Near-Field Characteristics of Focused Transmitarray Antennas
Payam Nayeri *, Atef Z. Elsherbeni*, Randy L. Haupt*, and Fan Yang†
*
Electrical Engineering and Computer Science Department,
Colorado School of Mines, Golden, CO 80401, USA
Email: pnayeri@mines.edu, aelsherb@mines.edu, rhaupt@mines.edu
†
Electronic Engineering Department
Tsinghua University, Beijing, China 100084
Email: fan_yang@tsinghua.edu.cn
Abstract — Near-field focusing characteristics
of
transmitarray antennas are investigated. The design procedure is
summarized and a Ku-band focused transmitarray antenna is
presented where the elements are aperture-coupled patches
interconnected with stripline delays. The focusing performance of
the transmitarray is compared with classic far-field designs. It is
shown that a near-field focused reflectarray can be a good
candidate for applications requiring high microwave power.
Index Terms — Array, aperture coupled, focused antenna,
microstrip, transmitarray.
I.
INTRODUCTION
Array lens antennas, also known as transmitarrays, combine
many of the favorable features of optical lens and array
antennas, and create a low profile and low mass design with
high radiation efficiency and diversified radiation performance
[1]. Similar to the lens antenna [2, 3], the elements of a
transmitarray produce specific phase shifts that, when properly
tuned, create a directive beam in the desired direction.
Conventionally transmitarray antennas are designed for farfield operation. However, numerous applications such as
remote sensing, wireless power transfer, medicine, and
imaging systems, require antennas that focus the microwave
power to a point in the near-field of the antenna.
Electromagnetic power can be focused into a small spot in
the near field by focusing the antenna radiation [4, 5]. In the
case of a transmitarray, the elements can produce any desired
value of phase shift. As such, to design a focused
transmitarray, one only needs to determine the phase
distribution that can focus the radiated power of the array to
the desired point. In comparison with classic antenna arrays
that require a complex feed network, which is usually quite
lossy for large arrays, the spatial feed system of the
transmitarray, combined with the individual phase control
capability of its elements, provides a myriad of design
capabilities.
The aim of our work here is to study the performance of
focused transmitarray antennas. We present the basic
procedure to design focused transmitarray antennas, and study
the feasibility of such designs through case study of a Ku-band
transmitarray. The focusing characteristics of the transmitarray
are also compared to a classic case designed for far-field
operation. We show that a focused transmitarray can be a good
candidate for applications requiring high microwave power.
II. PHASE DISTRIBUTION OF FOCUSED TRANSMITARRAY
The phase distribution on the aperture of classic array lens
antennas is designed to compensate for the spatial delay from
the feed antenna, and generate a highly directional beam in the
desired direction in the far-field zone of the antenna. For
focused antennas however, the phase distribution is different.
To focus the radiated power of the antenna array, the radiated
waves from all elements of the array should add up in phase at
the desired focusing point.
Consider a transmitarray antenna where the transmitting
aperture is placed on the xy-plane, centered at the center of the
coordinate system. To focus the beam of the antenna array at a
distance F along the z-direction, the focusing phase for each
element is obtained using
ϕ m , n = kd mn +
2π f F 2 + x′m2 + y ′n2
.
c
(1)
Here, x’m and y’n are the coordinates of the elements of the
array, f is the design frequency, k is the wavenumber, dmn is
the Euclidian distance from the feed phase center to the center
of each element, and c is the speed of light. Note that the total
phase shift for each element of the focused transmitarray is the
sum of the spatial delay and the focusing phase.
As an example, here we compare the phase distribution on
the aperture of a trasnmitarray designed for far-field operation,
and a focused transmitarray. Both transmitarrays have a
circular aperture with a diameter of 275 mm, and are designed
to operate at 14.25 GHz. The phase center of the feed antenna
is placed at x = 0, y = 0, z = -275 mm. The unit-cell size is
10.6×10.6 mm2. The transmitarray designed for far-field
operation produces a directive beam in the broadside direction.
The focused transmitarray is designed to focus the microwave
power at x = 0, y = 0, z = 275 mm. The required phase shift on
the apertures of both antennas is given in Fig. 1. Note that only
one phase wrap is observed with the far-field design, however
the focused transmitarray shows multiple phasse wraps.
now be designed using these elem
ments, where the required
phase distribution on the aperture, Fig. 1 (b), is achieved by
selecting the length of the sttripline accordingly. The
transmitarray antenna has 468 elem
ments. The 3D model of the
antenna is given in Fig. 4.
(a)
(b)
Fig. 1. Phase distribution on the aperture of transsmitarray antennas
designed for: (a) far-field operation, (b) near-field ffocusing.
III. DESIGN OF A KU-BAND FOCU
USED
TRANSMITARRAY ANTENNA
The required phase distribution on the apeerture of focused
transmitarray antennas was outlined in the pprevious section.
However, for a practical design, a key consideration is
designing an element that can achieve the deesired phase shift
with good transmission properties. While ddifferent types of
elements have been developed over the years, a popular design
is the aperture coupled microstrip patch propposed in [6]. For
our design we select the same element developped in [7], which
are aperture coupled rectangular patch elemeents with a phase
delay stripline. The element consists of four llayers of Taconic
RF-60A (İr = 6.15) substrates and has a total thickness of 120
mil. The element is designed for Ku-band opeeration and has a
unit-cell size of 10.6×10.6 mm2. A 3D imagee of this element
in Ansys HFSS [8] is given in Fig. 2.
(a)
(b)
Fig. 3. (a) Reflection and transmission magnitude of the element. (b)
Transmission phase of the element in deegrees.
(a)
(b)
Fig. 4. 3D model of the transmitarray in Ansys HFSS: (a) complete
model of the array, (b) zoomed view.
Fig. 2. 3D model of the transmitarray antenna elem
ment.
In this element, the U-shaped striplinne receives the
microwave power from the aperture coupled receiving patch,
and delivers it to the transmitting patch. The tootal length of the
stripline is L, which is used to adjust the transsmission phase of
the element. Magnitudes of the transmissioon and reflection
coefficient, and phase of the transmission coeefficient at 14.25
GHz are given in Fig. 3. It can be seen tthat this element
achieves a phase tuning range of about 5500 degrees, with a
good transmission magnitude. Note that in the results
presented here, material losses are not includeed in the analysis.
However, studies showed that the total elem
ment loss is less
than 1 dB when material losses are consideredd [7].
The focused transmitarray system describedd in section II can
IV. FOCUSING CHARAC
CTERISTICS OF
TRANSMITARRAY ANTENNAS
The basic steps in designing focu
used transmitarray antennas
were outlined in the previous secctions. Here we study the
focusing characteristics of these an
ntennas, and also compare
them with classic far-field designs. Different classic analysis
approaches are available to obtaain the far-field radiation
performance of antenna arrays [9]], and they typically show
good agreement when compared to measured results. However
accurate studies of the antenna near-field characteristics
typically requires a full-wave anallysis method. Here we use
the commercial electromagnetic sofftware Ansys HFSS for this
study. Electromagnetic modeling of
o large antenna array such
as transmitarrays is computationally
y challenging. In particular
for space-fed arrays, modeling the volume between the
primary feed and the aperture significantly increases the size
of the solution domain. To effectively model the antennas in
HFSS, we use the finite element boundary integral (FEBI)
method. With this approach, one bounding box is defined
around the feed antenna, and a second box is defined around
the array. The exterior for both these boundaries are defined as
HFSS-IE domains, and the interaction between them is
computed iteratively. Moreover, to further reduce the size of
the problem, the feed antenna simulation is solved separately
and linked to the array simulation problem. The 3D models of
these two simulations are given in Fig. 5. Note that in order to
evaluate the near-fields in the plane of interest, in the array
model, two additional boxes are defined in the xz and yz
planes. The feed is an A-INFO LB-62-15 pyramidal horn
antenna, where its phase center is set to be at x = 0, y = 0, z = 275 mm in the linked simulations.
The electric fields in the xz-plane of the focused
transmitarray antenna are shown in Fig. 7, where it can be
seen that the antenna is focusing the electromagnetic power at
the desired point. Similar observations were made for different
focal points by adjusting the phase of the transmitarray
elements according to (1).
(a)
(b)
Fig. 7. Electric fields in the xz-plane of the focused transmitarray at
14.25 GHz: (a) magnitude of Ex in V/m. (b) phase of Ex in radians.
V. CONCLUSIONGS
Focused transmitarray antennas are investigated in this
study. The basic design procedure is outlined, and a Ku-band
focused transmitarray antenna is studied. It is shown that
focused transmitarrays are a suitable choice for applications
requiring high power focused microwave energy.
ACKNOWLEDGEMENT
(a)
(b)
Fig. 5. Setup of the simulation models in Ansys HFSS: (a) feed
antenna, (b) transmitarray.
The electric fields in the xz-plane of the transmitarray
antenna designed for far-field operation are given in Fig. 6.
Similar results were observed in the yz-plane and are not
shown for brevity. Note that the parallel lines in the phase plot
clearly indicate that a plane wave is being formed, and this
antenna has a directive beam in the broadside direction.
However, the magnitude plot indicates that radiated power is
not focused in the near-zone of the antenna, thus this design
will not yield a good performance for near-field applications.
The authors acknowledge the generous contributions of
Ansys Inc. and Intel Corp. to Colorado School of Mines.
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Fig. 6. Electric fields in the xz-plane of the transmitarray designed for
far-field operation at 14.25 GHz: (a) magnitude of Ex in V/m. (b)
phase of Ex in radians.
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