ground plane should be at least 3λ/4. However, the ground plane can also be cupped in the form of a cylindrical cavity. The geometrical configuration of a helix consists usually of N turns, diameter D and spacing S between each turn. The total length of the antenna is L = NS while the total length of the wire is 2 2 NL N S where L n C 2 2 L S C is the length of the wire between each turn and C = πd is the circumference of the helix. Another important parameter is the pitch angle α which is defined by tan 1 S 1 tan D S C When α =, then the winding is flattened and the helix reduces to a loop antenna of N turns. On the other hand, when α = 9 then the helix reduces to a linear wire. When < α < 9, then a true helix is formed with a circumference greater than zero but less than the circumference when the helix is reduced to a loop (α = ). The radiation characteristics of the antenna can be varied by controlling the size of its geometrical properties compared to the wavelength. The input impedance is critically dependent upon the pitch angle and the size of the conducting wire, especially near the feed point, and it can be adjusted by controlling their values. The relations between S, C, α and the length of wire per turn, L, are obtained as L S Ltan C tan S C 1 S D 2 Therefore, the input impedance (purely resistive) is obtained by C R which is accurate to about ±2. All these relations are approximately valid provided 12 14, 3 4 C 4, N 3 3 V. DESIGN SPECIFICATIONS The Design of Archimedean spiral antenna and helical antenna includes the design of three basic components for spiral and five parameters for helix. The spiral components are spiral radiator, the backing cavity and the balun transformer and the helical parameters are number of turns, circumference, pitch angle, wire diameter and ground plane diameter. The spiral card is in Archimedean shape and its dimensions are decided by the frequency of operation. The smallest and largest sides of spiral card are deciding by the highest and lowest frequencies. The spiral cavity is designed to suit the spiral card. Diameter of the cavity is equal to largest diameter of spiral. Depth of the cavity varies according to spiral dimensions and maximum depth is /4 at lowest frequency. A printed circuit balun is designed using Tchebychev transformation. MATLAB programs are developed for the design of spiral radiator and balun. Same time helix is designed using CST studio suite software. Design of Archimedean Spiral Circuit The smallest and largest diameters of the Archimedean spiral cards are decided by the highest and lowest frequencies. The diameter is given by of the lowest frequency operation of Archimedean spiral antenna. To avoid end-effects (reflections from the spiral outer terminals), the sides are taken 1% more than that of theoretical calculations. 1. Lowest frequency of operation = 4GHz 2. Highest frequency of operation = 18GHz 3. Diameter of arm at highest frequency = 5.38mm 4. 1% of the theoretical size =.53mm 5. Diameter of arm at lowest frequency = 23.8mm 6. 1% of the actual size = 2.38mm 7. Final dimension of antenna = 32.9mm But maximum dimensions are taken as 37mm because of size restrictions imposed by platform. Design of Balun Different types of printed circuits baluns are available for feeding a two arm spiral antenna. But, the printed circuit balun using Tchebychev transformation is the optimum. A computer program is developed for the design of a broad band printed circuit balun.

4 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) Highest impedance to be matched, Z H= 11Ω Lowest impedance to be matched, Z L = 5 Ω Substrate thickness =.8mm Dielectric thickness, Є r = 2.2 Lowest frequency of operation= 1.2GHz Length of the balun =46mm Fig(iii):Balun schematic showing ground plane and centre conductor Assume that maximum reflection coefficient in the pass band should not exceed one-twentieth of ρ о max /12 Step (1): Calculation of reflection coefficient.5 ln( Z H / Z L ) Step (2): Calculation of A.5ln(11 / 5) cosh( A ) / max 2 Hence, A= Step (3): Calculation of length of the balun βi=a I=A/β =A/ (2Π/λ) I=.587λ 1 Where λ 1 is wavelength at lowest frequency of operation Step (4): Calculation of characteristic impedance Z e where ( Z, A) 1 ln( ZH ZL ) 2 z I 1 2 2X, cosh( ) A A A I 2 A 1 y dy; Z 1 A 1 y I 1 is Bessel function of first order. These are calculated using computer program developed. The values of characteristic impedance of different sections of tapered line are realized in micro strip configuration using the standard formulae. 2 Incorporating all these provisions, an artwork is prepared on a cut and strip material magnified by five times. This artwork is photo reduced to the actual size. This artwork is used for printing the balun circuit on a low loss Teflon fiberglass substrate (Є r =2.2). The micro strip printed circuit is etched on a.5mm (2 Thou) thick double sided copper clad material (RT Duroid 588). On top of the substrate centre conductor is printed and the ground plane is printed on the bottom surface of the substrate. Alignment marks are provided for aligning both the centre conductor and ground plane. When the printed circuit balun is realized in micro strip configuration, there is bound to be stray radiations, which can cause problems of beam squint, asymmetry an ripple in patterns which are undesirable. The stray radiation is eliminated by enclosing the balun inside the cylindrical tube loading with microwave absorbing material (ECCOSORB MF-117). This gave good radiation patterns up to 18GHz. Design of Cavity Spiral cavity was designed to suit the spiral card. The dimensions are chosen such that the side of the cavity equals to the largest square side of the spiral. Gain of the antenna will be maximum when the depth of the cavity is λ/4. But, this cannot be achieved over the full band of GHz. Hence cavity is loaded with honeycomb absorber (ECCOSORB HC-.5) to avoid destructive interference of the reflected signal from the bottom of the cavity. Depth of the cavity=λ/4 at lowest frequency. Honeycomb absorber is used because of high repeatable electrical performance along with high structural rigidity. Assembly of Spiral Antenna Input end of the balun is soldered to the connector and the other end is soldered to the feed points of the spiral card. The feed connection is very critical because it cannot be made through a naked eye. This was done under a microscope. Before the feed connection, the balun is loaded with absorber and spacer discs. The detailed procedure is discussed below. 1) An Archimedean cavity is made as per the sketch from the aluminum alloy. 2) The balun tube is loaded with rubber gasket, Teflon spacers and absorber discs as per the assembly drawing. 3)Front plate is fixed to the cavity 4) Balun along with the connector is inserted into the cavity and then fixed to the cavity with M2 screws. 321

5 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) 5) Honey comb absorber is bounded to the plate to the plate front of the cavity with suitable adhesive. 6) Spiral card is placed on the top of the honeycomb absorber such that the two leads of the balun will pass through the two hole drills on the pad. 7) The two leads are cut and bent onto the curves and soldered to the feed point Axial ratio : 2 db Squint : 3 VI. SIMULATION RESULTS OF SPIRAL HELIX ANTENNA a)return loss Design of Helix Fig(iv):Assembly of Spiral Antenna The optimized design data for the helical antennas located above an infinite ground plane is given below. The shape and dimensions of the ground conductor (reflector) have influence on helical antenna performance. The Optimized design parameters for helical antennas with cylindrical ground conductor are given below. Number of Turns = 5.5 Pitch Angle = 7 Radius Change = -.8 Polygon Radius =.5 Start helix radius = 2 Radius ratio =.65 Polygon Segments = 4 Angle = 5.5*36 Ground plane Diameter = 56.2mm By using the above parameters helix is designed and is wounded around the FR-4 material and is placed with its axis at 9 degrees to the spiral lies behind it. The outer ends of the spiral arms are terminated with a helix. In general spiral helix antenna provides frequency coverage unattainable in a single device. Thus the antenna size has been reduced to 5 percent used for the frequency band GHz. Aimed Specifications Frequency band of operation : GHz VSWR : 3.5:1 Polarization : Circular Nominal Beam width : 77 (Nom) 48 (Min) 17 (Max) Gain : better than -3dB 322 b) Gain c) Beam width d) Axial ratio Fig(v):Return loss of Spiral Helix Antenna Fig(vi):Gain of Spiral Helix Antenna Fig(vii): Polar plot of spiral helix antenna Fig(viii):Axial ratio of spiral helix antenna VII. MEASURED RESULTS FOR SPIRAL HELIX ANTENNA The antenna is tested using Network Analyzer in an anechoic chamber.

6 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) With network analyzer the VSWR plot is obtained and using anechoic chamber the radiation patterns for both vertical and horizontal polarizations are measured. Using the radiation patterns 3dB beam width, gain, squint of the antenna is measured. These measured results are also tabulated below. Radiation Pattern Measurements The Radiation patterns at different frequencies are measured using anechoic chamber. Here are some of the radiation patterns corresponding to some frequencies which are given below. e) At 8GHz f) At 12GHz a) At 1.2GHz e) At 18GHz b) At 2GHz c) At 4GHz VIII. MEASURED RESULTS FOR SPIRAL HELIX ANTENNA The results which are tabulated below are obtained from the radiation patterns measured at different frequencies in an anechoic chamber. Table I Measured Results Of An Antenna Which Are Obtained From The Radiation Patterns At Different Frequencies. Frequency Axial d) At 6GHz (GHz) 1.2 Beamwidth 9.28 Gain Squint -.1 ratio

7 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) The maximum VSWR which is obtained using network analyzer is observed to be 3.33 at 16.4GHZ and at all other frequencies it is <3.33 whose performance range is in acceptable limits for the antenna which is measured over the frequency range of GHZ is shown below. Fig(ix): VSWR Plot of Spiral Helix Antenna Thus the comparison table of specified, simulated and realized specifications is given below. Table II. Comparision Of Specified, Simulated And Realized Specifications Specifications Specified Simulated Realized Frequency band of operation GHz GHz GHz VSWR < Polarization circular circular circular Beam width 77(nom) 48(min) 17(max) 8.8(nom) 75(nom) 46.4(min) 13.31(max) IX. CONCLUSIONS A Tapered Spiral Helix Antenna covering GHz has been developed. The antenna that produces low gain and nearly circular polarization over a wide bandwidth has been presented. The antenna exhibited VSWR of 3.3(max) over the entire band of GHz and patterns are symmetric with beam width varying from 46º to 13º with a well balanced balun. Development of the antenna is very critical for development of future DF systems because of compact size and broadband characteristics. Computer programs are generated and different software s are used to design spiral helix antenna. This paper presents the complete theoretical analysis and design methodology of a tapered spiral helix antenna covering the frequency range of GHZ. REFERENCES [1 ] H.Nakano,Y.okabe,H Mimaki and J.Yamauchi, A spiral antenna excited through a helical wire, IEEE Tans Antenna Propag 51(23), [2 ] Press,letchworth,1987. Kraus, J.D., (W8JK), A Helical-Beam Antenna without a Ground Plane IEEE Antennas and Propagation Magazine April [3 ] H.Nakano, Helical and spiral antennas :A numerical Approach, Research Studies [4 ] M.N. Afsar, Y. Wang and R. Cheung, Analysis and Measurement of a Broadband Spiral Antenna, IEEE Antennas and Propagation Magazine, Vol. 46, No. 1, February 24, pp [5 ] H. Nakano, H. Takeda, T. Honma, H. Mimaki, and J. Yamauch, "Extremely Low-Profile Helix Radiating a Circularly Polarized Wave," IEEE Trans Antennas Propagat., Vol.39, No.6, pp June [6 ] M. Buck and D. Filipovic, Spiral Cavity Backing Effects on Pattern Symmetry and Modal Contamination, IEEE Ant. Wirel. Propagat. Let., vol. 5 (26): Gain better than - 3dB -8.44dB dB Axial ratio 2dB 2.3dB 1.5dB Squint

8 Website: (ISSN , ISO 91:28 Certified Journal, Volume 3, Issue 3, March 213) BIBLIOGRAPHY Sneha Talari received the B.Tech degree from JNTU, Hyderabad in 29, the M.E degree in Microwave and Radar Engineering from Osmania University in 211.She is currently working as a Assistant Professor in Gurunanak Institutions of Technical campus, Department of Electronics and Communications Engineering, Hyderabad. R. Naraiah received the B.Tech degree from JNTU, Hyderabad in 23,the M.Tech degree in Instrumentation and Control Systems form JNTU, Kakinada in 21. He is currently working as Associate Professor in Gurunanak Institutions Technical Campus, Hyderabad. His research activities are centered around Image Processing, Communciations, EMTL. 325