13. Analysis of Half TEm horn type antenna for High power Impulse radiation applications

3288 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014

Analysis of HTEM Horn-Type Antenna forHigh-Power Impulse Radiation Applications

Sachin Bhagwat Umbarkar, Harivittal A. Mangalvedekar, Member, IEEE, Sreedevi Bindu,Archana Sharma, Purnamasi Chotelal Saroj, and Kailash Chandra Mittal

Abstract— High-power microwaves are generated using highvoltage (HV) pulses, which are generated using a Marx generatorconnected to a peaking stage. The output of the peaking stageis the input to the half transverse electromagnetic (HTEM)horn-type antenna. This paper discusses the effect of isolationdistance between HV tapered arm and the grounded reflector onthe variation of characteristic impedance and gain with respectto the length, flair angle, and tapering angle of the antenna.This paper also discusses the radiation loss on the side and backof antenna. An experiment has been described using an availableMarx generator and HTEM antenna. Using the experimental andsimulation results, the optimal design parameters of antenna areobtained.

Index Terms— Antenna, pulse, radiation, transverseelectromagnetic (TEM).

I. INTRODUCTION

H IGH-POWER microwave (HPM) can be generated byfeeding a high voltage (HV) pulse with rise time in the

range of nano to subnanoseconds. The applications for suchHPM are target object detection, transient radar, mine clearing,detection of crack on underground pipeline, electronic effectstesting, jamming, and so on are reported in [1]–[3].

A HPM system generally consists of HV low rise time pulsegenerator, such as the Marx generator, magnetic compressioncircuits, pulse forming network, and so on. The developmentand analysis of pulse forming network-based Marx generatorusing finite integration techniques for an antenna load is givenin [4]. The input pulse to an antenna should have low rise timeand therefore, the Marx output is connected to the peakingstage. The peaking stage consists of a peaking capacitor andpeaking switch [5]. This low rise time HV pulse is then fedto an antenna. A block diagram of a HPM generator and itscalibration is shown in Fig. 1.

Manuscript received December 18, 2013; revised February 28, 2014 andMarch 31, 2014; accepted April 8, 2014. Date of publication May 12, 2014;date of current version October 21, 2014. This work was supported in partby the Board of Research in Nuclear Science and in part by the Centreof Excellence, Control and Nonlinear Dynamic System, Veermata JijabaiTechnological Institute, Mumbai, India.

S. B. Umbarkar and H. A. Mangalvedekar are with the Department ofElectrical Engineering, Veermata Jijabai Technological Institute, Mumbai400019, India (e-mail: [email protected]; [email protected]).

S. Bindu is with the Father Conceicao Rodrigues Institute of Technology,Navi Mumbai 400703, India (e-mail: [email protected]).

A. Sharma, P. C. Saroj, and K. C. Mittal are with Accelerator & Pulse PowerDivision, Bhabha Atomic Research Centre, Mumbai 400085, India (e-mail:[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2014.2320496

Fig. 1. Block diagram of HPM generator.

Fig. 2. HTEM horn antenna with flair angle α = 31°.

The current developed in the conducting arm of the antennagives rise to electromagnetic fields [6]. The reflector of theantenna which is at the ground potential focuses and directsthe magnetic field. The geometric shape of an antenna affectsthe field radiation pattern [7], [8]. The impedance of thesource, the antenna, and the free space decides the radiationintensity and patterns. The approximate step response of TEMhorn antenna is reported in [9]. Intermediate and far fieldcalculation of a reflector antenna, energized by a hydrogenspark-gap switched pulser is described in [10]. Lee et al. [11]have carried out the design study of TEM horn antenna. Theyhave discussed the interdependency and variation of gain, withrespect to flair angle, tapering angle, characteristic impedanceof TEM horn antenna. A practical half TEM horn antenna isshown in Fig. 2.

This paper has made an attempt to include the salientfeatures of the contributions given in [4]–[11]. The mathe-matical formulation for the characteristics impedance reportedin [11] and [12] has been modified by considering the isolationdistance between HV tapered arm and the grounded reflector.The variation of characteristic impedance and gain with respectto the flair angle and tapering angle of the antenna has beenplotted by considering the isolation distance (feeding height).This has been plotted for width/height ratio less than one andgreater than one. This paper has also estimated the radiation

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UMBARKAR et al.: ANALYSIS OF HTEM HORN-TYPE ANTENNA FOR HIGH-POWER IMPULSE RADIATION APPLICATIONS 3289

Fig. 3. Schematic of near field and far field region.

Fig. 4. Side view and top view of antenna.

intensity of half transverse electromagnetic (HTEM) antennaat the side and back, in order to get a feel of radiationloss.

To understand the design and analysis of the antenna, anexperiment was conducted using HV pulse generator havingan output pulse of rise time 6.0122 ns connected to a halfTEM horn antenna. The experimental results were used alongwith FIT-based software (CST microwave studio) and theoptimum geometrical parameters of the antenna were obtained.A comparison of the experimental results with simulation hasbeen shown.

The rest of this paper is organized as follows: 1) the detailsof antenna parameter calculation are reported in Section II;2) the experiment on HTEM antenna is reported in Section III;3) Section IV compares the experimental and simulationresults; and 4) Section V investigates the optimum geometricalvalues of the antenna using the simulation software so as toobtain maximum gain.

II. ANTENNA PARAMETER CALCULATIONS

An antenna generally consists of a near and far field regionas shown in Fig. 3. The near field region is defined by thespherical region whose radius is less than R

R = 2L2

ctr(1)

where L is the total length of the reflector of the antenna,tr is the rise time of the pulse, and c is speed of light. Thespherical region with radius greater than R is called as the farfield region [13].

The far field region calculations are given in [13]. Theantenna parameters, such as tapering angle (θa), apertureheight (a), flair angle (α), plate width (w), and arc curvature,are shown in Fig. 4 (for 3-D view, refer Fig. 2).

The various optimal geometrical relationships are given in(1)–(8) [8]

w = 2L tan(α/2) (2)

a = L sin θa (3)w

a= 2 tan(α/2)

sin θa(4)

α = 2 arctan

[w

2asin(θa)

]. (5)

The characteristic impedance (Zc) for w/a > 1 and w/a < 1are given by (6) and (7), respectively [12]

Zc = 2 × 377

(w/a) + 2(6)

Zc = 2 × 138 × log8

(w/a). (7)

It should be noted that w and a have to be chosen such thatZc = 377 � to match the output impedance of antenna tocharacteristic impedance of free space.

The geometric impedance ( fg) is then obtained from Zc

and intrinsic impedance Z0 given by (8)

fg = Zc

Z0. (8)

Equations (2)–(8) are discussed in [8] and these formulasneed to be rewrite by considering the feeding height. If antennahas feeding height (H f ) then its aperture heights will be

a = H f + L sin θa. (9)

Thus, (w/a) ratio will be

w

a= 2L tan(α/2)

H f + L sin θa(10)

α = 2 arctan

[1

2L

w

a(H f + L sin(θa)

]. (11)

The characteristic impedance (Zc) for w/a > 1 and w/a < 1are given by (12) and (13), respectively

Zc = 377(H f + L sin(θa))

H f + L(sin(α/2) + sin(θa))(12)

Zc = 2 × 138 × log

(8(H f + L sin θa)

2L sin(α/2)

). (13)

The variation of characteristic impedance for (w/a < 1)and (w/a > 1) for H f = 0.33 m, is shown in Figs. 5 and 6,respectively.

The antenna has been modeled as a transmission line model[14], [15] and the total radiated E field in bore-sight is

ETOTy (r, t)=− Vo

r

a

4πc fg

[δ(t)− c

2L

[u(t)−u

(t− 2L

c

)]](14)

where r is bore sight distance and δ(t) is the delta function.Equation (14) is now expressed in terms of tapering angle ofthe antenna. Equations (3) and (14) gives

ETOTy (r, θa, t)

=−Vo

r

a

4πc fg

⎡⎣δ(t)− c sin θa

2a

⎡⎣u(t)−

u(

t− 2c

(a

sin θa

))⎤⎦

⎤⎦. (15)

3290 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014

Fig. 5. Characteristics impedance, Zc, of the HTEM horn antenna as afunction of the angles α and θa for (w/a < 1).

Fig. 6. Characteristics impedance, Zc, of the HTEM horn antenna as afunction of the angles α and θa for (w/a > 1).

Equation (10) shows that the total radiated field depends onθa , r, f g , and dV /dt. Equation (15) is modified to (17)

θa = sin−1[2

( a

w

)tan

(α

2

)](16)

ETOTy (r, α, t)

= − Vo

r

a

4πc fg

[δ(t)− c

wtan(α/2)

[u(t)−u

(t− w

c1

tan(α/2)

) ]].

(17)

Orientation of antenna (refer Fig. 4) has tapered length alongz-axis and flared along y-axis. The magnetic vector potential(Az) is given by

Az = μL

4π

I(

t − rc

)r

cos θa cos α (18)

where μ and ε are the permeability and permittivity of freespace, I (t − r/c) is the retarded current for 0° < θa < 90°and 0° < α < 90°.

Equations (15)–(18), relate the antenna geometric parameterwith the electromagnetic field parameters. Further Maxwell’sequations and vector magnetic potential equations are used toobtain the electric and magnetic field equations

Hφ = μL cos θa cos α sin θ

4π

⎛⎝ I ′

(t − r

c

)rc

+I(

t − rc

)r2

⎞⎠ (19)

Fig. 7. Experimental setup.

Eθ = L sin θ

4πε

⎛⎝−

I ′(t − r

c

)rc2 +

I(

t − rc

)r2c

+I ′

(t − r

c

)r3

⎞⎠ (20)

Er = L cos θa sin α cos θ

2πε

⎛⎝I ′

(t − r

c

)r2c

+I(

t − rc

)r3

⎞⎠. (21)

It can be seen from (19)–(21) that Er , Eθ , Hφ are functionsof L, r, α, θa , c, ε and other current related quantity. The gainof antenna is equal to [r · Efar/V ]peak [3].

III. EXPERIMENT

The half TEM antenna has L = 1.5 m, α = 31°,θa = 30°, which corresponds to width w = 0.83 m, anda = 1.08 m. It is located on the top of Marx generator at aheight of 1.85 m from the ground. The 20 stage, 64 J, 360 pF,300 kV Marx generator along with the peaking stage, andantenna are shown in Fig. 7. The input pulse applied to theantenna was a pulse with 6.0122-ns rise time and half-width atfull-maximum (FWHM) 150 ns. This is the pulse output of theMarx generator peaking stage. The experiment described wasconducted to understand the shortcomings of the design andmodify the antenna.

Voltage and current output waveform of the Marx generatorpeaking stage is shown in Fig. 8. It is observed that theoutput pulse has 264-kV peak voltage and 1.4-kA peak current.In this experiment, the radiated magnetic field is measured byPRODYNE magnetic field sensor model B-24 (R) at variousdistances and angles from the antenna center.

A. Calculation of the Radiated Magnetic Field

The simplified mathematical equation reported in [20] formagnetic field measurement is given in

Voscilloscope = Aeq · dB

dt= sensor (Volt) (22)

UMBARKAR et al.: ANALYSIS OF HTEM HORN-TYPE ANTENNA FOR HIGH-POWER IMPULSE RADIATION APPLICATIONS 3291

Fig. 8. Output of 20 stages of the Marx generator with peaking stage(FWHM: 150 ns, Vch: 24 kV, RL: 160 �, time/div.: 100 ns, rise time: 3 ns).

Fig. 9. Observed radiated far field, measured at 15-m distance.

where VOscilloscope is the voltage measured on the oscilloscope,and B is the magnetic flux density. The B-dot sensor (ModelNo. B-24-R) has equivalent area (Aeq) = 9 × 10−6 m2.

B. Calculation of the Radiated Electric Field (E)

The relationship of electric field intensity and magnetic fieldis given in

E ≈ cB(Volt/meter). (23)

This sensor is connected to the oscilloscope using BayonetNeill–Concelman shielding cable, to avoid introduction ofthe external field effects. Experimental reading of radiatedfield at the 15-m distance is shown in Fig. 9, which givesthe peak amplitude of electric field intensity (5 kV/m). Theantenna feeding pulse has rise time of 6.0122 ns and has peakamplitude 264 kV. Thus, the maximum rate of rise of voltageis (dV/dt) = (264 kV/6.0122 ns) = 4.391 × 1013 V/s

(r Efar)peak ∝ (dV/dt) (24)

where r is the bore-sight distance [3].

IV. COMPARISON OF EXPERIMENTAL RESULTS

WITH CST SIMULATION

The FIT software gives 3-D platforms for design and analy-sis of high-frequency electromagnetic problems. The radiatedfree space propagation of the pulse is calculated using transientanalysis solver [19]. The simulation is carried out for differentfar field distances and azimuthal angles for the input feeding

Fig. 10. Marx generator output pulse with peaking capacitor.

Fig. 11. Radiated E-field (far field) at 15-m distance for 5-ns rise time inputpulse.

Fig. 12. Scaled up version of Fig. 11.

pulse shown in Fig. 10. This feeding pulse is obtained fromthe experiment.

The radiated pulse measured at 15-m bore-sight distancefrom the center is shown in Fig. 11. The scaled up version ofthe Fig. 11 is shown in Fig. 12 to measure the rise time ofradiated pulse.

The simulated results have first spike of 5.0025-kV/mamplitude with rise time of 1.84 ns and second spike of2.9655-kV/m with rise time of 3.16 ns. The 16.7481-nsdelayed second spike is probably due to ground plate reflection[17]. These two consecutive spikes correspond to the twodominant frequencies of 32.8 and 81.9 MHz, which can beobserved from Fig. 13 [16], [18].

The experimental results and simulation results for the farfield with respect to azimuthal angle and bore-sight distance

3292 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014

Fig. 13. FFT of radiated E-far field for 5-ns rise time pulse at 15-m distance.

Fig. 14. Electric field variation in azimuthal direction.

Fig. 15. Electric field variation with respect to distance.

Fig. 16. Electric field variation with height from ground floor.

are plotted, as shown in Figs. 14 and 15. It is observed thatboth the results match. The variation of electric field intensitywith azimuth angle at 15-m distance from HTEM horn and1.85-m height from ground was recorded as shown in Fig. 16.

Fig. 17. Gaussian pulse with 1.44-ns rise time.

Fig. 18. [r · Efar /V ]peak versus antenna length (L).

Fig. 19. [r · Efar/V ]peak versus antenna tapering angle (θa ) L = 4.5 m,α = 31°.

V. INVESTIGATION OF HTEM ANTENNA USING CST-MS

It is observed that the gain of the antenna is only 0.3, whichcould be improved. Equations (14)–(16) and [8] indicate thatthe gain is equal to (r · Efar /V )peak of antenna depends onthe electrical length of antenna (L/λ) and antenna parameters.To observe the variation of the gain with respect to the antennageometry simulation is carried out with a Gaussian input pulseof tr = 1.44 ns as shown in Fig. 17. Fig. 18 shows the variationof gain with respect to the length of antenna [8]. It is observedthat as length of antenna increases its gain improves. FromFig. 18, it is observed that the gain is 0.489 at 4.5-m lengthof the antenna. Then, from (2) and (3) width w = 2.4959 mand height a = 2.25 m. The simulation also shows that loss ofradiation reduces from the back and sides of the antenna forL ≥ 4.5 m. Similarly the gain for variation of θa , α, and trare shown in Figs. 19–21, respectively. It is observed that thegain is 0.444 at θa and α = 25°. It is to be noted that for thisantenna the maximum gain will be 0.5 as reported in [16].Thus, the simulation shows that the impulse generator-peaking

UMBARKAR et al.: ANALYSIS OF HTEM HORN-TYPE ANTENNA FOR HIGH-POWER IMPULSE RADIATION APPLICATIONS 3293

Fig. 20. [r · Efar/V ]peak versus antenna flair angle (α) for L = 4.5 m,θa = 25°.

Fig. 21. [r · Efar/V ]peak versus rise time for L = 4.5 m, θa = 23°, α = 25°.

switch stage output rise time should be modified to 2 ns. Theantenna geometry should be L = 4.5 m, α = 25°, θa = 23°to obtain an approx gain of 0.5.

VI. CONCLUSION

This paper discusses the salient features of HTEM antenna.It has modified the existing mathematical formula to includethe isolation distance between HV input arms and thegrounded reflector. The variation of characteristic impedancewith respect to flair angle and tapering angle have beendiscussed. The variation of gain for the length, flair angle andtapering angle of antenna, and rise time has been plotted. Theeffects of side and back radiations have also been discussed.An experiment has been conducted and it has been shown thatthe experimental and simulations results match. The simulationhas been used to obtain the parameters of antenna, which cangive optimum gain value of 0.5 pu for the antenna.

ACKNOWLEDGMENT

The authors would like to thank Prof. O. G. Kakde(Director-VJTI), Dr. L. M. Gantayet (Group Director-BARC),Dr. N. M. Singh, Dr. W. Sushma, Prof. F. S. Kazi,Dr. R N. Awale, and D. Aniket of VJTI, Mumbai, andS. Singh, Dr. A. K. Ray, D. P. Chakravarthy, S. Sandip,A. Ritu, T. Somesh, and C. S. Reddy, of BARC, Mumbai andH. Singh of Onus Engineering Group, for their encouragementand fabrication support.

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[3] C. E. Baum et al., “JOLT: A highly directive, very intensive,impulse-like radiator,” Dept. Elect. Comput. Eng., Univ. New Mexico,Albuquerque, NM, USA, Tech. Note 480, Nov. 2003.

[4] S. V. Tewari et al., “Development and analysis of PFN basedcompact Marx generator using finite integration technique for an antennaload,” IEEE Trans. Plasma Sci., vol. 41, no. 10, pp. 2684–2690,Oct. 2013.

[5] S. Bindu, H. A. Magalvedekar, M. Parekh, A. Sharma,D. P. Chakravarthy, and K. C. Mittal, “Electrodynamic simulationof high-voltage peaking switch,” IEEE Trans. Plasma Sci., vol. 40,no. 11, pp. 3093–3099, Nov. 2012.

[6] E. G. Farr and C. A. Frost, “Compact ultrashort pulse fuzing antennadesign and measurements,” Dept. Elect. Comput. Eng., Univ. NewMexico, Albuquerque, NM, USA, Tech. Note 380, Jun. 1995.

[7] J. D. Kraus, R. J. Marhefka, and A. S. Khan, Antennas and WavePropagation, 4th ed. New York, NY, USA: McGraw-Hill, 2012, ch. 2,pp. 33–35.

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[12] D. A. Kolokotronis, Y. Huang, and J. T. Zhang, “Design of TEM hornantennas for impulse radar,” in Proc. High Frequency PostgraduateStudent Colloq., England, U.K., Sep. 1999, pp. 120–126.

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[14] E. G. Far and C. E. Baum, “A model of small-angle TEM horns,” Dept.Elect. Comput. Eng., Univ. New Mexico, Albuquerque, NM, USA, Tech.Note 340, May 1992.

[15] C. E. Baum, “General properties of antennas,” Dept. Elect. Comput.Eng., Univ. New Mexico, Albuquerque, NM, USA, Tech. Note 330, Jul.1991.

[16] G. A. Mesyats, Pulse Power. New York, NY, USA: Springer-Verlag,2004, ch. 28, p. 550.

[17] J. R. Mayes, W. J. Carey, W. C. Nunnally, and L. Altgilbers, “The Marxgenerator as an ultra wideband source,” in Proc. IEEE Conf., PulsePower Plasma Sci., Jun. 2001.

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[19] E. Ben-Ari. (2014, Mar. 1). Design of ultra-wideband high-power-microwave travelling antenna. Appl. Article [Online]. Available:https://www.cst.com/Applications/Category/

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Sachin Bhagwat Umbarkar received the B.E.degree in electronics and telecommunication fromPravara Engineering College, University of Pune,Pune, India, in 2009, and the M.Tech. degree fromthe Veermata Jijabai Technological Institute (VJTI),Mumbai, India, in 2011, where he is currently pur-suing the Ph.D. degree in high-power microwaveapplication for UWB systems with the ElectricalEngineering Department.

He is currently a Research Fellow at VJTI.

3294 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014

Harivittal A. Mangalvedekar (M’14) received theB.E., M.E., and Ph.D. degrees in electrical engineer-ing from University of Mumbai, Mumbai, India, in1979, 1984, and 1995, respectively.

He has been with the Veermata Jijabai Tech-nological Institute (VJTI), Mumbai, India, for thelast 27 years, where he is currently a Professorwith the Electrical Engineering Department. He hasdeveloped the High Voltage Laboratory at VJTI.His current research interests include pulsed powersystems, and high-voltage and power systems

Sreedevi Bindu was born in Kerala, India, in 1970.She received the Degree in electrical and electronicsengineering and the master’s degree in power systemfrom the University of Mumbai, Mumbai, India, in1992 and 2001, respectively, where she is currentlypursuing the Ph.D. degree with the Department ofElectrical Engineering, Veermata Jijabai Technolog-ical Institute.

She is an Associate Professor with the Fr. Conce-icao Rodrigues Institute of Technology, Navi Mum-bai, India.

Archana Sharma received the B.E. degree inelectrical engineering from Regional EngineeringCollege, Bhopal, India, in 1987, and the M.Sc.(Eng.) and Ph.D. degrees from the Indian Instituteof Science Bangalore, India, in 1994 and 2003,respectively. Her specialization is in the design anddevelopment of single shot and repetitive pulsedelectron beam generators based on Marx generatorand linear induction accelerators.

She joined the Bhabha Atomic Research Center,Mumbai, India, as a Scientific Officer, where she

is currently the Head of the Energetics and Pulsed Power Systems Sectionwith the Accelerator and Pulse Power Division. Her current research inter-ests include compact pulsed power systems for HPM, FXR, and industrialapplications.

Purnamasi Chotelal Saroj was born in UttarPradesh, India, in 1966. He received the Diplomadegree in industrial electronics and the B.E. degreein electrical engineering from University of Mumbai,Mumbai, India, in 1992.

He is currently with the Accelerator and PulsePower Division, Bhabha Atomic Research Center,Mumbai. His current research interests include thedevelopment of high-voltage pulse.

Mr. Saroj is a member of VEDA Society in India.

Kailash Chandra Mittal received the M.Sc. (Hons.)degree in physics from Punjab University, Chandi-garh, India, in 1974, and the Ph.D. degree in physicsfrom the University of Mumbai, Mumbai, India,in 1986.

He joined the Plasma Physics Division at theBhabha Atomic Research Center, Mumbai, as aScientific Officer, in 1975. From 1989 to 1991, hewas with Cornell University, Ithaca, NY, USA, as aPost Doctoral Fellow, the University of New Mex-ico, Albuquerque, NM, USA, as a Senior Research

Associate, and the University of Paris, Paris, France, as an Invited Professor.In 2007, he was with Ecole Polytechnique, Paris, as an INSA Fellow. He hasbeen involved in high-power electron beam generation and its applications toflash X-ray generation, high-power microwave generation, and pulse neutrongeneration for strategic purposes. He is involved in the Industrial ElectronAccelerator Program, where high-power electron beams are employed forthe industrial applications. He is currently involved in the developmentof superconducting RF cavities for high-energy proton accelerators. He iscurrently the Head of the Accelerator and Pulse Power Division, the Headof the Particle Beam Generation and Diagnostics Section, and the ProjectManager of the Electron Beam Center. He has more than 175 scientificpublications/presentations in international/national journals/conferences.