Near-Field Scanning Characteristics of Focused Reflectarray Antennas Payam Nayeri , Atef Z. Elsherbeni

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Near-Field Scanning Characteristics of
Focused Reflectarray Antennas
Payam Nayeri 1, Atef Z. Elsherbeni 1, Randy L. Haupt 1, and Fan Yang 2
1
EECS Department, Colorado School of Mines, Golden, CO 80401, USA
[email protected], [email protected], [email protected]
2
Electronic Engineering Department
Tsinghua University, Beijing 100084, China
[email protected]
Abstract ņ Focused reflectarray antennas are
designed with the aim of investigating the
scanning capabilities of these systems. Near-field
scanning with a 30° scan coverage is demonstrated
with a Ku-band reflectarray system.
Index Terms - Focused antenna, reflectarray.
I. INTRODUCTION
Electromagnetic power can be concentrated
into a small spot by focusing the antenna [1].
Focused antennas arrays have numerous
applications in remote sensing, radar, medicine,
and imaging systems, and therefore have received
a great deal of attention over the years.
Reflectarray antennas on the other hand, combine
the many favorable features of reflectors and
arrays, and have now emerged as the new
generation of high-gain antennas [2]. The spatial
feed system of the reflectarray, combined with the
individual phase tunability of its elements,
provides a myriad of design capabilities for
applications requiring high-gain antennas. In this
paper we present computed far field patterns from
near-field scanned reflectarray antennas. Focusing
characteristics of reflectarrays are investigated and
a Ku-band focused reflectarray system achieving
30° scan is demonstrated. It is shown that a
focused reflectarray can be a good candidate for
limited field-of-view (FOV) applications.
II. NEAR-FIELD ANALYSIS OF
REFLECTARRAYS
Parabolic-phase
reflectarray
antennas
are
designed for highly directional beams in the farfield zone [2]. While different analysis approaches
are available for reflectarrays [3], accurate studies
of the antenna near-field characteristics typically
requires a full-wave analysis method. In this study
we use the commercial electromagnetic software
FEKO [4], and investigate the near-field
performance of Ku-band reflectarrays designed for
the operating frequency of 14.25 GHz.
(a)
(b)
Fig. 1. Electric fields of the reflectarray in the xzplane: (a) |Ex|, phase of Ex.
A circular aperture with a diameter of 360 mm
is selected for the reflectarray. An offset system
with a feed offset angle of 15 degrees is
considered. The reflectarray aperture is placed in
the xy-plane and the feed is pointing to the center
of the coordinate system. The phase center of the
feed antenna is at X = -91.88 mm, Y = 0, Z =
342.9 mm. The feed is an A-INFO LB-62-15
pyramidal horn antenna. Variable size patch
elements are used for the reflectarray phasing
elements [3], and the dimensions of the patches
are selected to scan the far-field beam to ș = 15°, ij
= 0°. The electric fields of this reflectarray are
shown in Fig. 1, where the formation of the plane
wave is clearly visible in the phase plot. Note that
however, the radiated power is not focused in the
near-zone of the antenna, thus this design will not
yield good performance for near-field applications.
III. NEAR-FIELD FOCUSED SCANNING
REFLECTARRAYS
To focus the radiated power of the reflectarray
antenna, the radiated waves from all elements of
the array should add up in phase at the desired
focusing point. The required phase shift on the
aperture which focuses the beam at the desired
point is computed as given in [5]. Here we study
near-field scanning capabilities of 3 reflectarrays
which focus the radiated power at ș = 0°, 15°, and
30°, in the xz-plane. The focal distance is set to
355 mm from the center of the aperture. The
masks of these reflectarrays are given in Fig. 2.
Note that frozen-state designs are considered in
the current study, however several designs of
reconfigurable reflectarray elements are available
that can be used for dynamic operation [6].
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. Electric fields of scanned reflectarray
antennas in the xz-plane. (a) |Ex| (ș = 0°), (b) phase
of Ex (ș = 0°), (c) |Ex| (ș = 15°), (d) phase of Ex (ș
= 15°), (e) |Ex| (ș = 30°), (f) phase of Ex (ș = 30°).
(a)
(b)
(c)
Fig. 2. Masks of the focused reflectarray antennas.
The electric fields of these focused reflectarrays
are shown in Fig. 3. It can be seen that all three
designs focus the electromagnetic power at the
desired point, and a stable scan performance is
achieved over a 30° scan range. This can be seen in
an almost constant peak field magnitude at the
desired focal point. In comparison between the
three designs, the array designed for ș = 15° shows
the best performance, which is similar to the case
for far-field operation. As such, very wide-angle
scanning with reflectarray may not be practical.
Nonetheless, for limited FOV systems, near-field
focused reflectarrays can be a suitable candidate.
IV. CONCLUSION
The feasibility of near-field scanning with
reflectarray antennas is investigated. A Ku-band
focused reflectarray system achieving 30° scan is
demonstrated.
ACKNOWLEDGMENT
The authors acknowledge the contributions of
Altair Inc. to Colorado School of Mines.
REFERENCES
[1] J. W. Sherman, “Properties of focused aperture in
the Fresnel region,” IRE Trans. Antennas Propag.,
vol. 10, no. 4, pp. 399–408, Jul. 1962.
[2] J. Huang and J. A. Encinar, Reflectarray Antennas.
New York, NY, USA: Wiley-IEEE, 2008.
[3] P. Nayeri, A. Z. Elsherbeni, and F. Yang,
“Radiation analysis approaches for reflectarray
antennas,” IEEE Antennas and Propagation
Magazine, vol. 55, no. 1, pp. 127 - 134, Feb. 2013.
[4] FEKO v 7.0, EM Software & Systems Inc., 2014.
[5] H. Chou, T. Hung, N. Wang, H. Chou, C. Tung,
and P. Nepa, “Design of a near-field focused
reflectarray antenna for 2.4 GHz RFID reader
applications,” IEEE Trans. Antennas Propag., vol.
59, no. 3, pp. 1013-1018, Mar. 2011.
[6] S. V. Hum and J. Perruisseau-Carrier,
“Reconfigurable reflectarrays and array lenses for
dynamic antenna beam control: a review,” IEEE
Trans. Antennas Propag., vol. 62, no. 1, pp. 183198, Jan. 2014.
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