Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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instances, the determinant of the radiated signal characteristics. Sidelobes generated
using TEM antennas tend to fall off sharply and are accompanied by a stretching of the
original pulse duration. The distance from the antenna to where the far field region
begins is defined such that the travel time of the differential distance is less than the
rise time of the pulse. Parameters such as gain and beam width for UWB antennas are
difficult to define because these concepts are from the narrowband world, where the
definitions are for a single frequency. For UWB antenna comparisons, the community
uses the previously described figure of merit concept to aid with this problem. The FOM
is the peak electric field measured at some distance multiplied by that distance, thus
the unit for FOM is volts. Using this concept, gain for UWB antennas is defined as:
UWB gain = FOM/Vp
where: Vp is the peak voltage of the driving pulse.
Typical gains are 3-4 but with extreme designs can reach 6 and above. These are the
antennas of choice for impulse radars since they can be small and lightweight but
radiate UWB pulses quite well. However, sidelobe pulse stretching makes the aiming
accuracy of the transmitting and receiving antennas crucial.
Another technique of interest in WB/UWB antennas uses a log-periodic antenna
designed with dispersion characteristics such that, when driven with the proper input
signal, it produces the fast rise time pulse required. The drive signal in this technique
must have a strong increase in frequency from start to end. No high-power designs
using this technique appear to have been accomplished.
A new type of antenna, the impulse radiating antenna (IRA), incorporates a parabolic
dish as a main component of the design (in fact, it is debatable whether this is actually
a new design because it incorporates a parabolic dish). The distinguishing feature of an
IRA is its use of a final fast switch located at the focal point of the dish to provide a
spherical waveform to the dish. The design also must include a high-voltage
transmission line to feed the peaking switch, which in all probability will not be
dispersion less. However, the frequency content of the feed signal most likely will be
less than that of the wave front from the peaking switch. The design also must include
some number of conical transmission lines from the peaking switch back to the dish,
providing something close to an impedance match for the feed pulser. The crucial
criterion here is that the IRA be driven by a spherical TEM wave front, in which case the
phase center of the wave is then fixed and the IRA is dispersion less. There is also a
much smaller pre-pulse that is radiated from the front side of the switch and is not
reflected from the dish. This signal will be radiated two focal lengths ahead of the main
pulse. For a 4-meter dish, the pre-pulse arrives about 10 ns before the main pulse.
Fast-acting semiconductor protection devices could in principle be effective in negating
the effects of the main pulse. The peaking switch at the focal point must be contained
in some insulating media and, thus, there is a reflection associated with the transition
to air. The wave front generally is not spherical as it enters the air beyond the peaking
switch owing to the physical dimensions of the switch and high-voltage insulation
requirements. Successful IRA designs use the switch insulating media and container to
form a lens designed to give a spherical wave at the air interface. The IRA, like the TEM
horn, transmits a differentiated signal from the applied pulse. This is why the rise time
of the driving pulse is so important to antenna response.
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