nas for Millimetre-Wave Asset Tracking
Integrated Antennas for Millimetre-Wave Asset Tracking
V.F. Fusco, D. Salameh, T.Brabetz
High Frequency Electronics Laboratories
School of Electrical and Electronic Engineering
The Queens University of Belfast
Ashby Building Stranmillis Rd
Belfast BT9 5AH
N.Ireland
Ph: 44 1232 274087
Fax: 44 1232 667023
Email:
v.fusco@ee.qub.ac.uk
Web: http://www.ee.qub.ac.uk/hfe
Abstract
A soft board-board millimetre wave subarrayed Van Atta
antenna has been designed fabricated and measured for
asset tracking applications at 62GHz. Soft-board, 254
µ
m
thickness and 2.19 relative permittivity, single and array
patch antennas were designed and characterised. The
elements and arrays are designed to be connected via
microstrip to WR-15 waveguide transitions. Measurements
of insertion loss for the transition have been performed and
the transition used to allow patch array and subarray
absolute gain values to be computed. The performance of
the Van Atta Array is also given and peculiarities of its
performance at mm-wave frequencies described. The
characteristics of a millimetre wave integrated Silicon
antenna are discussed also. The frequencies chosen for this
work are 62.5 and 65.5 GHz. These are transmit and receive
frequencies
allocated
for
broadband
Mobile
wireless
applications [1].
Silicon Antennas
Series silicon antenna arrays were designed using 50 half-wavelength dipoles separated by half-wavelength 50

lines metallized using 1.5
µ
m aluminium on patterned
SiO
2
, placed on 10 k cm Silicon, r=11.9, Figure 1. The
dipole antenna was matched using a /4 transformer.
Coplanar pads were designed to probe the antenna with 150
µ
m. The ground pads are connected to a large metallization,
0.1 impedance, in order to act as a ground plane. This
type of grounding occupies large area but eliminates the
need for thru-hole via fabrication.
Figure 1a shows the measured and simulated reflection loss
of the silicon dipole array (series configuration). The
measured resonant frequency is 65.6 GHz with -22.2 dB
reflection loss compared with 65.5 GHz designed frequency.
The measured resonant bandwidth is wider than the
simulated bandwidth for both resonant frequencies due to
dielectric and metallization losses i.e. reduced Q. After
extraction of bondwire and antipodal finline losses, the
antenna gain was found to be 8 dBi.
-30
-25
-20
-15
-10
-5
0
55
57
59
61
63
65
67
69
71
73
75
Frequency (GHz)
S
1
1
(
d
B
)
measured
simulated
65.5 GHz
65.6 GHz
-22.2 dB
0
1
2
3
4
5
6
7
8
mm
mm
65.5 GHz
(a) Series Array
-30
-25
-20
-15
-10
-5
0
-90
-65 -40
-15
10
35
60
85
Normalized RX
Power
(b) Radiation characteristic
Figure 1
Silicon half-wavelength dipole antenna array The radiation
characteristic for this antenna is squinted,
this is thought to be due to the close proximity of the
waveguide step with respect to the feedline of the antenna,
combined with secondary radiation from the pen waveguide
face.
Design of Softboard Array
Next a single patch antenna and then a 2X2 softboard
microstrip patch array were designed and fabricated. For
each patch the input impedance was 90

in order to
facilitate matching in the subarray to the 50

impedance to
be used in the Van Atta array to be described below.
The single patch element return loss response and that of
the 2X2 array is illustrated in Figure 2. From Figure 2 it
can be seen that the return loss for the 2X2 array is 10dB
at 65.1GHz while for the patch it is also 10 dB.
Single
Array
Figure 2
Softboard Microstrip Patch and
Array Return Loss
A measured typical radiation pattern is shown in Figure 3
for the patch element and the 2X2 array. The theoretical
beam width of a 2
×
2 planar patch array separated by 0
/ 2
edge to edge is 27º (measured 25º ) for = 0 with -8.5 dB
side lobe level (measured 7dB). the measured pattern for the
2X2 array exhibits 9º squint from broadside which is
thought to be due to the proximity of the horizontal feed
connection shown in Figure 3(b).
The measured gain for a single 62GHz microstrip patch on
Taconic material is 2.4dBi, while the 4 element patch array
gives around 9dBi gain. The 65 GHz silicon dipole array
gives around 4.5dBi gain. The lower than expected gain of
the silicon array is thought to be due to its increased
dielectric and metallization losses. A single GaAs patch
antenna (840
µ
m X 617
µ
m) designed for use at 65GHz
yielded a measured return loss of 9.4dB with 2.6dBi gain.
An array of GaAs elements is too expensive in area
requirement hence the need for the softboard and Silicon
alternatives discussed in this paper.
V Band Patch Array Gain at 62 GHz
-25
-20
-15
-10
-5
0
5
10
15
-100
-50
0
50
100
Scan Angle (degrees)
G
a
i
n
(
d
B
i
)
1 Ele Patch Gain (62 GHz)
4 Ele Patch Gain (62 GHz)
(a) Far field patterns
0
1
2
3
4
5
6
7
8
9
mm
mm
62.5 GHz
(b) physical circuit 2x2 subarray
Figure 3
Softboard radiation patterns
microscope 3
microscope 4
microscope 1
microscope 2
substrate
copper
back mask
front mask
alignment marks
alignment marks
alignment marks
front mask
alignment marks
back mask
Figure 4
Double Mask Alignment Procedures Antipodal Measurement Interface
In
order
to
create
a
stable
interface
for
antenna
measurement at mm-wave frequencies we investigated how
to construct a WR-15 to microstrip transition with integral
antenna attached. To do this we had to develop double sided
mask alignment for Silicon and for softboard antenna
variants. The procedure using a split optics approach is
shown below in Figure 4. The transitions formed by using
this assembly method have better than 15dB return loss
and 2.5dB insertion loss at 62 65GHz.
In figure 5 we show a typical installation of a microstrip
patch subarray connected via the transition to a section of
WR-15 waveguide, the antipodal transition is illustrated
also.
(a)
Figure 5
WR-15 to Microstrip Array Transition.
62 GHz Van Atta Array
Using the 2X2 sub-arrays described above a 62GHz Van
Atta array [2] was constructed and tested, Figure 6. From
Figure 6 it can be seen that reflections of the dielectric
material used to form the patch array will mask the arrays
performance due to auxiliary reflection [3]. Typically the
target gain of the 24mm X 33mm metal backed dielectric
plate on which the antenna elements is fabricated is
calculated to be about 35dB at 62GHz. This value is greater
by 17dB than the theoretical maximum gain of the Van Atta
array i.e. 18dBi. Therefore it is essential that as much
redundant dielectric is removed from the array face in order
to minimise this backscattering mechanism which tend to
swamp the desired self-tracking response of the passive Van
Atta Array, i.e. the Cu Van Atta response shown in figure
6b, (this undesirable effect can be minimised by using an
active self tracking array [4]). When surplus dielectric was
removed, figure 6a, the normalised results in figure 6b were
obtained. These show that self tracking by the antenna array
is actually occurring (with around 5dB ripple caused by
residual backscatter from the remaining substrate material).
Here it can also be seen that the array is capable of tracking
with a similar amplitude profile to that of a single patch
antenna but with 15dBi gain, c.f. 2.6 dBi for the single
element.
(a) 62GHz VanAtta Array
V Band Van Atta Patch Array
-60
-50
-40
-30
-20
0
20
40
60
80
100 120 140 160 180
Scan Angle (Degrees)
S
2
1
(
d
B
)
Cu Van Atta (62 GHz)
Substrate (62 GHz)
Chopped(3) Normalised, flipped about -32dB
Single Patch
(b)
response
Figure 6
62GHz Van Atta Array Conclusions
Silicon, GaAs and soft-board antennas have been designed,
fabricated and measured for use as radiating elements for V-
band millimeter wave wireless asset tracking front ends. It
has been shown that it is possible to construct the array to
operate to within 0.2% of the design frequency with 10dB
return loss. Results on the performance of an mm-wave
passive Van Atta array show that auxiliary scattering from
the dielectric material on which the radiating elements are
constructed play a critical role in its overall success as an
asset tracking antenna.
Acknowledgements
The authors would like to thank the IRTU for its
sponsorship of this work under grant No. ST173 , also
Philips LEP for the provision of their foundry for the
production of the GaAs Patch antenna MMIC, and to
Taconic for the soft board substrate materials used. Thanks
also go to Dr Neil Buchanan for the antenna measurements
presented here.
References
1.
Mobile
Broadband
Systems,
European
Radio
Communications Office, July 1997.
2. Van Atta, L.C., Electromagnetic Reflector, US Patent No.
2908002, Oct. 1959.
3. W.J. Tseng, S.J., Chung, Chang, K., A Planar Van Atta
Array Reflector with Retrodirectivity in Both E-Plane and
H-Plane, IEEE Trans. On Antennas and Prop., Vol,48,
pp173-175, Feb.2000.
4. Toh, B.Y., Fusco, V.F.,
Retrodirective Array Radar
Cross-section Performance Comparisons, IEEE 2000 High
Frequency Postgraduate Student Colloquium, Sept. 2000,
Dublin pp65-69.