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Design and testing of an active 190-GHz millimeter-wave imager

[+] Author Affiliations
Greg P. Timms, Michael L. Brothers, John D. Bunton, John W. Archer, Grahame C. Rosolen, Yue Li, Andrew D. Hellicar, Juan Y. Tello, Stuart G. Hay

Commonwealth Scientific and Industrial Research Organisation, Information and Communication Technologies Centre, P.O. Box 76, Epping, New South Wales 1710, Australia

J. Electron. Imaging. 19(4), 043019 (December 29, 2010). doi:10.1117/1.3514744
History: Received April 01, 2008; Revised August 18, 2010; Accepted September 27, 2010; Published December 29, 2010; Online December 29, 2010
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The design and testing of an active 190-GHz imaging system is presented. The system features two beam-scanning antennas, one of which transmits a vertical fan beam, and the other which receives a horizontal fan beam. By correlating the transmitted and received signals, an output is obtained that is proportional to the millimeter-wave reflectivity at the intersection of the two fan beams. Beam scanning is obtained by rotating a small subreflector within each antenna, allowing rapid scanning. The system has an angular resolution of 0.3 deg, a field of view of 14×14 deg, and operates at a standoff distance of 5 m.

Figures in this Article

The last ten years have seen increasing interest in millimeter-wave imaging systems,19 generally operating around 94 GHz. These imaging systems have less spatial resolution than optical or infrared systems, as a consequence of the much longer wavelength; however, they have an advantage over optical and infrared sensors in their ability to see through clouds, fog, smoke, and clothing materials. Most existing systems operate in a passive mode, relying on blackbody radiation emitted and reflected by objects in the scene to produce an image; however, there has also been some work on active systems89 that use a millimeter-wave source to illuminate the scene. These two methods are analogous to taking an optical photograph without or with a flash.

For both active and passive systems, three main approaches have generally been used to acquire the image. In the first, a large array of millimeter-wave receivers is placed in the focal plane of a lens.56 This allows high frame rates to be achieved because the entire image is acquired at once. The second approach is to build up an image over a 2-D plane by mechanically scanning an antenna with a single detector,2 greatly reducing the cost of the system. Both of these approaches have shortcomings; the high cost of millimeter-wave detectors makes the cost of focal-plane-array systems prohibitively high for most applications, and the long scanning time in the scanned-antenna approach results in very low frame rates, ruling out potential applications such as aircraft landing in fog or security screening of airline passengers. The third approach offers a compromise, with a subarray being mechanically scanned to build up the image.3 The subarray typically contains on the order of 20 detectors, meaning that cost remains an issue in these systems, although with improved processing techniques, improved chip yields, and the burgeoning market for automotive radars at nearby frequencies, the cost of detectors is beginning to fall.

An alternative approach is to use two orthogonally oriented scanning fan-beam antennas, each of which scans in one dimension. An image is built up by scanning both antennas so that the intersection of the two beams moves around a 2-D plane. The major advantage of this approach is that only two detectors are required. Cross-correlation of the output of the two detectors provides scene information from the intersection of the two antenna beams.

This fan-beam approach can be used as both an active and passive imager. When operating this system as an active imager, an alternative illumination strategy is possible. Here the scene is illuminated with one of the fan beams; the second fan beam then receives the field reflected by the scene. The transmitted and received signals are then cross-correlated to build the image.

This work details the design and testing of the active cross-correlating imager system. The system design generates unique requirements on antenna design, image generation, and receiver hardware. Antenna design aspects have been covered in Refs. 10 and 11, which discussed a shaped two-reflector design capable of scanning by rotating a small subreflector. Theoretical imaging aspects of the passive form of the system were covered in Ref. 13, and analyses of the system's point spread function and beam patterns were given in Ref. 14. The receiver hardware was discussed in Ref. 15. The system is covered by an international patent application.16 Beyond the design, this work is concerned with results generated by the whole system. Work presented in Refs. 17 and 18 is extended by discussing the theoretical performance of active imaging systems, and more specifically, the performance of the presented imaging system. Additional results are presented, and these results are now analyzed and compared with theoretical expectations.

The work is organized as follows. Section 2 provides the background and outlines the theory behind active imaging at millimeter wavelengths. The imager design is described in Sec. 3, the experimental setup in Sec. 4, and measurements are compared with theoretical estimates of system performance in Sec. 5.

Passive millimeter wave imaging has a number of advantages (particularly with respect to covertness, licensing, and public acceptance); however, the blackbody power emitted at these wavelengths is very small. For example, a 1 K temperature difference on the surface of an object corresponds to a difference in emitted power (over the band 190 to 198 GHz) of approximately 1.1 μW/m2. The low powers mean that relatively long integration times need to be used for each detector, and that fast frame rates can only be achieved by using significant numbers of detectors.

In active imaging,89 the situation is slightly different. The power of the source used to illuminate the scene is much greater than the blackbody powers emitted by objects in the scene, and the overwhelming contribution to the received signal from a pixel is the millimeter-wave reflectivity of the object in that pixel. Let us consider the active imaging scenario shown in Fig. 1. A power Pt is transmitted via an antenna with gain Gt. At a distance d meters, it strikes an object within the current pixel that has a bistatic radar scattering cross section σtr. The reflected signal is received by a second antenna, with gain Gr. The received power Pr is given by the radar equation: Display Formula

1Pr=PtGtArσtr(4πd2)2,
where Ar = Grλ2/4π is the effective area of the receive antenna.

Graphic Jump LocationF1 :

Key components of an active millimeter-wave imager.

The bistatic radar cross section σtr is dependent on the reflectivity, shape, and orientation of the object with respect to the transmitter and receiver. The received signal will be dependent on the bistatic radar cross section weighted over the angular extent of the source and receiver as viewed from the target. If the source is incoherent and physically large and the receiver aperture is large, then the received signal is only weakly dependent on target orientation, and the active approach approximates a passive approach, but with the surroundings at very high temperatures. This results in less orientation-induced variation in the temperature of the object and the temperature of the surroundings, and hence a much greater contrast within the image.

If the source is a point source or physically small and the receiver is small, the orientation of the object being imaged needs to be considered. An object oriented so that the incoming radiation reflects directly toward the receiver will produce large (specularly reflected) signals when compared to objects with other orientations. A number of techniques can be used to alleviate this effect. These include: 1. using multiple sources, 2. rotating the object being imaged or rotating the imager,19 and/or 3. compressing the dynamic range of the image so that the specular reflections do not dominate the processed image.

One additional advantage of active imaging with a physically small source is the potential to obtain range information in the scene. By transmitting a known signal (for example noise or a chirped signal) and utilizing a multilag correlator, the time of flight of the transmitted signal can be estimated, and this allows the distance of the object to be calculated.

A schematic of the imaging system is shown in Fig. 2. Millimeter-wave radiation was generated by a frequency-doubled Gunn oscillator, which was chirped from 93.425 to 93.55 GHz (the output of the frequency doubler varied from 186.85 to 187.10 GHz) and produced approximately 2 mW of cw power at 187 GHz. The 250 MHz of bandwidth allowed the time of flight to be estimated to within 4 ns, and hence distance to be estimated to within 0.6 m. A fraction of the 187-GHz signal was passed to a downconverting module, with the remainder fed to the horizontal pillbox antenna, which had a gain of 35 dBi. This antenna produced a vertical fan beam focused at a distance of 5 m from the imager. The second (vertical) antenna received radiation from a horizontal fan beam and passed it to a second downconverting module. The IF signals from the two downconverters were cross-correlated and a voltage was produced that was proportional to the magnitude of the reflected radiation at the intersection of the two fan beams. Using this approach, it was possible to cover a 2-D plane by scanning the beam from each antenna in one direction.1011 The two antennas were arranged in a T formation, with phase centers separated by approximately 277 mm. Because of the relatively close proximity of the antennas when the system is viewing targets at distances of five meters or greater, the system approximates a monostatic radar, and backscatter radar cross sections can be used in calculating the predicted returns from targets.

Graphic Jump LocationF2 :

Schematic showing the operation of the cross-correlating imager.

The antennas were constructed from dimensionally stable aluminium (MIC-6) to minimize warping of the parallel plates. This was critical, since the subreflector thickness of 4.7 mm allowed only 150-μm clearance on either side of the subreflector. The final dimensions of the pillbox antenna were approximately 700×600×50 mm with an aperture of 490×5 mm.

The integrated downconverter modules15 were assembled in-house using components designed at the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO, Epping, Australia) and manufactured by Northrop Grumman Space Technology (NGST, Redondo Beach, California). They consist of two InP low noise amplifiers and an InP subharmonically pumped mixer, which make use of the fourth harmonic of the local oscillator (LO).

The LO was an 11.75-GHz dielectric resonator oscillator multiplied four times to a frequency of 47 GHz (corresponding to a fourth harmonic frequency of 188 GHz at the mixer). The LO power was boosted through a GaAs amplifier before being passed to the mixer.

The intermediate frequency (IF) underwent 60 dB of amplification and went through 0.5-GHz high-pass and 8-GHz low-pass filters, followed by a bandpass filter with a central frequency of 1.067 GHz and bandwidth of 260 MHz. A second IF stage was used to demodulate the signals down to 25 MHz, each with in-phase and quadrature components. The filtered signals were passed through 250 Msample/s analog-to-digital Converters (ADCs) and fed into a field programmable gate array (FPGA) based multilag correlator. Figure 3 is a photograph of the assembled demonstrator.

Graphic Jump LocationF3 :

Photograph of the cross-correlating imager.

Once the instrument-specific parameters are inserted into Eq. 1, it reduces to: Display Formula

2Pr=2.08×105σtr,
with Pr in mW and σtr in m2. The parameters used to derive this expression were: d = 5 m, Pt = 1 mW (assuming ohmic losses of 3 dB in the transmit feed and antenna), Gt = Gr = 35 dBi = 3162 (linear), and Ar = 6.48×10–4 m2. The noise level of our receiver system at input was calculated from the measured system temperature Tsys of 35 279 K via:Display Formula
3Pn=kBT sys ,
where k is Boltzmann's constant and B is the system bandwidth.

The minimum detectable power is given by: Display Formula

4ΔP min =PnBτ,
and τ is the integration time per pixel. For an integration time of 8 ms and a bandwidth of 250 MHz, the minimum detectable power is calculated to be –100.7 dBm (or 0.086 pW). Hence, the expected signal-to-noise ratio (SNR) at a distance of five meters was equal to 2.41×105σtr for a 4-ms integration time; the expected SNR was 1.70×105σtr.

Experimental tests of the imaging system were performed both outdoors and in CSIRO's near-field anechoic chamber, with targets generally placed at a distance of 5 m from the imager. While the initial tests were conducted at 187 GHz, the transmitter and detectors have the capability to operate over the 180- to 210-GHz range.

Figure 4 is a photograph of a convex metal hemisphere supported by wires. The hemisphere had a diameter of 5 cm, a calculated backscatter cross section of 1.96×10–3 m2, and an expected SNR of 333 for a pixel integration time of 4 ms. An approximate 3 cm length of 1-mm-diam wire could be found in a given pixel and, assuming that the axis of the cylinder is perpendicular to the direction of the incoming radiation, the estimated radar cross section of the wire is 1.80×10–3 m2 and the expected SNR is 306. The measured data showed a SNR of 44 for the hemisphere and a SNR of less than 13 for the wires. In practice, the incoming radiation would not be precisely normal to the wire, and as the cross section is highly dependent on this angle, this would explain the low observed SNRs.

Graphic Jump LocationF4 :

Photograph and active 187-GHz image of a metal hemisphere suspended by wires.

Figure 5 is a 187-GHz image of one of the authors without any concealed objects. Figure 5 is the 187-GHz image, with an integration time of 8 ms per pixel, superimposed on the photograph of the scene. Despite image compression and bilinear interpolation, evidence of specular reflection is still present on the image. Bright spots are seen on the subject's left arm, chest, and right waist. The maximum SNR of 147 was achieved for the bright reflection in the center of the chest. The SNR of the received signal varied over the body, head, and arms but was typically 6 or 7.

Graphic Jump LocationF5 :

(a) Composite photograph with superimposed 187-GHz image and (b) active 187-GHz image of a subject with no concealed objects.

Figure 6 is a repeat of Fig. 5 with a metallic knife concealed beneath the subject's shirt. The location of the knife is clearly visible in the 187-GHz image, although it should be noted that the clarity of the knife is highly dependent on its orientation. The peak SNR of reflections from the knife was 195, with values above 80 being obtained over a 7.5×12.5-cm area. The radar cross section of a 2.5×2.5-cm section of flat plate at this frequency is 1.99 m2. Hence, we should expect a maximum SNR of approximately 4.80 × 105. However, as noted before, the cross section of a flat plate is highly dependent on orientation, and a very small change in angle could be responsible for the discrepancy between the predicted and observed SNRs.

Graphic Jump LocationF6 :

(a) Composite photograph with superimposed 187-GHz image and (b) active 187-GHz image of a subject with a concealed metallic knife.

The knife signal level in Fig. 6 is sensitive to the orientation of the knife. This sensitivity is of interest when detecting concealed weapons, because it is important to know when the knife will appear in the image. Figure 7 shows an experimental setup where the knife used in Fig. 6 is mounted on a rotator to allow for radar return measurements. The knife was rotated around its longer and shorter axes. The shorter axis is oriented horizontally in Fig. 7 and is the axis around which rotation occurs in this setup. Images of the scene were completed for each rotation, and an example is shown in Fig. 7 (bottom). The pixel containing the maximum value in each image did not change location during the experiment, and the value of this pixel is plotted in Fig. 8. As can be seen, the knife exhibits a half power beam width of approximately 5 deg. The difference between the patterns in Fig. 8 is likely attributable to the asymmetry in the knife. When rotated around the knife's long axis, there is little variation in the size of the knife's surface intercepted by the beam. Whereas when rotation occurs around the knife's short axis, the location of specular reflection will move along the long axis of the knife; along this axis the knife width narrows (toward the tip) and hence the radar return will be asymmetrical along the length. To interpret the result, the oblique monostatic radar cross section (RCS) of a flat plate was calculated using a physical optics approximation.20 The results are plotted in Fig. 9 for two different plate sizes. The flat plate corresponding to the 2.5×2.5-cm pixel size of the imaging system is found to have a narrower beam width than measured; the 1×1-cm flat plate seems a better model for the knife's interaction with the beam. The side lobes in the RCS for the flat plates are due to the plane wave illumination used in the RCS calculation. The measured beam width of the knife may be attributable to the curvature on the knife surface, which would have the effect of broadening the radar return.

Graphic Jump LocationF7 :

(Top) Photograph of a metallic knife mounted for rotation around the knife's short (S) axis, and (bottom) active 187-GHz image of mounted metallic knife.

Graphic Jump LocationF8 :

Signal power return from knife as it is rotated around its short (S) and long (L) axis normalized to maximum signal power.

Graphic Jump LocationF9 :

Normalized signal power return from knife with long (L) axis rotation against theoretical normalized monostatic RCS for two different sized plates.

The range-measurement capability of the system is demonstrated in Fig. 10, which shows a photograph [Fig. 10] and millimeter-wave image [Fig. 10] for a number of corner reflectors at different distances from the imager. The two side corner reflectors were located 20 m from the imager, while the front corner reflector was only 15 m from the imager. Figure 10 shows the lag of the multilag correlator in which the peak signal occurred. A peak in lag 1 indicates the object is located between 12.4 and 13.0 m from the imager, a peak in lag 2 indicates the object is between 13.0 and 13.6 m from the imager, etc. The two side reflectors are seen in lag 12 (19.6 to 20.2 m from the imager), and the front reflector is seen in lag 4 (14.8 to 15.4 m from the imager), both in good agreement with the measured distances of 20 and 15 m, respectively.

Graphic Jump LocationF10 :

Image of three reflectors (side reflectors at 20 m, front reflector at 15 m). (a) photograph of scene, (b) 187-GHz image, and (c) distance information.

A cross-correlating millimeter-wave imager operating at a frequency of 187 GHz is designed and tested. The system has an angular resolution of 0.3 deg and a field of view of 14×14 deg, and is used to obtain images of a subject with a concealed weapon, as well as to obtain a pseudo-3-D image of a scene (a 2-D image with range information to objects in the image).

Future work is needed to reduce the effect of object orientation on the images obtained.

The authors would like to thank K. Smart, F. Ceccato, and S. Barker for their assistance with the measurement program, S. L. Smith for her major contribution to the testing of the scanning pillbox antennas, and R. Forsyth and C. Holmesby for their assistance with mounting and aligning the system.

Yujiri  L., , Agravante  H., , Biedenbender  M., , Dow  G. S., , Flannery  M., , Fornaca  S., , Hauss  B., , Johnson  R., , Kuroda  R., , Jordan  K., , Lee  P., , Lo  D., , Quon  B., , Rowe  A., , Samec  T., , Shoucri  M., , Yokoyama  K., , and Yun  J., “ Passive millimeter-wave camera. ,” Proc. SPIE. 3064, , 15–22  ((1997)).
Smith  R., , Sundstrom  B., , and Belcher  B., “ Radiometric one second camera (ROSCAM) airborne evaluation. ,” Proc. SPIE. 3703, , 2–12  ((1999)).
Appleby  R., , Anderton  R. N., , Price  S., , Salmon  N. A., , Sinclair  G. N., , Borrill  J. R., , Coward  P. R., , Papakosta  P., , Lettington  A. H., , and Robertson  D. A., “ Compact real-time (video rate) passive millimeter-wave imager. ,” Proc. SPIE. 3703, , 13–19  ((1999)).
Ferris  D. D.  Jr., and Currie  N. C., “ Overview of current technology in MMW radiometric sensors for law enforcement applications. ,” Proc. SPIE. 4032, , 61–71  ((2000)).
Moffa  P., , Yujiri  L., , Jordan  K., , Chu  R., , Agravante  H., , and Fornaca  S., “ Passive millimeter wave camera flight tests. ,” Proc. SPIE. 4032, , 14–21  ((2000)).
Martin  C., , Lovberg  J., , Clark  S., , and Galliano  J., “ Real time passive millimeter-wave imaging from a helicopter platform. ,” Proc. SPIE. , 4032, , 22–28  ((2000)).
Yujiri  L., , Shoucri  M., , and Moffa  P., “ Passive millimeter-wave imaging. ,” IEEE Microwave Mag.. 4, , 39–50  ((2003)).
Sheen  D. M., , McMakin  D. L., , and Hall  T. E., “ Cylindrical millimeter-wave imaging technique and applications. ,” Proc. SPIE. 6211, , 62110A  ((2006)).
Grossman  E. N., and Miller  A. J., “ Active millimeter-wave imaging for concealed weapons detection. ,” Proc. SPIE. 5077, , 62–70  ((2003)).
Hay  S. G., , Archer  J. W., , Timms  G. P., , and Smith  S. L., “ A beam-scanning dual-polarized fan-beam antenna suitable for millimeter wavelengths. ,” IEEE Trans. Antennas Propagation. AP-53, , 2516–2524  ((2005)).
Hay  S. G., “ Multibeam-reflector approach to beam-scanning feed for pillbox antenna. ,” 9th Aust. Symp. Antennas. , pp. 16 ((2005)).
Li  Y., , Archer  J., , Rosolen  G., , Hay  S., , Timms  G., , and Guo  Y. J., “ Fringe management for a T-shaped millimeter-wave imaging system. ,” IEEE Trans. Microwave Theory Tech.. 55, (6 ), 1246–1254  (June (2007)).
Li  Y., , Archer  J., , Tello  J., , Rosolen  G., , Ceccato  F., , Hay  S., , Hellicar  A., , and Guo  Y., “ Performance evaluation of a passive millimeter-wave imager. ,” IEEE Trans. Microwave Theory Tech.. 57, (10 ), 2391–2405  ((2009)).
Archer  J. W., and Shen  M. G., “ 176–200 GHz receiver module using indium phosphide and gallium arsenide MMICs. ,” Microwave Opt. Technol. Lett.. 43, (6 ), 458–462  ((2004)).
Archer  J. W., , Sevimli  O., , and James  J. C., “ Real-time cross-correlating millimetre-wave imaging system. ,” PCT publication date: 22 January (2004); provisional patent filed: 11 July 2002.
Timms  G. P., , Bunton  J. D., , Brothers  M. L., , and Archer  J. W., “ 190 GHz millimetre-wave imaging using MMIC-based heterodyne receivers. ,” in 2nd Int. Conf. on Wireless Broadband and Ultra Wideband Communications (AusWireless 2007), 27–30 Aug. 2007, pp. 32–32  ((2007)).
Brothers  M. L., , Timms  G. P., , Bunton  J. D., , Archer  J. W., , Tello  J. Y., , Rosolen  G. C., , Li  Y., , and Hellicar  A. D., “ A 190 GHz active millimeter-wave imager. ,” Proc. SPIE. 6548, , 654804  ((2007)).
Sinclair  G. N., , Anderton  R. N., , and Appleby  R., “ Outdoor passive millimeter wave security screening. ,” IEEE 35th Intl. Carnahan Conf. Security Technol.. , pp. 172–179  ((2001)).
Balanis  C.,  Advanced Engineering Electromagnetics. ,  Wiley ,  New York  ((1989)).
© 2010 SPIE and IS&T

Citation

Greg P. Timms ; Michael L. Brothers ; John D. Bunton ; John W. Archer ; Grahame C. Rosolen, et al.
"Design and testing of an active 190-GHz millimeter-wave imager", J. Electron. Imaging. 19(4), 043019 (December 29, 2010). ; http://dx.doi.org/10.1117/1.3514744


Figures

Graphic Jump LocationF1 :

Key components of an active millimeter-wave imager.

Graphic Jump LocationF2 :

Schematic showing the operation of the cross-correlating imager.

Graphic Jump LocationF3 :

Photograph of the cross-correlating imager.

Graphic Jump LocationF4 :

Photograph and active 187-GHz image of a metal hemisphere suspended by wires.

Graphic Jump LocationF5 :

(a) Composite photograph with superimposed 187-GHz image and (b) active 187-GHz image of a subject with no concealed objects.

Graphic Jump LocationF6 :

(a) Composite photograph with superimposed 187-GHz image and (b) active 187-GHz image of a subject with a concealed metallic knife.

Graphic Jump LocationF7 :

(Top) Photograph of a metallic knife mounted for rotation around the knife's short (S) axis, and (bottom) active 187-GHz image of mounted metallic knife.

Graphic Jump LocationF8 :

Signal power return from knife as it is rotated around its short (S) and long (L) axis normalized to maximum signal power.

Graphic Jump LocationF9 :

Normalized signal power return from knife with long (L) axis rotation against theoretical normalized monostatic RCS for two different sized plates.

Graphic Jump LocationF10 :

Image of three reflectors (side reflectors at 20 m, front reflector at 15 m). (a) photograph of scene, (b) 187-GHz image, and (c) distance information.

Tables

References

Yujiri  L., , Agravante  H., , Biedenbender  M., , Dow  G. S., , Flannery  M., , Fornaca  S., , Hauss  B., , Johnson  R., , Kuroda  R., , Jordan  K., , Lee  P., , Lo  D., , Quon  B., , Rowe  A., , Samec  T., , Shoucri  M., , Yokoyama  K., , and Yun  J., “ Passive millimeter-wave camera. ,” Proc. SPIE. 3064, , 15–22  ((1997)).
Smith  R., , Sundstrom  B., , and Belcher  B., “ Radiometric one second camera (ROSCAM) airborne evaluation. ,” Proc. SPIE. 3703, , 2–12  ((1999)).
Appleby  R., , Anderton  R. N., , Price  S., , Salmon  N. A., , Sinclair  G. N., , Borrill  J. R., , Coward  P. R., , Papakosta  P., , Lettington  A. H., , and Robertson  D. A., “ Compact real-time (video rate) passive millimeter-wave imager. ,” Proc. SPIE. 3703, , 13–19  ((1999)).
Ferris  D. D.  Jr., and Currie  N. C., “ Overview of current technology in MMW radiometric sensors for law enforcement applications. ,” Proc. SPIE. 4032, , 61–71  ((2000)).
Moffa  P., , Yujiri  L., , Jordan  K., , Chu  R., , Agravante  H., , and Fornaca  S., “ Passive millimeter wave camera flight tests. ,” Proc. SPIE. 4032, , 14–21  ((2000)).
Martin  C., , Lovberg  J., , Clark  S., , and Galliano  J., “ Real time passive millimeter-wave imaging from a helicopter platform. ,” Proc. SPIE. , 4032, , 22–28  ((2000)).
Yujiri  L., , Shoucri  M., , and Moffa  P., “ Passive millimeter-wave imaging. ,” IEEE Microwave Mag.. 4, , 39–50  ((2003)).
Sheen  D. M., , McMakin  D. L., , and Hall  T. E., “ Cylindrical millimeter-wave imaging technique and applications. ,” Proc. SPIE. 6211, , 62110A  ((2006)).
Grossman  E. N., and Miller  A. J., “ Active millimeter-wave imaging for concealed weapons detection. ,” Proc. SPIE. 5077, , 62–70  ((2003)).
Hay  S. G., , Archer  J. W., , Timms  G. P., , and Smith  S. L., “ A beam-scanning dual-polarized fan-beam antenna suitable for millimeter wavelengths. ,” IEEE Trans. Antennas Propagation. AP-53, , 2516–2524  ((2005)).
Hay  S. G., “ Multibeam-reflector approach to beam-scanning feed for pillbox antenna. ,” 9th Aust. Symp. Antennas. , pp. 16 ((2005)).
Li  Y., , Archer  J., , Rosolen  G., , Hay  S., , Timms  G., , and Guo  Y. J., “ Fringe management for a T-shaped millimeter-wave imaging system. ,” IEEE Trans. Microwave Theory Tech.. 55, (6 ), 1246–1254  (June (2007)).
Li  Y., , Archer  J., , Tello  J., , Rosolen  G., , Ceccato  F., , Hay  S., , Hellicar  A., , and Guo  Y., “ Performance evaluation of a passive millimeter-wave imager. ,” IEEE Trans. Microwave Theory Tech.. 57, (10 ), 2391–2405  ((2009)).
Archer  J. W., and Shen  M. G., “ 176–200 GHz receiver module using indium phosphide and gallium arsenide MMICs. ,” Microwave Opt. Technol. Lett.. 43, (6 ), 458–462  ((2004)).
Archer  J. W., , Sevimli  O., , and James  J. C., “ Real-time cross-correlating millimetre-wave imaging system. ,” PCT publication date: 22 January (2004); provisional patent filed: 11 July 2002.
Timms  G. P., , Bunton  J. D., , Brothers  M. L., , and Archer  J. W., “ 190 GHz millimetre-wave imaging using MMIC-based heterodyne receivers. ,” in 2nd Int. Conf. on Wireless Broadband and Ultra Wideband Communications (AusWireless 2007), 27–30 Aug. 2007, pp. 32–32  ((2007)).
Brothers  M. L., , Timms  G. P., , Bunton  J. D., , Archer  J. W., , Tello  J. Y., , Rosolen  G. C., , Li  Y., , and Hellicar  A. D., “ A 190 GHz active millimeter-wave imager. ,” Proc. SPIE. 6548, , 654804  ((2007)).
Sinclair  G. N., , Anderton  R. N., , and Appleby  R., “ Outdoor passive millimeter wave security screening. ,” IEEE 35th Intl. Carnahan Conf. Security Technol.. , pp. 172–179  ((2001)).
Balanis  C.,  Advanced Engineering Electromagnetics. ,  Wiley ,  New York  ((1989)).

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