A 300 GHz radar imaging system is presented, including descriptions of the radar sensor and antenna subsystems. The antenna consists of a Bifocal Ellipsoidal Gregorian Reflector whose beam is scanned by a combination of the rotation and vertical tilting of a flat small secondary mirror. A prototype is being mounted and its characterization will be presented

Besada Sanmartin, Jose Luis

García Pino, Antonio

González Valdes, Borja

Grajal de la Fuente, Jesús

Mencia Oliva, Beatriz

Rubiños López, Oscar

Besada Sanmartín, José Luis

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Bifocal reflector antenna system for radar imaging at 300 GHz

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Bifocal Reflector Antenna System for Radar Imaging 
at 300 GHz 
Antonio García-Pino Borja Gonzalez-Valdes Beatriz Mencia-Oliva Jesús Graj al de la Fuente 
Osear Rubiños López and José Luis Besada Sanmartín 
Abstract—A 300 GHz radar imaging system is presented, 
including descriptions of the radar sensor and antenna 
subsystems. The antenna consists of a Bifocal Ellipsoidal 
Gregorian Reflector whose beam is scanned by a combination of 
the rotation and vertical tilting of a flat small secondary mirror. 
A prototype is being mounted and its characterization will be 
presented. 
Index Terms— Reflector antennas, scanning antennas, 
submillimeter-wavelength imaging 
different parts, corresponding to the technologies involved in 
the design (Fig. 1). 
I. INTRODUCTION 
The detection of weapons or contraband concealed on 
persons have been solved by methods that require the sensor 
and the target to be in close proximity, such as trace chemical 
sniffers or X-ray imaging systems. For situations that call for 
standoff observation, concealed weapons detection is 
extremely difficult to accomplish. Active sub-millimeter-wave 
imaging has demonstrated to be a promising technique to 
achieve through-clothing imaging at standoff ranges from 5 to 
100 m. Between 100 GHz and 1 THz, high image resolution of 
about 1cm can be achieved with a compact aperture. Besides, a 
wide range of materials (clothing, paper, plastic, wood ...) is 
reasonably transmissive [1,2] in this band. 
Due to the technological difficulty of having focal plane 
arrays at these frequencies, mechanical fast beam scanning has 
been proposed to interrogate the interest area in a pixel by pixel 
imaging configuration. This communication presents the 
description of the imaging system, including the details of the 
antenna and radar subsystems 
II. RADAR SENSOR AT 300 GHz 
This section presents a continuous-wave linear-frequency 
modulation (CW-LFM) high-resolution radar sensor. The 
operational bandwidth goes from 286.2 to 313.2 GHz, within a 
window of atmospheric attenuation. Sub-cm range resolution is 
attained by the use of 27-GHz bandwidth. This homodyne 
prototype is composed of commercial-off-the-shelf (COTS) 
components and designed-specific circuits. This system is the 
upgrade of a previously reported radar sensor working at 100 
GHz [3,4]. Its architecture, based on frequency-multiplying and 
amplification stages, makes the system easily scalable to higher 
frequencies. The radar structure can be divided into three 
Chirp signal Chirp signal Tx chirp signal 
(15.9-17.4 GHz) (95.4-104.4 GHz) (286.2-313.2 GHz) 
Millimeter-wave subsystem 
Figure 1. Block diagram of the 300-GHz radar sensor 
The digital electronics subsystem includes the chirp signal 
generation by means of a DDS module, which ensures the 
linearity of the transmitted chirp waveform, a digitizer for data 
acquisition, a microcontroller in charge of the initial 
configuration and a conventional PC. The microwave 
subsystem is based on encapsulated MMICs and other 
commercial surface-mount technology (SMT) components, 
assembled on custom-made microstrip technology circuits. The 
chirp signal ramps over 1.5 GHz, from 15.9 to 17.4 GHz. 
Finally, the millimeter-wave subsystem includes both 
transmitter and receiver (Fig. 1). The former is based on a ><6 
frequency multiplier-amplifier - isolator - x3 frequency 
multiplier chain. The latter is based on a subharmonic mixer, 
which is in charge of the demodulation of the received signal. 
The LO input signal (143.1-156.6 GHz) is obtained by 
means of frequency multiplication from the output signal of the 
microwave subsystem. The chirp signal at 286.2-313.2 GHz 
with 0-dBm power level is transmitted through a silicon 
beamsplitter, acting as a circulator to isolate transmitter and 
receiver, towards the antenna subsystem. The reflected signal 
reaches the subharmonic mixer to be demodulated. It should be 
pointed out that this subsystem is based on components 
implemented in waveguide technology since it is beneficial in 
terms of power handling and shock and corrosion resistance. 
Fig. 2 shows (a) the signal generation module as part of the 
digital electronics subsystem and the microwave modules, and 
(b) the transmitter and receiver chains, corresponding to the 
millimeter-wave subsystem. 
Figure 2. Radar sensor: (a) signal generation and microwave modules; (b) 
millimeter-wave subsystem: transmitter (top) and receiver (bottom) chains. 
III. REFLECTOR ANTENNNA SYSTEM 
A General description of the antenna configuration 
The radiating subsystem (tx/rx) has critical impact on the 
design of the radar to meet the aforementioned objective (high 
image resolution and fast image acquisition with a compact 
aperture at a standoff range of 8 m at 300 GHz). Due to the 
technological difficulty of fabricating focal plane arrays at this 
band, a multi-reflector antenna (Fig. 3) based on the rotation of 
a small, flat and lightweight mirror has been proposed [5-6]: 
(a-b) The main reflector and the subreflector are shaped 
surfaces configured as the Bifocal Ellipsoidal Gregorian 
Reflector System (BEGRS) described in [6], in order to 
improve the scanning behavior of the antenna, provide higher 
field of view, increase the antenna gain in the target region and 
allow better control of the scanning operation. In addition, the 
subreflector must be oversized in order to span the field of 
view to the size of the human torso. 
(c) In order to provide scanning characteristics, the illuminating 
wave arrives at the subreflector after being reflected in the 
aforementioned mirror. The fast rotation of this mirror and the 
slow tilting of its rotation axis allow the fast scanning operation 
of the antenna. 
(d) A feed reflector designed to illuminate the subreflector 
with a tapered collimated beam. 
(e) A silicon beamsplitter to separate the transmitted and 
received beams. 
B. BEGRS shaped design 
The baseline configuration of the main reflector and 
subreflector is shown in Fig. 4, which consists of an ellipsoid 
and a paraboloid sharing their axes along z-axis and with a 
common focus O at the origin. The main reflector and 
subreflector surfaces have been reshaped under the bifocal 
concept as described in [6], After developing the iterative 
shaping procedure, which produces a number of points in the 
offset profile of the main reflector and subreflector, rotationally 
symmetric surfaces are defined and represented numerically as 
follows: 
ZM(x,y) = C-A^-^2^ + A0+A1(x2+y2) + A2(x2+y2)2 (1) 
Bl 
Zs(x,y)=B0 + B1(x2 + y2) + B2(x2+y2)2 (2) 
Figure 3. Offset profile of the reflector system 
fm=2c 
FoV 
- Xt, 
Rotating 
H s
° . / . \ h\j\Dsf2si 
mirror / N.^ , / J Ds'2 (p 
Subreflector 
araboloid) 
Tertiary Feed Reflector 
(Paraboloid) 
Figure 4. Baseline antenna configuration: offset plane profile view 
The first two terms of the main reflector surface equation ZiJx) 
describe the original ellipsoid while the last three terms 
represent the surface modification due to the shaping 
procedure. Similarly the subreflector surface, which originally 
is a second order polynomial corresponding to a paraboloidal 
one is modified to a fourth order polynomial for representing 
the shaped surface. The shaped configuration provides a focal 
ring parallel to the XY plane with radius 30 cm around the 
target point. An offset section is then selected to avoid 
blockage problems. 
C. Flat mirror motion for beam scanning 
The mirror surface can be oriented according two axes 
mechanisms. The mirror is tilted an angle ¡3 respect to its ideal 
unscanned orientation to deviate the beam 25 cm upwards in 
the offset plane (which is the horizontal one in the prototype 
picture of Fig. 3). Then, a rapid rotation of the mirror is 
available about an axis which is coincident with the normal 
vector of the mirror for its unscanned orientation. The rotation 
angle is denoted by <j> and the mirror rotation produces an 
elliptical trajectory (due to the offset configuration) of the spot 
in the target plane. The second mechanism is able to tilt the 
rotation axis an angle y up and down (perpendicularly to the 
offset plane) in order to displace the elliptical trajectory 
vertically, allowing the target plane scanning of the spot and 
the vertical extension of the scanning area in the target plane, 
as shown in Fig. 5. Due to the mirror scanning, the subreflector 
surface must be oversized with more emphasis in its vertical 
dimension. 
xt = a0 + axu + a2u + a3v + a4u + a5uv 
y, = b{v + b2uv + b3u 2v + b4 v3 
(4) 
0.1 
1 
(' 
/£• 
^ 
^ 
*~— 
- 0 . 6 - 0 . 5 - 0 . 4 - 0 . 3 - 0 . 2 - 0 . 1 0 0.1 0.2 0.3 0.4 0.5 0.6 
X ( m ) 
Figure 5. Scanning trajectory of the beam 
A center of rotation of the mirror remains in the same location 
while (/> and y mechanisms actuate. A local reference system 
axis is defined at the mirror for angular reference: the mirror z 
axis is coincident with the rotation axis for y=Q, the mirror y 
axis is perpendicular to the offset plane and the local x axis is 
coincident with the tilting axis for the ^motion. The x and y 
components of the normal vector to the mirror are denoted as 
auxiliary u and v variables describing the mirror motion: 
-sin/3 cos0 -sin ycos/7-cos y sin/3 sin^ (3) 
The mapping transformation between the virtual wv-plane and 
the target plane has been predicted by Geometrical Optics and 
approximated by simple polynomials in order to simplify the 
calibration of the system. The polynomial approximation of the 
mapping transformation is as follows: 
Fig. 6 represents the mapping transformation across the target 
plane when a regular 20x20 lattice in the virtual wv-plane is 
considered with -0.1<M<0.1 and -0.2<v<0.2. Both the GO ray 
tracing simulated transformation and the polynomial 
approximation are represented showing almost perfect 
agreement. 
By GO ray tracing 
By polynomial approximation 
T 
Figure 6. Mapping transformation between mirror motion and target plane. 
The target plane lattice is represented for a 20x20 lattice in the virtual uv-
plane with -0.1<w<0.1 and -0.2<v<0.2 
D. Geometrical Optics simulation of the antenna scanning 
performance 
For a helical scanning trajectory as described in Fig. 5, a 
ray tracing procedure has been developed for 720 different 
mirror orientations along 20 0-turns of the mirror. After 
tracing up to 127 rays for each mirror orientation, the mean 
impact point of the bundle of rays on the target plane is an 
estimation of the spot trajectory represented in Fig. 5. The path 
length r.m.s. (PLRMS) of the bundle of rays is also accounted 
as a quality parameter of the scanning because is directly 
related to the phase aberration of the antenna pattern as 
described in [6]. The resulting PLRMS of the antenna vs. the 
radial scan in the target plane is presented in Fig. 7, showing a 
minimum at 0.3 m corresponding to the radius of the focal ring 
provided by the bifocal shaping. This behavior allows 
improving the field of view of the antenna as mentioned in [6]. 
: .:Í 
:."Í 
: . : Í 
r ^ 
A y 
n i r 
: 0.3 
radial sean (m) 
Figure 7. Path length r.m.s. of the antenna by GO simulation 
E. Physical Optics simulation of the antenna scanning 
performance 
The Physical Optics performance of the antenna has been 
simulated, based on [7]. Fig. 8 describes the antenna behavior 
for eight points of the central trajectory (^0°) showing the 
spot beams, their amplitude and beamwidth. Fig. 9 shows a 
similar result for a displaced trajectory. In all cases the 
beamwidth of the antenna spot remains in reasonable values. 
Ü 
0.2 
-> 
(b) 
E field (dB) 
- - • Beamwidth (cm 
i i 
2 
1.9 
1.6 
1 
Ü 
- ? 
(b) 
^ / 
r—"^ 
„ - - -
E field (dB) 
- - • Beamwidth (cnf 
Figure 9. 
90 135 180 225 270 
Rotation Angle (deg) 
PO simulation for a ^-displaced elliptical trajectory of the spot: (a) 
scanned beams and (b) Relative E-field and beamwidth 
IV. CONCLUSION 
An imaging radar system at 300 GHz has been developed 
and constructed. The radar and antenna subsystems have been 
described, showing images of the prototype. The antenna 
configuration consists of a confocal reflector system shaped 
under the bifocal criterion for improving of the field of view. A 
two axis rotating mirror allows the scanning operation of the 
imaging radar by implementing an helical trajectory across the 
target plane. The mapping transformation between the angular 
motion of the mirror and the beam spot across the target plane 
has been predicted as well as the antenna performance under 
GO and PO methods. Now the prototype is being calibrated 
and tested and preliminary experimental results are expected 
soon. 
ACKNOWLEDGMENT 
This work is partially supported by the Spanish National 
Research and Development Program under project TEC2011-
28683-C02-01/02 and Terasense CSD2008-00068 (Consolider-
Ingenio2010). 
Rotation Angle (deg) 
Figure 8. PO simulation for a centered elliptical trajectory of the spot: (a) 
scanned beams and (b) Relative E-field and beamwidth 
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Bifocal reflector antenna system for radar imaging at 300 GHz

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