Broadband Focusing Using Aperture-Coupled
Microstrip Patch Antenna Arrays
Payam Nayeri, Atef Z. Elsherbeni, and Randy L. Haupt
EECS, Colorado School of Mines, Golden, CO 80401
Abstract—A new design for a broadband focused planar
array using aperture-coupled stacked patches is presented. The
array has a Dolph-Tschebyscheff amplitude taper. Mutual
coupling is compensated by using the active scattering matrix. A
4×4 focused planar array antenna having a bandwidth of 26.5%
is demonstrated.
Index Terms—Antenna arrays, broadband, focused array,
microstrip antenna, mutual coupling.
I.
INTRODUCTION
Focused antennas concentrate the radiated power at a point
in the near field [1]. These antennas have numerous
applications in remote (non-contact) sensing, radar, medicine,
and imaging systems, and therefore have received a great deal
of attention over the years [2, 3]. While many different types
of antennas, such as reflectors, lenses, and arrays, can be
focused; arrays have the highest degree of design freedom, and
thus have received the most attention. In addition, phase and
amplitude control at the elements allows for a movable focal
point through electrical signals rather than mechanical
movement. Many applications of focused antennas requires a
wide bandwidth, and while microstrip arrays are typically
considered to be the favorite choice, the bandwidth of these
arrays is usually quite narrow [2, 3]. In this work we present a
new design for broadband focused arrays using a stacked patch
aperture coupled configuration. A two-dimensional DolphTschebyscheff amplitude taper lowers the side-lobe levels.
Mutual coupling compensation based on the active scattering
matrix is implemented to ensure that the desired phase and
amplitude taper is realized. Moreover, the importance of
mutual coupling compensation in small focused array antennas
is delineated by studying the near electric fields of the array for
compensated and uncompensated excitations. A 4×4 focused
planar array with a 26.5% bandwidth, achieving good focusing
properties is demonstrated.
II.
DESIGN OF THE BROADBAND FOCUSED MICROSTRIP
PATCH ARRAY
Microstrip antenna technology has been the most rapidly
developing topic in the antenna field in the last few decades
[4]. Owing to their low cost, low mass, low profile, and
conformability, microstrip patch antennas have found
numerous applications in high-performance microwave
systems over the years. While the classical microstrip antenna
is inherently narrowband, many broadband techniques have
been developed over the years to overcome this primary
barrier. Amongst these techniques, the aperture coupled
stacked patch configuration [5], not only yields a wide
bandwidth, but it also provides the advantage of isolating
spurious feed radiation through the use of common ground
plane. This feature is particularly desirable for microstrip patch
arrays, and thus selected here.
The array is designed for X-band operation. The isolated
element patch model in Ansys HFSS [6] is depicted in Fig. 1
(a). The dimensions of the top and bottom patch are 6.5×3.5
mm2, and 8.6×5.4 mm2, respectively. The dielectric substrates
used for this design are 62 mils Rogers 5880, and 120 mils
Rogers 6002, for the top and bottom layers, respectively. The
slot has a length of 6.5 mm. Microwave power is coupled
through this slot to the stacked patch antenna, by a microstrip
transmission line. The E- and H-plane patterns at 10 GHz,
obtained by Ansys HFSS, are given in Fig. 1 (b). The far-field
gain of the isolated array element is 7.14 dBi at 10 GHz.
z
x
y
(a)
(b)
Fig. 1. The aperture coupled stacked patch: (a) 3D model in Ansys HFSS, (b)
isolated element pattern at 10 GHz (dashed is E-plane, solid is H-plane).
In the next stage, we construct a 4×4 planar antenna array
with an element spacing of 20 mm. The model of the array in
Ansys HFSS is given in Fig. 2. To focus the radiated power of
the antenna, the radiated waves from all elements of the array
should add up in phase at the desired focusing point. The array
is placed along the xy-plane; thus at a focusing distance F
along the z-direction, the required phase shift for each element
is obtained using
ϕ n = 2π
f
c
2
2
2
F + x′n + y ′n ,
(1)
where f is the center frequency. We select a focal distance of
80 mm, and assign a Dolph-Tschebyscheff amplitude taper to
the elements in order to achieve low side-lobe levels in the
near field of the antenna. It should be noted that in this setup,
each element of the array is excited with a microstrip port. The
input reflection coefficient for 4 elements on one quarter of the
array (marked with a dashed square in Fig. 2) are given in Fig.
3, where it can be seen that this microstrip antenna array is
matched over a 26.5% frequency band.
z
4
x
2
y
phase of each element is provided through time delay
microstrip lines, and therefore are inherently adjusted at all
frequencies. More discussion on the feed network for this array
will be presented at the time of the conference. Fabrication and
test of the prototype is also in progress.
3
2000
1
Fig. 2. 3D model of the microstrip array in Ansys HFSS.
|Ex| (V/m)
0
-20
1000
|S11|
|S |
-30
22
500
0
|S |
33
-40
|S44|
9
10
11
Frequency (GHz)
= (I + S )
−1
Vdesired ,
(2)
which ensures that a proper excitation is assigned to each port.
Here I is the identity matrix, S is the scattering matrix obtained
by full-wave simulation, and Vdesired is the uncompensated
amplitude and phase of the array element excitations [7].
z-axis (mm)
An important consideration in antenna arrays is the effect
of mutual coupling on the performance of the array [7]. In
general for large arrays that don’t require exact patterns, this is
less of an issue and typically mutual coupling effects are only
included in the pattern computation. However when low sidelobes are required, or when the array is small, such as this case,
mutual coupling effects must be compensated. Mutual
coupling only changes the element excitation, not the shape of
the current distribution on the element; thus one only has to
determine the correct incident signal voltage vector (V+) to the
array. To compensate for the mutual coupling in our design,
the incident voltage vector is determined using
+
150
Fig. 4. The magnitude of electric field along the focal line at 10 GHz.
12
Fig. 3. Input reflection coefficient magnitude of the array elements.
V
uncompensated
compensated
50
100
z-axis (mm)
0
140
120
100
80
60
40
20
-50
-10
-20
0
50
y-axis (mm)
z-axis (mm)
dB
-10
1500
140
120
100
80
60
40
20
-30
-50
100
0
-100
0
50
y-axis (mm)
(a)
(b)
Fig. 5. The focused electric fields at 10 GHz in the yz-plane: (a) normalized |Ex|
in dB, (b) phase of Ex in degrees.
IV.
CONCLUSIONS
We present a new design for broadband focused planar
array antennas. A stacked configuration of aperture coupled
patches are used for the elements in order to achieve wide
bandwidth, and a 4×4 array with 26.5% bandwidth is
demonstrated. The importance of mutual coupling
compensation in focused arrays is also investigated, and it is
shown that by proper element excitation, the radiated nearfield power of the antenna can be increased.
ACKNOWLEDGMENT
III.
FOCUSING PROPERTIES OF THE ANTENNA ARRAY
In the previous section, detailed information on the
construction of the focused array was presented. Here we study
the focusing features of this design. As discussed earlier,
mutual coupling compensation has a large impact on the
performance of small arrays. To demonstrate the importance of
this, we show the magnitude of the electric field along the zaxis in Fig. 4, for both uncompensated and compensated
excitation. It can be seen that a notable improvement in peak
near electric field magnitude is achieved, which is essentially
analogues to the improvement in peak gain for far-field arrays
when mutual coupling is compensated.
The amplitude and phase of the electric field (Ex) in the yzplane are also given in Fig. 5, where it can be seen that the
focal point is exactly located at z = 80 mm as desired. Note
that the peak magnitude of the electric field is closer to the
aperture of the array, however the location of the focal point is
observed in the phase plot. Similar observations were made at
other frequencies across the band, indicating wideband
focusing characteristics for the array. It is important to note
that in the corporate feed network of the array, the excitation
The authors gratefully acknowledge the contributions of
Ansys Inc. and Intel Corporation to Colorado School of Mines.
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