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Study of a New Millimeter-Wave Imaging Scheme Suitable for Fast Personal Screening

December 2012
IEEE Antennas and Wireless Propagation Letters 11:787-790

December 2012
11:787-790

DOI:10.1109/LAWP.2012.2203574

Authors:

Xiang Gao

Beijing Institute of Technology

Chao Li

University of Birmingham

Fang Guangyou

Chinese Academy of Sciences,Institute of Electronics

A new millimeter-wave (MMW) imaging scheme is presented, which has potential applications for fast personal screening. The proposed design employs a special pillbox-like quasi-optics that can generate a spot beam with nearly translational scanning pattern to ensure the image uniformity. The 200-GHz heterodyne transceiver was fabricated based on a microwave vector network analyzer (VNA) to couple the MMW into the quasi-optics and to extract the return wave for imaging. Experimental results based on a prototype system demonstrate the good performance of the new imaging scheme and its capability for concealed threat objects detection.

Novel active millimeter-wave imager. (a) Schematic illustration. (b) Scanning manner.

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Inner structure of the beam-scanning antenna.

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Block diagram of the millimeter-wave transceiver.

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Imaging results with the novel MMW imager.

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Content uploaded by Xiang Gao

Content may be subject to copyright.

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012 787

Study of a New Millimeter-Wave Imaging Scheme

Suitable for Fast Personal Screening

Xiang Gao, Chao Li, Shengming Gu, and Guangyou Fang

Abstract—A new millimeter-wave (MMW) imaging scheme

is presented, which has potential applications for fast personal

screening. The proposed design employs a special pillbox-like

quasi-optics that can generate a spot beam with nearly trans-

lational scanning pattern to ensure the image uniformity. The

200-GHz heterodyne transceiver was fabricated based on a mi-

crowave vector network analyzer (VNA) to couple the MMW

into the quasi-optics and to extract the return wave for imaging.

Experimental results based on a prototype system demonstrate the

good performance of the new imaging scheme and its capability

for concealed threat objects detection.

Index Terms—Image uniformity, millimeter-wave (MMW)

imaging, personal screening, pillbox-like quasi-optics, spot beam.

I. INTRODUCTION

AS AN effective detection instrument, X-ray imager has

been widely applied in lossless detection, security check,

and medical treatment for its high spatial resolution. However,

due to its intrinsic ionization effect harmful to body health [1],

X-ray may not be the best alternative for personal screening.

In comparison, millimeter wave (MMW) is a more secure

choice since the radiation effects of MMW are minor compared

to X-ray. Meanwhile, MMW is not only easy to penetrate

obscuring materials [2] such as clothing like microwave, but

can obtain enough resolution with relatively small antenna

sizes to detect concealed dangerous targets on a person. There-

fore, imaging with MMW plays an important role in personal

screening.

To develop a feasible MMW imager, both imaging speed

and cost of system have to be considered. An effective way

to achieve relatively high frame rates with relatively low

cost is to develop an imaging system with beam-scanning

function [3]–[7]. A novel active MMW imaging scheme with

fast scanning spot-beam is presented in this letter. The small

rotating subreﬂector inside the antenna structure makes it easy

to realize high-speed beam-scanning and imaging. Moreover,

the consistency of the antenna’s beam direction during scan-

ning is optimized based on the combination of a Reversed Ray

Manuscript received January 28, 2012; revised April 01, 2012 and May 18,

2012; accepted June 01, 2012. Date of publication June 08, 2012; date of current

version July 17, 2012. This work was supported by the National Natural Science

Foundation of China under Grants 11174280, 60990323, and 60990320, and the

Knowledge Innovation Program of Chinese Academy of Sciences under Grant

YYYJ-1123.

The authors are with the Key Laboratory of Electromagnetic Radiation and

Sensing Technology, Institute of Electronics, Chinese Academy of Sciences,

Beijing 100190, China (e-mail: cli@mail.ie.ac.cn).

Color versions of one or more of the ﬁgures in this letter are available online

at http://ieeexplore.ieee.org.

Digital Object Identiﬁer 10.1109/LAWP.2012.2203574

Tracing Algorithm (RRTA) and a modiﬁed Physical Optics

Method developed in our previous work [8]. This letter details

the schematic design of the novel MMW imaging system.

Section II describes the details of the design of the beam-scan-

ning antenna, MMW transceiver, and data processing after

acquisition and image processing. In the end, some imaging

results on mannequin and person based on a state-of-proof

system are shown and discussed, which demonstrate the good

performance of the new imaging scheme and its potential

applications in fast personal screening for concealed threat

objects detection.

II. SCHEMATIC DESIGN OF THE NOVEL MILLIMETER-WAV E

IMAGER

A. Imaging Scheme Description

The working schematic diagram of the 200-GHz active

MMW imager is shown in Fig. 1(a). A pillbox-like beam-scan-

ning antenna is employed to transmit the MMW and to receive

the returning wave, with its inner beam-splitter to realize

isolation. By taking advantage of its thin and ﬂat proﬁle, the

antenna is easily installedonaplatformtopanaquicklinescan

along the vertical -direction. Additionally, with its special and

elaborative quasi-optics design, a spot-beam with small size

can be formed with nearly translational scanning pattern in the

horizontal -direction based on the focusing of the concave

main reﬂector and the rotation of the small subreﬂector. The

nearly translational scanning pattern in the -direction avoids

the quality loss resulted from the undesirable effect of image

nonuniformity, which exists in most of the beam-rotating

imaging systems.

Basedonthequ

ick rotation of the subreﬂector (driven by a

servo motor) with a typical speed of 60 cycles per second, the

scanning of the spot-beam in the -direction can be repeated

rapidly with equivalent maximum speed of 120 rows per second

due to the symmetry of the subreﬂector. Synchronously, the pan

scan of the whole box antenna along vertical direction is per-

formed only once during the whole time of imaging. Such a pan

scan can be readily realized based on a lead screw driven by a

servo motor. The resulted scanning trace that is effective to the

imaging is illustrated as the solid lines in Fig. 1(b). A computer

is used to process the acquired data from the MMW transceiver

and communicate with the scanning controller. The imaging

scheme is suitable for conducting fast personal screening in sev-

eral seconds.

Generally, MMW imaging system with fast beam-scanning

may also be implemented with electronic scanning technique

such as the phased array antennas. However, the high cost and

788 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 1. Novel active millimeter-wave imager. (a) Schematic illustration. (b)

Scanning manner.

complexity limit the application of the MMW electronic scan-

ning system as compared to the mechanical scanning alterna-

tives with relatively simple hardware architectures, especially

for 200-GHz or higher bands.

B. Beam-Scanning Antenna

Fig. 2 shows the inner structure of the beam-scanning an-

tenna. It consists of two cylindrical reﬂectors, two identical

pyramid horns, a beam splitter, and an MMW absorber. All

these components are embedded between two parallel metal

plates. The beam splitter is a semi-permeable one to separate

the transmitting and returning wave with a piece of MMW

absorber to avoid unwanted reﬂections. The subreﬂector is

a rotating one with small size, capable of steering the beam

quickly in the horizontal -direction through its fast rotation

around the -axis. One time of beam scanning in the -direction

can be realized by the rotation of subreﬂector within 180 .

Fig. 2. Inner structure of the beam-scanning antenna.

Thanks to the symmetric structure of the subreﬂector, one cycle

rotation of subreﬂector with 360 results in two identical beam

scannings in the -direction, which doubles the speed of the

beam scanning. For personal screening with high resolution, a

ﬁxed concave main reﬂector is employed to focus the reﬂecting

beam from the subreﬂector to a suitable distance away from

the antenna aperture. As long as is small, which is the case

for portal screening applications, the beam leaving the aperture

would hardly diffuse in the -direction. Therefore, a spot-beam

that can scan rapidly along the -direction can be generated

from the pillbox antenna.

To avoid the quality loss resulting from the undesirable effect

of the image nonuniformity that exists in most beam-rotating

imaging systems, the combination of an RRTA and a modiﬁed

Physical Optics Method developed in our previous work [8] was

introduced to optimize the consistency of the antenna’s beam

direction and to minimize the aberration during the beam scan-

ning. As a high-frequency approximation algorithm, the main

concept of the RRTA is to reversely trace the rays emitted from

the focusing points on the imaging plane to ﬁnd the ideal con-

tour of the mirror images of the feed horn with respect to the

subreﬂector. Then, the practical contour of the mirror images

is optimized to ﬁt the ideal contour by optimizing the positions

and dimensions of the subreﬂector and the pyramid horn. In ad-

dition, a modiﬁed Physical Optics based on Discrete Real Mirror

Image Theory (DRMI-PO) [8] helps to efﬁciently analyze and

further optimize the performance of the designed antenna with

special electrically large structures embedded between two par-

allel metal plates.

The ﬁnal designed antenna has a size of

1100 780 60 mm , with 10-mm separation between

parallel plates and an aperture length of 600 mm. The

beam-scanning antenna produces a spot-beam with half-power

beamwidth (HPBW) of 7 10 mm , falling on the imaging

plane at a distance of cm from the antenna aperture. With

the subreﬂector rotating anticlockwise 28.2 , the spot-beam

scansover50cminthe -direction. It should be especially

pointed out that, with RRTA and DRMI-PO, the nearly

GAO et al.: NEW MMW IMAGING SCHEME SUITABLE FOR FAST PERSONAL SCREENING 789

Fig. 3. Block diagram of the millimeter-wave transceiver.

translational scanning pattern in the -direction was achieved

with the beam-direction discrepancy less than 1 within

45 cm scanning range. With such an optimized design, the

discrepancy of the scattering coefﬁcients in different scanning

angles can be reduced within 2-dB level, as computed for a

plane metal object with dimension of resolution unit cell.

C. MMW Transceiver

To achieve high-quality images in the application of fast per-

sonal screening, it is more reasonable to adopt a coherent plan

than the incoherent direct detection for the former one provides

much higher sensitivity. In the design of the novel MMW im-

ager, the heterodyne coherent detection is employed.

Fig. 3 is the block diagram of the MMW transceiver with

heterodyne detection. Both of two two-way Ku-band oscil-

lating sources are provided by a microwave vector network

analyzer (VNA) to realize frequency sweeping with a band-

width of 830 MHz and a step of 4.15 MHz. The two-way radio

frequency (RF) signals and ,withﬁxed frequency

difference of 25 MHz, are both separated into two branches by

two power dividers. A couple of branch signals, coming respec-

tively from and ,aremixedwithaKu-bandmixer.

After downconversion, the 25-MHz differential frequency

output is upconverted into a 300-MHz signal with a 12

multiplier, then via ampliﬁcation circuit and ﬁlter, becoming

the local oscillating (LO) signal for coherent demodulation

inside the VNA. Another branch Ku-band signal from is

directly ampliﬁedandmultipliedby12toobtain200GHz,then

transmittedthroughthehornfeed.Anisolatorisusedtoprevent

the transmitter from being damaged by the unwanted reﬂecting

power in the transmitting link. Another branch Ku-band signal

from ,afterbeingampliﬁed and upconverted to 200 GHz

with a 12 multiplier, is then mixed with the receiving MMW

signal. The 300-MHz output is ampliﬁed and ﬁltered as the

measured intermediate frequency (IF) signal for coherent de-

modulation inside the VNA. The resulted scattering parameters

in MMW band are transferred to a computer for data processing

and image processing.

Fig. 4. Imaging results with the novel MMW imager.

D. Data and Image Processing

The acquired scattering parameter at MMW band is the

function of the MMW signal’s frequency . The range-com-

pressed signal can be obtained using the inverse fast

Fourier transform (IFFT) of .

For the beam-scanning antenna shown in Fig. 2, some small

interference power is also received by the horn, which includes

the coupling between the two horns, scattering from the sub-

reﬂector, and discontinuous reﬂection on the antenna aperture.

All these interference terms can be removed by adding a

proper range window to . The size of the range

window is chosen to cover the extended range of the targets

we concern, such as the whole front surface of a person.

To preferably display a 2-D image, the data to be dis-

played is chosen as the mean power received from each pixel

point within the range window

(1)

In comparison to the single-frequency operation, the combi-

nation of multiple-frequencies scheme with range window has

several advantages. First, the darkness effects resulting from

specular reﬂections by smooth targets may be reduced based on

the averaging from the scattering in different frequencies. More-

over, the noise and the unwanted interference can be reduced by

applying the window in the effective extended range of the tar-

gets in which we are interested.

III. IMAGING RESULTS AND DISCUSSION

To demonstrate the performance of the novel active MMW

imager to detect concealed weapons, some imaging experi-

ments have been carried out for a state-of-proof purpose. Fig. 4

shows the imaging results with the novel active MMW imager.

Fig. 4(a) and (b) is the MMW image and the photograph of a

mannequin wearing a white T-shirt, with a hidden plastic cap

gun that has been shown in Fig. 4(c). As in Fig. 4(e) and (f), a

metal knife is concealed underneath a person’s shirt. Fig. 4(d)

shows the corresponding MMW image of the man concealing

the dangerous weapon.

Because of the limited moving lengths (60 cm max) along

vertical direction of the elevating platform, the images shown in

790 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 4 are made up of imaging results for different parts. In the

experiments, the sample intervals in -and -directions are both

4 mm, and the number of sweep points is set as 201. Consid-

ering the restriction of the VNA’s intermediate frequency band-

width (IFBW), which has been chosen as its maximum value

360 kHz, the total time for a single image is about 66 s. The

measured reﬂecting power from a normal-placed metal knife

reaches 15 dBm. Due to the high sensitivity of the MMW re-

ceiver ( 95 dBm at 1 kHz), the imaging time can be greatly

reduced with the image quality remaining high enough. For ex-

ample, if the IFBW is increased to 10 MHz to greatly reduce the

frequency sweep period, due to the signal-to-noise ratio (SNR)

for the MMW imaging system being in inverse proportion to

the IFBW of the receiver, the noise level of the MMW receiver

will become 55 dBm. As a result, the ﬁnal imaging time can

be reduced to about several seconds with the dynamic range of

the imager about 40 dB, which is enough for the application of

fast personal screening.

IV. CONCLUSION

A new MMW imaging scheme, which employs a special

pillbox-like quasi-optics to generate a spot-beam with nearly

translational scanning pattern, is presented. The design ensures

image uniformity that may not exist in conventional imaging

systems. A state-of-proof system was designed at 200 GHz

based on a microwave VNA. The experimental results demon-

strate the good performance of the new imaging scheme and its

potential application in fast personal screening for concealed

threat objects detection.

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Conference Paper

Jul 2003

An objective of developing intelligent transport systems (ITS) is to enhance driving convenience. As an ITS technology that meets this objective, an adaptive cruise control (ACC) system has already been commercialized. Another objective is to enhance the vehicle safety performance. With the aim of attaining this objective, we have developed a Pre-crash Safety system by evolving the ITS technology used in ACC. The Pre-crash Safety system is designed to predict an unavoidable collision and activate Pre-crash Seatbelt and Pre-crash Brake Assist well in advance of the collision to mitigate occupant injury caused by the collision. A millimeter-wave radar sensor is indispensable to the Pre-crash Safety system. This present paper describes the electronically scanned millimeter-wave radar sensor developed for use in the system. This radar is physically compact while providing high object recognition performance. This is due to the phased array technology employed for the first time in the world for automotive radar. Other features of the radar include a "high-efficiency planar antenna, "high-power and low-loss monolithic microwave ICs (MMICs) and millimeter-wave circuit" and a "high-speed digital-beam-forming (DBF) algorithm".

A Beam-Scanning Dual-Polarized Fan-Beam Antenna Suitable for Millimeter Wavelengths

Article

Sep 2005

The design and testing of a beam-scanning pillbox antenna for use in a 200 GHz imaging system is presented. The antenna features a rotating subreflector that scans the antenna's fan beam and a multimode structure that reduces ohmic loss and allows use of two orthogonal polarizations. The antenna has an aperture size of 450 $,times,$ 5 mm, half-power beam widths of approximately 0.4 $^circ$ and 18 $^circ$ and a beam-scan range of 10 $^circ$ . The gain of the antenna was measured to be greater than 33 dBi for both polarizations. The radiation patterns of the pillbox antenna have been measured in the radiating near field using an outdoor range. Good agreement between measured and calculated patterns has been obtained.