features occur in the angular directions ψ = sin1
2.5 cycles. The 50-MHz surveys were performed
with a pair of unshielded (metallic shielding miti-
(1/n) measured from vertical, where n is the real
gates back radiation) strings of resistors. The trans-
part of the refractive index of the propagation
medium. For frozen alluvium with n = 2.3, ψ = 26.
mit and receive antennas were separated by 3.3 m
to prevent overloading the receiver by the 800-W
The effective transmitreceive directivity pat-
peak power transmitter. The transmitter keyed
terns are for a horizontal, unshielded, finite-size
the receiver along a fiber optic cable to reduce
radar antenna operating monostatically on the
cable noise.
ground. The H-plane pattern is perpendicular to
The 100-MHz antennas were commercially ob-
the antenna axis and, therefore, along our profile
tained (Model 3207, GSSI, Inc.) flared dipoles with
directions. This orientation helps to ensure that
a tapered resistive loading. The antennas were
we were profiling the ground directly beneath us
backshielded and housed in separate units placed
rather than to the sides. These patterns are not
2 m apart. The transmitter is rated at 800 W peak
affected significantly by the finite separation of
transmit and receive antennas, which is much
about 35%. The received pulse waveforms of the
less than the ground water depths investigated.
100-MHz antennas were similar to those of the 50
The patterns are more directive than for a single
MHz.
transmit antenna owing to multiplication of the
Theoretical transmit radiation patterns for point
identical transmit and receive directivities. The
dipoles are nonuniform, with nulls and lobes in
patterns may be considered to be the response to
planes parallel (E-plane) and perpendicular (H-
an isotropic, polarization-insensitive, nondisper-
plane), respectively, to the antenna axis. These
sive point target.
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