Download The high voltage (HV) pulse radiation of a 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, India
[email protected]
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.
Amplitude( Normalised)
1
Gaussian Pulse
First Derivative
Second Derivative
0.5
-0.5
-4
-3
-2
-1
0
Time
1
2
3
4
5
Fig. 1 Time domain Gaussian pulse and the first and second
derivatives
Mathematical expression for the Gaussian Pulse is
𝑡 2
𝜏
[−( ) ]
(1a)
First derivative Gaussian Pulse can be expressed as
2𝑡
𝑠 ′ (𝑡) = − ( 2 ) 𝑒
𝑡 2
𝜏
[−( ) ]
𝜏
(1b)
and the second derivative Gaussian Pulse can be expressed
as
𝑠 ′′ (𝑡) =
2
2𝑡
𝜏
𝜏
2 ( 2 − 1) 𝑒
𝑡 2
𝜏
[−( ) ]
(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
0
-1
-5
𝑠(𝑡) = 𝑒
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).
III. PULSE RADIATION
Type of Antenna
Antenna
Transient Antennas
TEM Horn
Monocone/Discone
Planar monopole
Spiral & sinous
Logarithmic
Broadband antennas
No dispersion
Very low
Vivaldi
Frequency
independent antennas
Dispersion
Elliptical dipole
Multimode slot
Extremely low
Very low
High
High
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].
Medium
Medium
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.
D- Cone diameter
Fig. 3(a) :Idealized Gaussian
Pulse
Fig. 3(b) :Transmitted signal by
short dipole configuration
d- Disc diameter
a- Cone slant length
- Cone half angle
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
𝑥
𝐼(𝑥, 𝑡) = 𝛿 (𝑡 − ) [𝑢(𝑥) − 𝑢(𝑥 − 𝐿)]
𝑐
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.
(2)
The surface current travels along the cone, with maximum
radiation at the vertex. The characteristic impedance of the
discone is given by
Fig. 4 Schematic of the experimental set up
𝜂
𝜃
2𝜋
2
𝑍𝑐ℎ𝑎𝑟 = ( ) ln [𝑐𝑜𝑡 ( )]
(3)
where 𝜂 is the free space impedance of 120 𝜋.
The electric field generated at a distance r, and angle φ , is
given by
𝑟
𝐸∅ (𝑟, 𝑡) = {𝑟×ln
𝑉(𝑡−𝑐 )
cot(θ⁄2)×sin ∅}
(4)
valid for t<L/c and V is voltage applied near the vertex and
c is velocity of light
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.
shown in Fig.9(a) is given and the broadband damped
sinusoidal signal , Fig.9(b) is received using a free field Ddot sensor.
1. Cone
2. Disc
3. Gap
4. Oil Medium
Fig.5 Schematic of the discone configuration for High Voltage Pulse
Radiation
IV. HIGH DIELECTRIC MEDIUM- TRANSFORMER OIL
Fig.7 Breakdown performance of 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.
Fig.8 Discone antenna with finite
gap
4
5
4
4
2
0
2
E-Field (V/m)
Voltage (volts)
3
x 10
1
0
-2
-4
-6
-1
-8
-2
-3
-1
Fig.6 Results from four similar series of breakdown measurements
with 6.25mm brass electrodes for 2/60 μs pulse
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
-10
-0.5
0
0.5
time (sec)
1
1.5
-8
x 10
Fig 9(a) Measured High voltage
input pulse to the discone
antenna
-12
0.6
0.8
1
1.2
1.4
Time (sec)
1.6
1.8
-8
x 10
Fig 9(b). Measured E-field at a
distance of 1m from the antenna
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
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Range", Proceedings of the 6th German Microwave Conference,
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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.