‘1 Sensor and Simulation Notes Note 382 4 July 1995 A Reflector Antenna for Radiating Impulse-Like Waveforms D. V. Giri and H. Lackner Pro-Tech, 3708 Mt. Diablo 131vd,Suite 215 Lafayette, CA 94549 I. D. Smith and D. W. Morton Pulse Sciences, Inc., 600 McCormick Street, San Leandro, CA 94577 and C. E. Baum, J. R. Marek, D. Scholfleld, and W. D. Prather CLEARED Phillips Laboratory, Kirtl=d .4FB, NM 87117 FOR PUBLIC RELEASE P@% $v $(L 73” &L y’J 27 fy/ Abstract P2ww’d’el e @@& .4 paraboloidal reflector antenna fed by a pyramidal horn has found widespread appli- cation in radar and communication engineering. However, the reflector antenna has very useful characteristics when it is fed or illuminated by two or four-conductor transmission lines. We have am.lyzed, designed, fabricated, and tested a reflector antenna fed by a pair of conical transmission Iines connected in parallel at the focal point. The voltage waveform at the apex of the lines, is a fast rising (~ 100 ps), slowly decaying (N 20 ns e-fold) pulse with a peak amplitude of about 125 kV. The calculated and measured radiated field at a distance of 304 m is impulse-Like with amplitude of about 4.2 kV/m and a pulse duration of < 200 ps. The resultant wide spectrum of the radiated field, extending from about 50 MHz to few GHz, is expected to fl.nd many applications in areas such as hostile target . identification, buried object detection, high-power j amming etc.
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‘1Sensor and Simulation Notes
Note 382
4 July 1995
A Reflector Antenna for Radiating Impulse-Like Waveforms
D. V. Giri and H. Lackner
Pro-Tech, 3708 Mt. Diablo 131vd,Suite 215 Lafayette, CA 94549
I. D. Smith and D. W. Morton
Pulse Sciences, Inc., 600 McCormick Street, San Leandro, CA 94577
and
C. E. Baum, J. R. Marek, D. Scholfleld, and W. D. Prather CLEARED
Phillips Laboratory, Kirtl=d .4FB, NM 87117 FOR PUBLIC RELEASE
P@% $v $(L 73”
&L y’J 27 fy/Abstract
P2ww’d’el e @@&.4 paraboloidal reflector antenna fed by a pyramidal horn has found widespread appli-
cation in radar and communication engineering. However, the reflector antenna has very
useful characteristics when it is fed or illuminated by two or four-conductor transmission
lines. We have am.lyzed, designed, fabricated, and tested a reflector antenna fed by a pair
of conical transmission Iines connected in parallel at the focal point. The voltage waveform
at the apex of the lines, is a fast rising (~ 100 ps), slowly decaying (N 20 ns e-fold) pulse
with a peak amplitude of about 125 kV. The calculated and measured radiated field at a
distance of 304 m is impulse-Like with amplitude of about 4.2 kV/m and a pulse duration
of < 200 ps. The resultant wide spectrum of the radiated field, extending from about
50 MHz to few GHz, is expected to fl.nd many applications in areas such as hostile target .
identification, buried object detection, high-power j amming etc.
1.
ble,
*
Introduction
A finite sized antenna that can radiate a true impulse into the far field is impossi- 0
from physical considerations. However, a dispersionless and wideband antenna with a
nearly flat radiated spectrum is desirable for many applications, such as hostile target iden-
tification, buried object detection, electronic warfare etc. It is the purpose here to describe
the working principles, design considerations, fabricational details and performance data of
an impulse radiating antenna (IRA) system. This type of an IRA employs a paraboloidal
reflector fed by transverse electromagnetic (TEM) lines. Such a radiating system has been
termed “the reflector IRA” for ease of reference. It is ml example of aperture type of
antennas. It is well known that the radiated field from an aperture antenna consists of a
spatial integration of the aperture fields over the aperture, while the temporal behavior
of the aperture field gets differentiated in the far field. For example, if the aperture is
illuminated by a step-function electric field of constant amplitude, then the radiated field
would be an impulse function. Since, ideal step functions and ideal impulse functions are
impractical, the physical antenna radiates an impulse-fike functiomwith an extremely high
bandwidth, satisfying the physical constraint that there be no dc component in the radi-0
ated spectrum. No radiated dc component also means that the total area under the time
domain radiated electric or magnetic field must vanish,
Since we are dealing with impulse-like radiated fields, the spectral content can range
horn 10’s of MHz to several GHz. The low frequency (~~) radiation is limited by the antenna
size, while the high frequency (~~) limitation is imposed by the non-zero risetime of the
volt age pulse fed to the TEM lines that illuminate the paraboloidaI reflector. Since the
feed is TEM, the antenna is dispersionless, unlike other “frequency-independent antennas”
such as log-periodic antennas. Some of the bandwidth definitions in the present context
are
bandwidth = ~~ – ftfh – f!percentage bandwidth =
[(fh + .fl)/21x 100
{
<170 narrowband (e.g., AM radio)1 to 25% wideband (e.g., TV signals)=> 25% ultrawideband (e.g., none in the
IEEE dictionary)
The reflector IRA described here has a percentage bandwidth in excess of 180% out of a
2
*
,
0maximum of 20070, suggesting the need to at least add another category in.the percentage
bandwidth definition. One could define a bandratio (= ~~/jt) instead of the bandwidth,
in which case, the bandratio for the prototype IRA is about 60.
Since it was first proposed in 1989 [I], many aspects of the reflector IRA have been
analyzed in the past, These include feed configurations [2], aperture efficiencies [3], an-
tenna analysis [4 to 6], low-frequency performance [7] and feed impedance [8]. Additional
analytical expressions, quantifying the diffractions from the launcher plates and the cir-
cular rim of the reflector were developed in [9], and certain fabricational details described
in [10 to 13]. Based on the analytical and design considerations described in [1 to 13], a
prototype IRA using a 3,66 m diameter reflector fed by a pair of 400 Q TEM conical trans-
mission lines connected in parallel, has been built and tested. We describe the expected
and measured performance as well as some optimization (e.g., use of electromagnetic lens
in the feed) techniques, in the following sections.
2. Working Principles of a Reflector IRA
The reflector IRA under consideration consists of a paraboloidal reflector fed by two
9 pairs of coplanar feed plates as illustrated in figure 1. Coplanar feed plates are chosen over
the more conventional facing-plate geometry to minimize the aperture blockage effects.
Again, to reduce the aperture blockage, the feed plates are required to be narrow, result-
ing in high-values (several 100’ of 0) of feed impedance. Two 400 0 lines are connected in
parallel resulting in a net feed impedance of 200 Q. The aperture area should be as large
as practical, since the far field is proportional to the square root of this area for a con-
stant voltage at the feed. The far field is proportional to the aperture area for a constant
aperture field. The pulse generator has to be of the differential type to avoid common
mode currents on the feed plates, which could distort the desired features in the far field.
The pulse generator can be represented by a single switch near the focal point. The two
electrodes of the switch have a potential difference of V(t) = &( VO/2)u(t). Since the far
field on axis is proportional to (dV/t%), it is desirable to maximize this rate of rise. We
have achieved a rate of rise of > 101s V/s. The combination of requiring physicaUy small
switches, high voltages and fast rise times implies the use of electromagnetic lenses in the
@
switch region. The lens can be made of an oil medium which serves the dual purposes of
high-voltage insulation and ensuring a spherical TEM wave launch on to the feed plates.
3
a) Side view x = O plane
iY
z
inatingimpedance
‘\\$l = (3Tr/4)4 = (lr/4) ,/”\
[
./ \ ./ .
//
‘; “ (57r/4)
\\
\
“.$ = (7Tr/4)\
‘\b) End view z = -(F - d) plane
\
Figure 1. An Illustration ofareflectorfed by apairofcoplanarconicalTENf lines
* o . .
●
0 The detailed design considerations are described in later section.
Next, we look at an estimation of boresight waveforms. For analysis purposes, one
could consider a 2 coplanar plate feed (figure 2), although in practice we used 2 such
feed lines connected in parallel for a more uniform illumination of the reflector. When
the reflector IRA was originally proposed [I], the boresight radiation was predicted to
consist of a feed-step followed by an impulse-like behavior as indicated in figure 3. A more
recent analysis [9] has extended the results of figure 3, by chronologically considering the
various temporal elements of the boresight radiation. Let us assume that the voltage pulse
generator is switched on at t = O, and the observer is at a distance r (= z) to the right of
the focal point of the paraboloid. These elements are:
A. Prepulse
1) feed step
B. Main pulse of interest
2) imp&e
C. Postpulse
0 3) feed plate diffraction consisting of two parts
a) plate edge on plate of finite width, large compared to wavelength
b) plate of finite width, small compared to wavelength, modelled by circular
cylinder
4) edge diffraction fkom the circular rim of the paraboloidal reflector
D. Con.9traint8 on entire puhe
5) low-frequency dipole moment radiation and no radiation at zero frequency (de).
All of the above, have been analyzed in the past, but summarized here for completeness.
Prepulse or feed step
This is a direct radiation from source or the “switch” towards the observer. It has a
negative amplitude for the assumed signs of voltages on launcher plates. For the geometry
in figure 2, under the assumption of narrow plates (plate width << plate separation), the