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2015

The 8th Annual International Conference ATMS -2015, Bangalore

© ATMS (INDIA)

Design of a Wideband Widebeam Vivaldi Phased Array Antenna for X-band Airborne Radar

Applications Babu Saraswathi K.Lekshmi

Research Scholar, EEE Dept Noorul Islam University

Kumaracoil, Thuckalay, India kavitha20012@gmail.com

I. Jacob Raglend

Professor, EEE Dept. Noorul Islam University

Kumaracoil, Thuckalay, India jacobraglend@rediffmail.com

Abstract

This paper presents the design, testing and measurement of Vivaldi tapered slot antenna (VTSA) with stripline feed which works at X-band frequency with widescan capability needed for airborne radar systems. This antenna have high gain, wider beamwidth, wide bandwidth and easy in fabrication. VTSA satisfies the requirements on the maximum reflection coefficient which is less than -10dB for beam width 120o in H-plane and 110o in E-plane with gain of 3-dB for single element, and satisfies the requirements of Voltage Standing Wave Ratio < 2 for X-band frequency. The S-parameter, radiation patterns and gain simulations for the VTSA are presented to verify the design concepts featuring a very satisfactory performance. The measured beamwidth around 90o is achieved in vertical polarization at x-band frequency. Design of linear array of 7-elements is presented and simulated return loss for center element is achieved less than -10dB including mutual coupling effects. The good radiation characteristics are obtained with gain 9.2 dB for linear array. In this work a Vivaldi notch antenna element is fed by Stripline and may be coupled electromagnetically by tapered transitions. The radiation characteristics, return loss, beamwidth and scan angle, VSWR are obtained through the use of electromagnetic simulation software Ansys HFSS. The simulated and measured results are in good agreement.

Index Terms Phased Array antenna, Tapered slot antenna, Radar, stripline, slotline, Vivaldi antenna.

I. INTRODUCTION

Phased array antenna systems can be used in numerous applications, where one of the oldest is radar systems. The first phased array radar system dates back to the Second World War, and today phased array radar systems are increasingly used on naval ships and aircrafts. Modern phased array radar systems can perform several tasks simultaneously, like keeping track of ground and air targets while at the same time communicating with other units. The modern shared aperture radar concept explored the development of multi-function wideband arrays capable of simultaneous and time interleaving, electronic warfare, and communications functions. This necessitated the need of

frequency independent wide band antennas. With the term frequency independent it is meant that the antenna pattern and impedance remain constant over a relatively wide frequency bandwidth. The stripline-fed tapered slot antenna (TSA) array was introduced by Lewis in 1974[1] and its potential for wideband and wide-scan arrays makes it a prime candidate for high-performance phased-array systems. Phased array antennas are attractive for applications that require rapid scanning of the beam or multiple simultaneous beams. A tapered slot antenna uses a slot line etched on a dielectric material, which is widening through its length to produce an endfire radiation. An electromagnetic [EM] wave propagates through the surface of the antenna substrate with a velocity less than the speed of light which makes TSAs gain slow wave antenna properties. The EM wave moves along the increasingly separated metallization tapers until the separation is such that the wave detaches from the antenna structure and radiates into the free space from the substrate end. The E-plane of the antenna is the plane containing the electric field vectors of the radiated electromagnetic waves. For TSAs, this is parallel to the substrate since the electric field is established between two conductors that are separated by the tapered slot. The H-plane containing the magnetic component of the radiated EM wave runs perpendicular to the substrate.

A tapered slot antenna has a slotline flare from a small gap to a large opening, matching to free space wave impedance. TSAs have moderately high directivity and narrow beamwidth because of the traveling wave properties and almost symmetric E-plane and H-plane radiation patterns over a wide frequency band as long as antenna parameters like shape, total length, dielectric thickness and dielectric constant are chosen properly. Other important advantages of TSAs are that they exhibit broadband operation, low sidelobes, planar footprints and ease of fabrication. A TSA can have large bandwidth if it exhibits a good match both at the input side (transition from the feed line to slot line) and the radiation side (transition from the antenna to free space) of the antenna. The gain of a TSA is proportional to the length of the antenna in terms of wavelength. Tapered slot antennas are also suitable to be used at high operating frequencies (greater than 10 GHz), where a long electrical length corresponds to a considerably short

ATMS INDIA

2015

The 8th Annual International Conference ATMS -2015, Bangalore

© ATMS (INDIA)

geometrical length. These antennas offer a wide bandwidth, significant gain and symmetric patterns in both co-polarization and cross-polarization. TSAs are efficient and light weight. In addition, TSAs are appreciably simple in geometry making them more advantageous. The most commonly used class of TSA in Wide Band technology is Vivaldi antenna. Vivaldi antenna, first introduced by Gibson [2] in 1979, has an exponentially tapered slotline. As a member of the class of TSA, Vivaldi antenna provides broad bandwidth, low cross polarization and directive propagation at microwave frequencies. Vivaldi antennas are low cost, easy to fabricate and fairly insensitive to dimensional tolerances in fabrication process due to printed circuit technology used for the construction of these antennas. Moreover, Vivaldi arrays are small size and low weight enabling compact arrays. It shall be also noted that the beamwidth and directivity of a Vivaldi antenna might be considerably improved varying the design parameters [3], [10]. Designing a wide-band scanning array is also difficult. Typically, the element spacing should be less than one-half free-space wavelength of the highest frequency to avoid grating lobes. Mutual coupling between the radiating elements may be quite large and this coupling may cause scan anmalies within the desired bandwidth and scan volume [12] [13].

II. DESIGN PARAMETERS OF A VIVALDI TAPERED SLOT

ANTENNA

The design of a Vivaldi notch-antenna is shown in Fig. 1. It shows the schematic of the three layers forming the complete antenna assembly viz., namely; bottom layer, top layer and the middle layer (stripline feed). The design parameters can be classified into two categories: substrate parameters (relative dielectric constant r, and thickness t) and antenna element parameters, which can be subdivided into the stripline /slotline transition, the tapered slot, and the stripline stub and slotline cavity.

Fig. 1 Design of a Vivaldi Tapered Slot Antenna

The Fig. 2 illustrates the geometry of the proposed antenna with the associated parameters. The model illustrates two substrates back to back consisting of radiating flare geometries on the two opposite faces. One of the substrate is etched completely on the opposite side of the flare and the other substrate consists of stripline feed being printed and sandwiched between the two substrates containing the flares. The FR4 dielectric substrate used for this design with r = 4.4 and t = 1.6mm). The flares act as an impedance transformation network between free space and the stripline feed. Radiation from the antenna occurs when the slotline impedance is matched to the impedance of free space.

Fig 2. Definition of parameters of Vivaldi Tapered Slot antenna with square cavity linearly tapered slot and radial stub stripline feed.

The stripline/slotline transition is specified WST (stripline width) and WSL (slotline width). The exponential taper profile is defined by the opening rate R and two points P 1(z1, y1) and P2 (z2,y2)

y = c1eRz +c2 (1)

Where

c1 =

c2 =

H

b

d

LG

L

R

P2

P1

LTC

LTA

AR

Rr

WST

WSL

DSL

Stripline feed

Top layer

Bottom layer

Dielectric substrate 1

Dielectric substrate 2

ATMS INDIA

2015

The 8th Annual International Conference ATMS -2015, Bangalore

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The taper length L is z2-z1 and the aperture height H is 2(y2-y1) +WSL. In the limiting case where opening rate R approaches zero, the exponential taper results in a so-called linearly tapered slot antenna (LTSA) for which the taper slope is constant and given by so= (y2-y1)/(z2-z1). For the exponential taper defined by (1), the taper slope s changes continuously from s1 to s2, where s1 and s2 are the taper slope at z=z1 and z=z2, respectively, and s1<s<s2 for R>0. The taper flare angle is defined by = tan-1s. The flare angles, however, are interrelated with other defined parameters, i.e H, L, R and WSL. In this study, Distance from the transition to the taper (LTA) and Distance from the transition to the slotline cavity (LTC) are taken as 1mm. The Radius of radial stripline stub (Rr=2.02 mm) and square slotline cavity (Dsl = 2.6 mm x 2.6mm) are investigated in this parametric study. The bandwidth of the antenna was improved with these non-uniform stubs and also noted that radial stub was more advantageous regarding the overlapping between stripline and slotline stubs. The stripline feeding increased the antenna bandwidth compared with the microstrip feeding.

III. ANTENNA TESTING AND MEASUREMENT RESULTS, DISCUSSIONS AND PERFORMANCE

The Vivaldi Tapered Slot antenna is fabricated with optimized dimensions and measured. The photograph of fabricated VTSA is shown in Fig. 3. The antenna is fabricated on FR4 substrate with r =4.4 with thickness 1.6mm. The antenna is having two substrates such as top and bottom consisting of radiating flare geometries on the two opposite faces. One of the substrate is etched completely on the opposite faces. One of the substrate is etched completely on the opposite side of the flare and the other substrate consists of stripline feed being printed and sandwiched between the two substrates containing the flares. The top, bottom and middle layers are forming the complete antenna. It is fabricated using printed circuit processing techniques. The two substrates were bonded together with the glue them together without any air gaps. The

simulated and measured results of S11 parameter, VSWR and Radiation patterns are shown in Figures 4, 5 and 6. A slight difference is observed between the measurement value and simulated value. The results from simulation and measurement are in good agreement.

VTSA has been designed to operate only in X-band frequency. The measured return loss of single element is achieved less than -10dB for the band of frequency 9GHz to 9.8GHz with the center frequency 9.6GHz shown in Figure 4 and the VSWR is less than 2 units for the same frequency band shown in Figure 5. Figure 6 shows the simulated radiation pattern plots in E-plane and H-planes, respectively of isolated VTSA element illustrating the achieved 3dB Half power beamwidth (HPBW) greater than 120o in H-plane and 110o in E-plane with gain of approximately 3-dB for single isolated element. The figure 7 illustrating the measured radiation results with gain 3dB and beamwidth 82.84 deg Azimuth vertical polarization.

Fig 3. Fabricated Vivaldi Tapered Slot Antenna with Stripline Feed.

8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00Freq [GHz]

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

Y1

HFSSDesign5S Parameter

m2

m3

m1 m4

m5

m6

Simulated Return Loss

Measured Return Loss

Curve InfodB(S(1,1))

Setup1 : SweepY Component 1

Imported

Name X Ym1 8.4000 -9.7884m2 10.4000 -9.4231m3 9.4000 -25.8525m4 9.0000 -9.7838m5 9.5000 -24.5520m6 9.8912 -9.7348

Fig. 4. Simulated and Measured Results of Return Loss

ATMS INDIA

2015

The 8th Annual International Conference ATMS -2015, Bangalore

© ATMS (INDIA)

8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00Freq [GHz]

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

Y1

HFSSDesign5S Parameter

m2

m3

m1 m4

m5

m6

Simulated Return Loss

Measured Return Loss

Curve InfodB(S(1,1))

Setup1 : SweepY Component 1

Imported

Name X Ym1 8.4000 -9.7884m2 10.4000 -9.4231m3 9.4000 -25.8525m4 9.0000 -9.7838m5 9.5000 -24.5520m6 9.8912 -9.7348

Fig. 5 Simulated and Measured Results of VSWR

-200.00 -150.00 -100.00 -50.00 0.00 50.00 100.00 150.00 200.00Theta [deg]

-10.00

-8.00

-6.00

-4.00

-2.00

-0.00

2.00

4.00

dB(G

ainT

otal

)

HFSSDesign5XY Plot 2

Radiation Pattern

m1

m2 m3 m4m5

Curve InfodB(GainTotal)

Setup1 : LastAdaptiveFreq='10GHz' Phi='0deg'

dB(GainTotal)Setup1 : LastAdaptiveFreq='10GHz' Phi='90deg'

Name X Ym1 0.0000 2.8756m2 -55.0000 -0.7562m3 56.0000 -0.9123m4 63.0000 -0.8523m5 -63.0000 -0.6602

-40

-35

-30

-25

-20

-15

-10

-5

0

-150 -100 -50 0 50 100 150

Far-field amplitude of X-Band FR4 Antenna - VP.NSI

Am

plitu

de (d

B)

Theta (deg)

Fig. 6. Simulated Radiation Patterns of Isolated single element VTSA in E-Plane and H-Plane. Fig. 7. Measured Radiation Patterns of Isolated single element VTSA in Azimuth Vertical polarization

IV. ANALYSIS OF VIVALDI TAPERED SLOT ANTENNA ARRAY

The linear array antenna is designed Array performance strongly depends on the mutual coupling in a wide bandwidth and wide scanning arrays. The linear array is designed with 7-elements, displaced a distance, d, all the elements are excited uniformly. All the coupling effects are accounted for in the scan element pattern. The Figure 8 shows the HFSS model of one-dimensional antenna array with 7-elements arranged in a linear grid with spacing, dy =16mm as per the desired specifications. The inter-element spacing is defined by the Equation (2).

------------- (2)

Where is the wavelength.

Therefore, the physical element spacing is restricted by the minimum operating wavelength (highest frequency) and maximum scan angle. When the antenna is in array environment S11is the impedance matching parameter for an antenna element, the principal performance parameter for a wideband array is the active reflection coefficient. That is the percentage of power reflected back from the radiating element when all the elements in an array are fully excited. It can be calculated by the following function

m( ) =

When k = (2 / ), m is the mth element and Smn is the S parameter between the mth element and nth element. In this function the mutual coupling between elements Smn is taken

ATMS INDIA

2015

The 8th Annual International Conference ATMS -2015, Bangalore

© ATMS (INDIA)

into account. The simulated active reflection coefficient is less than -10dB over the X-band frequency when the array scans to greater than ±45o from 8.4 GHz to 10.4 GHz. The HPBW of 16 degrees in = 90deg and HPBW of 126deg. This work reveals that the strong mutual coupling between elements has

good effects to the overall performance of the array. These studies have clearly established the suitability of the designed array element, VTSA, as a candidate for wideband widebeam active phased array for airborne radar applications.

Fig 8. One Dimensional Vivaldi Antenna Array with 7-elements. Fig. 9. 3-D fan shape beam Pattern.

-200.00 -150.00 -100.00 -50.00 0.00 50.00 100.00 150.00 200.00

Theta [deg]

-25.00

-20.00

-15.00

-10.00

-5.00

-0.00

5.00

10.00

dB(G

ainT

otal

)

HFSSDesign1XY Plot 15m1

m2m3

Radiation Pattern

Curve InfodB(GainTotal)

Setup1 : LastAdaptiveFreq='10GHz' Phi='0deg'

dB(GainTotal)Setup1 : LastAdaptiveFreq='10GHz' Phi='90deg'

Name X Ym1 0.0000 9.1692m2 8.0000 6.0766m3 -8.0000 6.0631

1.60

3.20

4.80

6.40

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120

Radiation Pattern 6

HFSSDesign1Curve Info

RealizedGainTotalSetup1 : LastAdaptiveFreq='10GHz' Phi='0deg'

RealizedGainTotalSetup1 : LastAdaptiveFreq='10GHz' Phi='90deg'

(a) (b)

Fig. 10. Simulated Radiation Patterns of Linear Array with 7-Elements in E-Plane and H-Plane and Polar plot

V. ACKNOWLEDGEMENT

The authors will like to acknowledge the support extended by DRDO in carrying out the work. They would like to thank the antenna measurement and fabrication divsions at LRDE for their support in realizing the structure.

IV. CONCLUSION

A compact, wideband dual layered exponentially tapered slot antenna with Stripline feed with the wide scan capabilities are presented in this paper. By transforming the

Stripline feed to Slotline, the bandwidth and radiation characteristics are preserved. Reasonable amount of reduction in size is achieved when compared to the antennas existing. The effects of the various parameters on the antenna performance are studied. The experimental results show that the realized antenna with a very compact size and good radiation characteristics has a wide bandwidth from operating frequency from 9GHz to 9.8 GHz in X-band. The scanned beam patterns of uniformly illuminated antenna array presents the ability of the array to scan over the desired scan volume of ± 60o in E-plane and ± 55o in H-plane. These studies have clearly established the suitability of the designed array

ATMS INDIA

2015

The 8th Annual International Conference ATMS -2015, Bangalore

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element, VTSA, as a candidate for wide scanning active phased array for the airborne active phased array radars.

.

REFERENCES

[1] L.R. Lewis, M. Fasset, and J.Hunt, A broadband stripline array element, in IEEE international Symposium Antennas Propagation dig., 1974, pp. 335-337.

[2] P. J. Gibson, The Vivaldi Aerial,

Proc. 9th European Microwave

Conf. Brighton, U.K, pp. 101-105, 1979.

[3] J. Shin and D.H. Schaubert," A parameter study of stripline-fed vivaldi notch antenna arrays," IEEE Trans. Antennas Propagation, Vol. 47, 879-886, 1999.

[4] Khabat Ebnabbasi, Student Member, IEEE, Dan Busuioc, Ralf Birken, and Ming Wang, "Taper Design of Vivaldi and Co-Planar Tapered Slot Antenna (TSA) by Chebyshev Transformer," IEEE Transactions on Antennas and Propagation, vol. 60, no. 5, May 2012.

[5] A.Kedar and K.S.Beenamole, "Widebeam tapered slot antenna for wide angle scanning phased array antenna," Progress in Electromagnetics Research," vol.27, 235-251, 2011.

[6] Skolnik, M. I., Introduction to Radar Systems, McGraw Hill, Book Company, 1981.

[7] Babu Saraswathi Kavitha L., Jacob Rglend. I, " A wide-scan phased array antenna for a small AESA antenna

A Review," Proceedings of

IEEE international conference on circuit, Power and Computing technologies, ICCPCT 2013, art.no.652910, pp 1008-1016.

[8] C. Hemmi, R.T.Dover, F.German and A.Vespa, "Multifunction wideband array design," IEEE Trans. Antenna Propagation, Vol.47, Mar.1999

[9] Balanis, C. A., Modern Antenna Handbook, John Wiley and Sons, 2008.

[10] Tan-HuatChio and Daniel H. Schaubert, "Parameter Study and Design of Wide-Band Wide-scan Dual-Polarized Tapered Slot Antenna Arrays," IEEE Transactions on antennas and propagation, vol. 48, no. 6, June 2000.

[11] Y. Yao M. Liu W. Chen Z. Feng, "Analysis and design of wideband widescan planar tapered slot antenna array," IET Microw. Antennas Propagation, 2010, Vol. 4, Iss. 10, pp. 1632 1638.

[12] A. Ellgardt, A Scan Blindness Model for Single-Polarized Tapered-Slot Arrays in Triangular Grids, IEEE Transactions on Antennas and Propagation, Vol. 56, No. 9, pp 2937-2942, September 2008.

M. A. Ellgardt and H. Steyskal, Antenna elements for wide-angle phased arrays, Division of Electromagnetic Theory, Royal Institute of Technology, Tech. Rep., 2005.