228 research outputs found
Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar
We show experimentally that a high-resolution imaging radar operating at 576â605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4â25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources
Confocal Ellipsoidal Reflector System for a Mechanically Scanned Active Terahertz Imager
We present the design of a reflector system that can rapidly scan and refocus a terahertz beam for high-resolution standoff imaging applications. The proposed optical system utilizes a confocal Gregorian geometry with a small mechanical rotating mirror and an axial displacement of the feed. For operation at submillimeter wavelengths and standoff ranges of many meters, the imaging targets are electrically very close to the antenna aperture. Therefore the main reflector surface must be an ellipse, instead of a parabola, in order to achieve the best imaging performance. Here we demonstrate how a simple design equivalence can be used to generalize the design of a Gregorian reflector system based on a paraboloidal main reflector to one with an ellipsoidal main reflector. The system parameters are determined by minimizing the optical path length error, and the results are validated with numerical simulations from the commercial antenna software package GRASP. The system is able to scan the beam over 0.5 m in cross-range at a 25 m standoff range with less than 1% increase of the half-power beam-width
Time-Delay Multiplexing of Two Beams in a Terahertz Imaging Radar
We demonstrate a time-delay multiplexing technique
that doubles the frame rate of a 660â690-GHz imaging radar
with minimal additional instrument complexity. This is done by
simultaneously projecting two offset, orthogonally polarized radar
beams generated and detected by a common source and receiver.
Beam splitting and polarization rotation is accomplished with a
custom designed waveguide hybrid coupler and twist. A relative
time lag of approximately 2 ns between the beamsâ waveforms is
introduced using a quasi-optical delay line, followed by spatial
recombination using a selectively reflective wire grid. This delay is
much longer than the approximately 20-ps time-of-flight resolution
of the 30-GHz bandwidth radar, permitting the two beamsâ
reflected signals from a compact target to be easily distinguished
in digital post-processing of the single receiver channel
Scanning the issue: T-ray imaging, sensing, and retection
Copyright © 2007 IEEEDerek Abbott, Xi-Cheng Zhan
Compact duplexing for a 680-GHz radar using a waveguide orthomode transducer
A compact 680-GHz waveguide orthomode transducer (OMT) and circular horn combination has been designed, tested, and characterized in a radar transceiver's duplexer. The duplexing capability is implemented by a hybrid waveguide quasi optical solution, combining a linear polarization OMT and an external grating polarizer. Isolation between the OMT's orthogonal ports' flanges was measured with a vector network analyzer to exceed 33 dB over a >10% bandwidth between 630 and 710 GHz. Calibrated Y-factor measurements using a mixer attached to the OMT ports reveal losses through the transmit and receive paths that sum to an average of 4.7 dB of two-way loss over 660-690 GHz. This is consistent with radar sensitivity measurements comparing the new OMT/horn with a quasi-optical wire grid beam splitter. Moreover, the radar performance assessment validates the OMT as a suitable compact substitute of the wire grid for the JPL's short-range 680-GHz imaging radar
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Remote Detection of Chemicals by Millimeter-Wave Spectroscopy
This paper discusses the development and field testing of a remote chemical detection system that is based on millimeter-wave (mm-wave) spectroscopy. The mm-wave system is a monostatic swept-frequency radar that consists of a mm-wave sweeper, a hot-electron-bolometer detector, and a trihedral reflector. The chemical plume to be detected is situated between the transmitter/detector and the reflector. Millimeter-wave absorption spectra of chemicals in the plume are determined by measuring the swept-frequency radar return signals with and without the plume in the beam path. The problem of pressure broadening, which hampered open-path spectroscopy in the past, has been mitigated in this work by designing a fast sweeping source over a broad frequency range. The heart of the system is a Russian backward-wave oscillator (BWO) tube that can be tuned over 225--315 GHz. A mm-wave sweeper that includes the BWO tube was built to sweep the entire frequency range within 10 ms. The radar system was field-tested at the DOE Nevada Test Site at a standoff distance of 60 m. Methyl chloride was released from a wind tunnel that produced a 2-m diameter plume at its exit point. The mm-wave system detected methyl chloride plumes down to a concentration of 12 ppm. The measurement results agree well with model-fitted data. Remote or standoff sensing of airborne chemicals is gaining importance for arms control and treaty verification, intelligence collection, and environmental monitoring
Sub-millimiter wave receivers
In this article a review is carried out of how
technological aspects are affecting the system configuration of a
sub-millimeterwave receiver for imaging applications.
Throughout the discussion we will emphasize the role that the Schottky diode plays in this type of applications
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