High Voltage Pulse Radiation from Discone Antenna

High Voltage Pulse Radiation from Discone Antenna

Ratan Sanjay D, Ratna Raju M, Sandeep M Satav, Borkar V G

Directorate of Electromagnetics , Research Centre Imarat,

Hyderabad,

Abstract — The high voltage (HV) pulse radiation of a discone antenna is presented. The generalized Gaussian pulse of nearly 50 kV is applied to the antenna and the pulse radiation is recorded at 3m from the antenna , strictly following the far field criterion for pulsed radiations. Discone antenna is a widely used Ultra wideband(UWB) antenna and has its applications in various fields of engineering and science. The present application is in the high voltage( HV) impulse radiation, in the order of hundreds of kV and vary fast rise time , in the order of sub nano second. The paper deals through the response of the discone antenna to the above said pulse. The high voltage scenario with transformer oil medium is shown. Prior to the practical implementation, simulations were carried out on high speed computers using one of the available Electromagnetic Simulator.

Index Terms: High voltage, Gaussian Pulse, Ultra wide band

I. Introduction

Ultra Wide Band ( UWB) antennas have occupied a very important place in the low voltage and high voltage pulse transmissions. Generally, UWB antennas differentiate the input pulse in transmitting mode and receive the incident pulse as it is. Very few UWB antennas (e.g. biconical) transmit the input pulse as it is and integrate the received pulse in receiving mode. The widely used inputs to the UWB antennas are the Gaussian Impulse and the Heaviside Charging/ discharging signals. The Gaussian signal and its first ,second derivatives are shown in Fig.1. Ideal antennas used in an UWB TX-RX system should faithfully replicate the transmitted pulse at the receiving end.

Mathematical expression for the Gaussian Pulse is

(1a)

First derivative Gaussian Pulse can be expressed as

(1b)

and the second derivative Gaussian Pulse can be expressed as

(1c)

There is a great variety of antennas that can be used for UWB applications, depending on their requirement, including:

- frequency independent antennas,

- aperture antennas,

- reflector antennas,

- travelling wave antennas.

Most frequently used UWB antennas are monopoles, dipoles, ridge horns, TEM horns, Impulse radiating antennas, vivaldi antennas, disc cone and bicones. Properties of the UWB antennas depends on the frequency. However, most of these antennas suffer from dispersion. Dispersion is a variation in pulse waveform as a function of phase angle of the signal. If the phase centre moves as a function of frequency, the resulting radiated waveform will be dispersive. Dispersion gives rise to extended waveform. Table 1 gives the dispersive nature of various UWB antennas. The antennas shown under the frequency independent antennas and broadband antennas cannot be used for high voltage applications for obvious reasons.

II. DISCONE ANTENNA

Among the transient antennas TEM horn and discone antenna would be best options to used in high voltage applications. The discone antenna, which is a monocone with a finite ground plane , is considered to be a better option for the following reasons:

1. It has low standing wave ratio versus frequency , as well as a fairly constant shaped pattern , promised some degree of radiation efficiency over a broad frequency range.

2. Its construction is very simple.

3. It can be used without any terminating impedances which otherwise are difficult to obtain ( for a smaller size).

Table 1. UWB antennas and their dispersion

4. The phase centre variation of the discone is very minimal[3]. If the discone is electrically small , it can be fed to a reflector and also could be easily immersed in a medium which has high dielectric breakdown.

Fig 2. Schematic of a discone antenna

The radiation characteristics of the discone antenna depends upon the dimension, whether the height, L is less than 0.3λ or greater, it behaves as a dipole and a conical horn, respectively. When behaving as a dipole(mono pole in this case), the radiation occurs at the origin. If we assume the axial current along the cone be

(2)

The surface current travels along the cone, with maximum radiation at the vertex. The characteristic impedance of the discone is given by

(3)

where is the free space impedance of 120.

The electric field generated at a distance r, and angle φ , is

given by

(4)

valid for t<L/c and V is voltage applied near the vertex and c is velocity of light

III. PULSE RADIATION

The pulse transmission characteristics of a discone antenna are as per stated[14] , that it differentiates the input as long as the slant length is less than 0.3λ. For, example, a generalized gaussian pulse will be transmitted as its first differentiation, eq.1. However, for a short dipole ( as we view the discone in the dipole configuration), the radiated far field is proportional to the second derivative of the input voltage on the structure[4].

Papas and King[5,6], clearly described about the impedance and radiation characteristics of the monocone antennas. Many configurations have be tried for various applications. When to be used for high voltage pulse radiation, the coaxial feed is not an acceptable option. Radiating broad band pulses using corona charging was stated in [10]. The disc and the cone are maintained with a ground - HV isolation by a finite gap. Once the voltage exceeds the breakdown voltage for the respective gap, a clean, broadband , damped sinusoidal signal is radiated. The generation, radiation and reception scenario is shown in the Fig 4.

Fig. 4 Schematic of the experimental set up

For the High Voltage (HV) pulse radiation ,of the order of few hundreds of kilo volts, it is quite obvious that the antenna cannot be fed like any other antenna, with a coaxial cable as in the case of [6]. Hence, here radiation is intended to achieve by corona charging mechanism[10]. Here the cone element of the antenna is grounded while the disc is fed with high voltage. The switch isolates the disc and the cone. disc is charged by a high voltage pulser. Once the voltage across the switch crosses the breakdown E-field for the respective gap, the high voltage, discharges itself into the ground , during when, the broad band pulse is radiated as damped sinusoid.

The arrangement is shown clearly in Fig.5.

IV. HIGH DIELECTRIC MEDIUM- TRANSFORMER OIL

In order to hold the high voltage in the order of hundreds of kV, the switch is placed in transformer oil. Transformer oil is stable at high temperatures and has very good electrical insulating properties. The dielectric breakdown strength of oil is reported in many references[15], which is always more than 500kV/cm going even upto 750kV/cm in certain conditions. For better performance and long life of the oil used in the antenna, it is advisable to use a sulfur-free Naphthenic transformer oil.

The breakdown is not only influenced by the dielectric medium and the gap between the electrodes, but also the geometry of the electrodes being used. Fig.7[13] shows the standard procedure results to estimate the breakdown voltage of transformer oil with a 1.5µs rise time and 40µs fall from peak to half amplitude for the three cases of profiles, conical, cylindrical and spherical. The spherical profile is the better chosen one, and its value still enhances with the diameter.

V. RESULTS

The discone antenna is fed fabricated as shown in Fig 8. The two portions, cone and disc are given ground and HV , respectively, maintaining the required gap for 50kV. An impulse signal generated from a high voltage pulser, as shown in Fig.9(a) is given and the broadband damped sinusoidal signal , Fig.9(b) is received using a free field D-dot sensor.

VI. CONCLUSION

A small comparison is made between various dispersive and non-dispersive antennas. The discone antenna with extremely low dispersive characteristics is modified to be used at very high voltages. The effective use of transformer oil as an insulating medium is explained. The discone antenna thus proves to be a very effective antenna element in the applications of high voltage pulse radiations. The applications range from ground penetrating radars to high power electromagnetic technologies.

Acknowledgement

The authors wish to acknowledge Ms. Anusha, SRF and Mr.S.Chakravarthy, SRF for their assistance in the measurement of high voltage radiation fields

REFERENCES:

1. Aixin Chen,Tiehua Jiang, Zhizhang Chen, Donglin Su, Wenxuan Wei, and Yanjun Zhang ,"A Wideband VHF/UHF Discone-Based Antenna" IEEE Antennas and Wireless Propagation Letters, vol. 10, 2011

2. Peter Knott, Tomasz Nowicki , Heiner Kuschel," Design of a Disc-Cone Antenna for Passive Radar in the DVB-T Frequency Range", Proceedings of the 6th German Microwave Conference, March 2011, Germany

3. Marco Dionigi, Mauro Mongiardo, Cristiano Tomassoni, "Investigation on the Phase Center of UltraWideband Discone Antennas", German Microwave Conference 2010

4. Debalina Ghosh, Arijit De,Mary C Taylor, Tapan K Sarkar,Michael C. Wicks,and Eric L. Mokole," Transmission and Reception by Ultra-Wideband (UWB) Antennas" , IEEE Antennas and Propagation Magazine, Vol. 48, No. 5, October 2006

5. Charles H. Papas, and Ronold King,Input Impedance of Wide-Angle Conical Antennas Fed by a Coaxial Line, Proceedings of the I.R.E. November 1949

6.Charles H. Papas, and Ronold King, "Radiation from Wide-Angle Conical Antennas Fed by a Coaxial Line", Proceedings of the I.R.E. January 1951

7. Xianming Qing, Zhi Ning Chen, Michael Yan Wah Chia, " UWB Characteristics of Disc Cone Antenna" IEEE Antennas and Propagation, 2005

8.Surendra N. Samaddar, and Eric L. Mokole, "Biconical Antennas with Unequal Cone Angles", IEEE Transactions on Antennas And Propagation, vol. 46, no. 2, February 1998

9.Jinu Kim and Seong-Ook Park, " Novel Ultra-Wideband Discone Antenna" Microwave And Optical Technology Letters / Vol. 42, No. 2, July 20 2004

10. F. Roman , N. Peña , J. Herrera, F. Santamaría , N. Mora, E. Aguilera , O. Díaz, F. Vega, "Radiating broad-band pulse generator with corona charging mechanism"Proceedings of the EUROEM 2008 Conference, Switzerland, 2008

11. D Ghosh and T K Sarkar," Design of a wide-angle biconical antenna for wideband communications", PIER B, Vol.16,229-245,2009

12. G.S.Smith, " Teaching antenna radiation from a time-domain perspective" Amer.J.Phys.,vol.69,no.3, pp 288-300., Mar 2001

13. William North,"High Power Microwave Tube Transmitters", Los Alamos National Lab

14. Karumudi Rambabu, Adrian Eng-Choon Tan, Kevin Khee-Meng Chan and Michael Yan-Wah Chia, Study of Antenna Effect on UWB Pulse Shape inTransmission and Reception" International Symposium on Antennas and Propagation— ISAP 2006

15. R. Hancox, H. Tropper, The Breakdown of transformer oil under impulse voltages, The Institution of Electrical Engineers,Paper No. 2408 M,Sept. 1957

D Ratan Sanjay was born in India, in 1976. He is graduated in Electronics & Communication Engineering from Andhra University, Vishakhaptnam, India , in 1998. He joined DRDO as a Scientist in 1999 and is presently working in RCI, Hyderabad . He is working in the areas of design of antennas, UWB and high voltage antennas, antenna measurements and High Power Microwave Systems . He is member of IEEE,AP, Hyderabad chapter.

M. Ratna Raju received the B.Tech Degree in Electronics & Communication Engineering from Sri Venkateswara University, Tirupati. He Joined DRDO, Hyderabad in the year 2000 and he worked on the development of Ground systems for Project ‘TRISHUL’. Since 2006, he has been with Research Centre Imarat. He is responsible for achieving the Electromagnetic compatibility of defense related systems. His research interests include design and development of High Voltage Fast Impulse generators, Impulse Radiating antennas and Vulnerability studies on defense electronic systems against NEMP. He is one of the main members in establishing the EMP facilities in DRDO.

Sandeep M. Satav received the B.E. degree from Amravati University in 1991, post graduate diploma in business management from Indore University in 1995 and the M.Tech. degree from IIT-Bombay in 2005.During 1992 to 1999, he worked with Scientific Mes-Technik, in the field of test and measuring instruments. Since 1999 he has been with Research Centre Imarat, a pioneer laboratory of DRDO as scientist, where he is responsible for the electromagnetic compatibility of the systems of Indian missile program. His research interests include electromagnetic pulse and its protection, high intensity transient electromagnetics, design and development of sensors and test and measuring instruments for EMI, EMC pre-compliance and transient electromagnetics. He is life member of Society of EMC Engineers of India (SEMCEI).

V G Borkar was born in India, in 1954. He did his Msc(Physics) from Bhopal University in 1976 and PhD from Osmania University, Hyderabad in 1994. He joined as a Scientist in DRDL in 1981 and later joined RCI in 1989. He worked in the fields of antenna development, antenna measurements, Radar Cross Section measurements and high power electromagnetic systems. He is presently, Director, Directorate of Electromagnetics, RCI. He is a member of IEEE-AP Hyderabad chapter and member of IETE.