Enhanced feeds in focal plane imaging array systems: off
focus configurations
Citation for published version (APA):
Bonnedal, M., Neto, A., Llombart, N., Gerini, G., & de Maagt, P. J. I. (2007). Enhanced feeds in focal plane
imaging array systems: off focus configurations. In
Proceedings of European Microwave Conference, 2007,
Munich, 8-10 Oct. 2007
(pp. 253-256). Institute of Electrical and Electronics Engineers.
https://doi.org/10.1109/ECWT.2007.4403994
DOI:
10.1109/ECWT.2007.4403994
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Published: 01/01/2007
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Enhanced Feeds in Focal Plane Imaging Array
Systems: Off Focus Configurations
M. Bonnedal
∗1
, N. Llombart
#2
, A. Neto
#3
, G. Gerini
#4
, P. De Maagt
+5
∗
Saab Space
Gothenburg, Sweden
1
magnus.bonnedal@space.se
#
TNO Defence, Security and Safety
Oude Waalsdorperweg 63, 2597 AK The Hague, The Netherlands
2
nuria.llombartjuan@tno.nl
3
andrea.neto@tno.nl
4
giampiero.gerini@tno.nl
+
ESTEC, European Space Agency
PO Box 299, NL 2200 AG Noordwijk, The Netherlands
5
peter.de.maagt@esa.int
Abstract— The performances of focal plane arrays that imple-
ment multi-beam coverage from a single reflector antenna are
usually reduced by conflicting requirements on the feed elements.
Dense packing is required to minimize the beam separation, while
typically large apertures provide the high feed directivity which
in turn translates in low spill over losses from the reflector. In
this paper the use of a dielectric super-layer is proposed to shape
the radiation pattern of each feed so that the spill over from
the reflector is reduced without increasing the dimensions of
each aperture. A prototype with 19 wave-guides arranged in a
hexagonal lattice has been designed. The simulated embedded
patterns provide an increase of the edge of coverage gain, with
respect to the free space case, of at least 1dB in an operating
bandwidth of ≈ 6%. Moreover when a frequency and polarization
reuse scheme is adopted the increase in the edge of coverage is
2.1 dB leading to a fully efficient use of the reflector.
I. INTRODUCTION
Present and next generation telecommunication satellite
systems often require multiple beam capability. The three fun-
damental ways to achieve high coverage gain are overlapping
feed arrays with expensive feed networks, interleaved beams
with multiple apertures, and designs with a single aperture
and a single feed per beam [1]. This last approach requires
small aperture feeds in order to obtain a small beam separation,
however these small feeds have low gain which results in 2
or 3 dB of power lost in spill over from the reflector. For this
reason this approach is usually not adopted for the military
[1] or commercial telecommunication satellite systems. This
paper specifically addresses and solves this problem.
G. V. Trentini [2] was the first to propose the use of a
partially reflecting screen to increase the directivity of single
aperture. A point source would generate an ensemble of waves
that impinge on a screen that is partially reflective and partially
transmitting. The reflected rays propagate laterally and then
are redirected toward the partially reflecting screen, so that
overall they generate a unique wave that mainly radiates
in the broadside direction. Since then this technique has
been proposed in many other configurations characterized by
periodic super-layers realized in dielectric or metal materials
[3]-[4]. In these papers the fact that the enhancement in gain
is due to the excitation of leaky waves has been clarified.
Unfortunately most of these attempts were characterized by
narrow frequency bandwidths (BW) that are not suited for
broad band or even moderate band scenarios. Most recently,
[5] studied the compromise between bandwidth and directivity
for printed antennas and arrays in the presence of periodic
super-layers.
In the present contribution leaky waves are used in order
to enhance the performances of each of the small feeds used
in an array for multi beam reflector systems. The spill over is
targeted and significantly reduced. Since the enhancement of
multi-beam antenna performances is a basic need for realistic
next generation telecom systems but also for imaging arrays in
the mm and sub-mm wave regimes, two prototypes have been
built. The first one is of general applicability and assumes a
worst case scenario with significant coupling between neigh-
boring radiators. The second one focuses on a specific satellite
telecommunication scenario and there the trade offs between
the BW, the isolation of the beams and the efficiency drives
the electrical design.
II. SYSTEM PARAMETERS AND REFERENCE ARRAY
The generic scenario introduced in this section involves an
area to be covered by independent beams, arranged in an
hexagonal lattice and separated by an angle ∆θ. To achieve
this coverage a corresponding hexagonal array placed in the
focal plane of a Cassegrain off focus parabolic reflector
is considered, as shown in Fig. 1. The array elements are
designed to be compatible with circular polarization in order
to maintain the highest usability. The basic parameters charac-
terizing an imaging system are: the beam separation ∆θ, the
reflector diameter, D, and the focal distance, F (see Fig. 1).
F/D defines the subtended angle θ
sub
. All the power that is
978-2-87487-003-3 © 2007 EuMA October 2007, Munich Germany
Proceedings of the 10th European Conference on Wireless Technology
253
Fig. 1. Schematic view of a reflector configuration
launched by the feed but is not intercepted by the reflector is
effectively lost for the system (spill over). In order to enhance
the spill over efficiency one would like to use directive feeds,
however the dimensions of the apertures are limited by the
period. For this reason most imaging systems need to perform
a trade off between the efficiency with which each element
of the array excites the reflector and the capacity to sample
the available field of view. In the following sections a single
dielectric super-layer will be proposed to allow the overlap of
equivalent apertures.
In order to establish a reference to which compare the
performances of the leaky wave enhanced arrays, a standard
waveguide array is investigated first, Fig. 2(a). It consists of
an hexagonal grid array composed of 19 circular waveguide
horns that open in an finite ground plane of dimensions
(12λ
0
× 12λ
0
). To accurately evaluate the performances of
a reflector system fed by this reference array, the secondary
patterns are obtained resorting to a complete Grasp analysis
starting from the fields radiated by the reference array, which
in turn have been calculated using CST Microwave Studio.
The key merit parameter for these multi-beam systems is the
edge of coverage gain, which is defined at the cross over
between three adjacent beams: G
eoc
= G(∆θ/
√
3). Figure
2(b) shows the edge of coverage gain for a reflector that
provides ∆θ ≈ 1
o
, which corresponds to F ≈ 66.66λ
0
.
WAVEGUIDE ARRAY PROTOTYPES
When the distance between a dielectric super-layer and
a ground plane is about half of a free space wavelength,
leaky waves can propagate between the sandwiched area. The
enhancement of the broadside directivity of a single antenna
is obtained when the super-layer thickness is a quarter of the
dielectric wavelength [6]. The proposed prototype is shown
Fig. 3(a). The array is composed, like the reference array, by
19 waveguides, this time of square cross section with width,
w = 0.67λ
0
, and separation d = 1.2λ
0
. Note that in this
case the array of apertures does not fully occupy the central
part of the ground plane. The dielectric constant of the slab is
y
z
t
D
s
y
t
x
d
D
a
(a)
(b)
Fig. 2. (a) Hexagonal grid array with periodicity d = 1.2λ
0
of circular
metallic waveguide D
s
opening in a infinitely extended ground plane. The
apertures D
a
= 1.1λ
0
fully sample the array. (b) Edge of coverage gain as
a function of the frequency.
4.5. In this kind of super-layer structures, there is a TM leaky
wave pointing to large angles that could contribute to spill
over losses. In order to cancel its contribution, it is convenient
to load the waveguide with a double irises configuration.
polarization by properly phase shifting the two polarizations.
The two slots are excited in phase and located at distance such
that their contributions cancel out exactly at the pointing angle
of this leaky wave. Each of the couples of slots is associated
to one polarization so that each waveguide can be operated
in circular polarization by properly phase shifting the two
polarizations. The slots are shaped as an arc of a circle in
order to achieve the desired cancellation over the maximum
azimuthal angle, [7].
The impact of the mutual coupling on the radiation patterns
254
h
1
h
x
y
z
e
r
w
l
s
w
i
s
w
1
4
2
5
4
e
r
4
h
1
e
r
1
4
e
r
3
5
6
7
d
w
s
(a)
10 20 30 40 50 60 70 80
Isolated
q[ ]
o
Array
E(0 )-plane
o
H(90 )-plane
o
D [dB]
0 90
-15
-10
-5
0
5
10
15
(b)
Fig. 3. (a) Prototype waveguide array. The area of each unit cell is
significantly larger than the dimension of each waveguide. (b) Amplitude of
the calculated radiation patterns.
can be seen in Fig. 3(b). The calculated patterns are shown for
the embedded and the isolated cases. The maximum directivity
in the array environment is lower than the one obtained in
isolation. That is because the distance between the array
elements is such that the scattered field from the neighboring
waveguides contributes almost out of phase with respect to the
central element at broadside. While the embedded patterns are
representative of the worst case scenario, the isolated patterns,
in first approximation, can be representative of an ideal case,
in which the adjacent waveguides are loaded with properly
tuned reactive loads to create an equivalent ground plane on
the apertures.
A. Worst Case Scenario
The worst case scenario corresponds to an imaging con-
figuration in which the isolation between beams associated
to adjacent wave-guides is not a driving parameters. For
instance in radiometry at mm or sub-mm waves. Here all the
neighboring wave-guides are closed in matched loads
A parametric study of the G
eoc
equivalent to of the reference
array is plotted in Fig. 4. For all three F/D the gain presents
a weak frequency dependence. It is apparent that there is a
significant improvement that can be as high as 1.7 dB for
F/D = 0.67.
Fig. 4. Edge of coverage gain calculated from the simulated patterns for the
worst case scenario.
B. Best Case Scenario
For the best case a satellite based telecommunication sce-
nario, in Ka band which makes use of two frequency bands
and two linear polarizations to obtain independent, interleaved
beams (separated by 1
o
) is imagined. The implementation of
interleaved beams and of single aperture corresponding focal
plane array is based on a 4 channel reuse scheme (see Fig.
5), with two linear polarizations (performed at the iris level)
and two frequencies. The distance of the frequency filter from
the aperture is tuned to achieve an effective short circuit at
the aperture. For budget reasons and thanks to the smooth
behavior as a function of the frequency of the phase of the
reflection coefficient of the frequency filter, the reactive load
can be assumed to be represented by a piece of waveguide
loaded at a certain distance by a real short circuit.
I
II
III
IV
III
II
IV
I
IV
I
I
I
III
III
IV
II
I
II
I
Fig. 5. Frequency and polarization re-use scheme. I → co/f
1
;II →
co/f
2
;III → cx/f
1
; IV → cx/f
2
.
255
The increase in edge of coverage in this second scenario
(see Fig. 6) is as high as 2.2 dB, which essentially means that
the no power is lost in spill over.
Fig. 6. Edge of coverage gain calculated from the simulated patterns for the
best case scenario.
CONCLUSIONS
A novel strategy has been presented to design multi-beam
reflector antenna system based on the excitation of a couple
of leaky waves by covering the focal plane array with a single
thin dielectric layer and tuning the array design to maximize
their positive effect. In the worst case scenario of neighboring
co-frequency and co-polarized beams an increase of the G
eoc
of 1 dB over a 6% BW with respect to the standard free space
designs could be obtained. In the scenario of a multi beam
system for a telecommunication satellite with four channels an
edge of coverage increase larger than 1.7 dB has been shown
over a 3% BW.
REFERENCES
[1] S. K. Rao, “Design and analysis of multiple-beam reflector,” IEEE
Antennas and Propagation Magazine, vol. 41, pp. 53–59, Aug. 1999.
[2] G. V. Trentini, “Partially reflecting sheet arrays,” IEEE Antennas and
Propagation Magazine, vol. 4, no. 4, pp. 666–671, Oct 1956.
[3] D. Jackson, A. A. Oliner, and A. Ip, “Leaky wave propagation and
radiation for a narrow-beam multiple layer dielectric structure,” IEEE
Transactions on Antennas and Propagation, vol. 41, no. 3, pp. 344–348,
March 1993.
[4] C. A. Balanis, Antenna Theory: Analysis and Design. New Jersey: Wiley
Interscience, 2005.
[5] R. Gardelli, M. Albani, and F. Capolino, “Array thinning by using a fabry
perot cavity for gain enhancement,” IEEE Transactions on Antennas and
Propagation, vol. 54, no. 7, pp. 1979–1990, July 2006.
[6] D. Jackson and A. A. Oliner, “A leaky-wave analysis of the high-
gain printed antenna configuration,” IEEE Transactions on Antennas and
Propagation, vol. 36, no. 7, pp. 905–909, July 1988.
[7] M. Qiu and G. Eleftheriades, “A reduced surface-wave twin arc-slot an-
tenna element on electrically thick substrates,” IEEE Microwave Wireless
Components Lett., pp. 459–461, Nov. 2001.
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