Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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AFRL, called the Matrix, is simply a quarter-wave, high-voltage coax line that is
switched onto a half-IRA antenna by a high-pressure hydrogen switch. This produces a
damped sinusoid with a frequency variable from 180 to 600 MHz by means of different
coax line lengths.
DIEHL Munitiossysteme in Germany manufactures several mesoband sources for sale to
the public. One is a suitcase-sized system using a compact 600-kV Marx generator to
provide an impulse to a loop antenna. It is made tunable by changing the size of the
loop antenna and can radiate a damped sinusoid at 70 kV/mat 2 meters' distance.
BAE Systems in the United Kingdom also sells mesoband sources made using nonlinear
transmission lines and solid-state modulators. These are repetition-rate operated to
more than 1 kHz.
HPM Antennas
Many types of antennas are used for radiating HPM signals, and the choice of which to
use depends on many factors, including power level, bandwidth, size constraints,
efficiency and range requirements, and fratricide concerns. This section discusses the
most common antennas used for high-power applications: horns, parallel-plate
antennas, and impulse radiating varieties.
NARROWBAND ANTENNAS
HPM narrowband antennas have typically been extrapolations of conventional types
modified in some way to prevent air breakdown and to support higher electric fields.
Antenna arrays are in development, albeit mainly for phase-locked multioscillator
systems. The obstacle to using arrays in single-oscillator systems is that little has been
done to develop the required antenna subelements (phase shi~ers and phase splitters)
capable of high-power operation. Eventually, however, antenna arrays will be dominant
in narrowband HPM because higher powers will require larger area antennas to prevent
breakdown and arrays are the only antennas that are compatible with electronic control
for tracking targets. To achieve good effectiveness and gain, the array size should be
larger than the wavelength to be radiated. The beam width is approximately 2;.JL, and
the gain is L2/), 2 . Also of importance is the separation between elements in the array.
Larger distances between elements cause grating lobes offcenter from the main beam.
If the distance between elements can be reduced to less than the wavelength, grating
lobes will be minimal. In practice, however, breakdown constraints make this hard to
accomplish.
In practice, only a very small number of antenna types have been used in narrowband
HPM with any success. By far the most common type is the horn in its many forms,
including transverse electromagnetic (TEM), pyramidal, and conical. One advantage of
horn antennas for narrowband use is that the radiation pattern and the gain can be
calculated precisely.
For TEM and pyramidal horns, the gain is given by:
G = 2nab/A2
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