PLANAR SATCOM ANTENNA SYSTEMS IN KA-BAND
ESA/ESTEC, NOORDWIJK, THE NETHERLANDS
3-5 OCTOBER 2012
S. Otto, I. Nistal, S. Holzwarth, O. Litschke, R. Baggen
IMST, Carl-Friedrich-Gauss-Str. 2-4, 47475 Kamp-Lintfort, Germany.
Email: [email protected], [email protected], [email protected], [email protected], [email protected]
ABSTRACT
Nomadic and mobile satellite communication (satcom)
services for broadband applications are becoming more
and more popular every day. Ka-band terminals are
favored for the next generation of satcom terminals.
Key results of two Ka-band projects (SANTANA &
KASANOVA) are summarized and typical features of
such satcom systems are addressed.
1.
INTRODUCTION
Nomadic and mobile satcom services for broadband
applications are becoming increasingly attractive
nowadays; users demand multimedia access at any time
at any place. In order to accommodate for the large
bandwidths required and the growing number of users,
higher frequency bands such as Ka-band are opened for
the next generation of satcom terminals. Such terminals
have to operate in nomadic or mobile user scenarios and
require in most cases smart and agile antenna solutions
that either can be easily deployed, or are capable of
tracking the satellite on-the-move.
Through the years, satcom antenna frontends have
become a key area at IMST. In this paper, two satcom
projects (from the past and on-going) in Ka-Band are
presented. First, some key results of a project from the
past, SANTANA, will be shown. Secondly, an on-going
joint project of Astrium and IMST, KASANOVA, is
introduced and preliminary results are presented.
2.
SANTANA
SANTANA (I-II-III) focused on Digital Beam Forming
(DBF) [1][2][3] and was supported and funded by the
German Ministry of Education and Research
(BMBF/DLR), in cooperation with the Technical
University of Hamburg, DLR and Astrium. The
application are airborne Ka-band satellite terminals for
broadband multimedia services (internet in the sky). The
DBF-terminal itself has been divided into a separate Tx
(30 GHz) and Rx (20 GHz) antenna array. The
frequency bands are based on upcoming satellite
multimedia applications and the polarization is circular,
so only the beam has to be steered; the polarization
Figure 1. Realized LTCC tile and a complete frontend
composed of 32x32 antenna elements (16 tiles), artist
impression
needs not be agile.
The RF-topology applied here is a mix of tile and brick
configuration [4], and a modular approach is used. The
antenna is realized in modules of 8x8 patch arrays using
a LTCC (Low-Temperature Co-fired Ceramics)
multilayer buildup. In Fig. 1 a picture of a realized tile
for Tx and a first impression of a complete array are
shown.
The modular approach allows adapting the size of the
antenna to different applications and data rates. The
basic antenna element design is based on circularly
polarized patches and allows a scan range of maximal
60° from boresight (typical restrictions of a planar
layout of the array). The circular polarization of the
patch elements is achieved by using two decoupled
feeds and one 90° hybrid ring per element. In addition,
the antenna elements are sequentially rotated to improve
the polarization behavior of the array. One multilayer
LTCC Tx-module consists of 17 layers of FERRO A6
and contains the following key components:
o
o
o
o
64 antenna elements,
hybrid ring feeds,
internal calibration network,
cooling channels (liquid cooling).
All are integrated in one complex LTCC tile (Fig. 2)
with the antenna elements located on one side, and on
the other side, the 64 PA’s (Power Amplifier),
corresponding peripheries and connectors for LO (Local
Oscillator), DC, IF and liquid cooling (Fig. 3). One tile
can be calibrated automatically using an internal
calibration receiver. The components used are all offthe-shelf in order to reduce development costs.
Figure 2. Layout of the 17 layer LTCC buildup
(backside view with all PA & connectors visible)
The antenna can be pointed in any fixed azimuth-plane
within an elevation from boresight of 60° without any
scan blindness problems. The side lobe levels are
maintained at levels of approximately 13 dB (no
amplitude taper applied) and a very good suppression
(> 20 dB) of the cross-polar component can also be
observed. The good polarization performance is the
result of several factors: (i) the feeding of the antenna
elements using the hybrid ring coupler allows for a very
good axial ratio of each single element, (ii) in addition,
the hybrid ring feeding also attenuates the cross-polar
components resulting from coupling effects, (iii) finally,
Figure 3. Realization of the 8x8 LTCC antenna tile
the sequential rotation of the antenna elements improves
the axial ratio at the array level. The complete buildup
consists not only of the antenna frontend realized in
LTCC and an up-converting RF-circuitry, but also of
the complete digital hardware necessary for the
controlling of the antenna signals at digital level.
The DBF-architecture considered for SANTANA
requires a high degree of integration with respect to the
large number of components and a high speed digital
processing unit. The real-time processing of thousands
of antenna element signals is however not feasible.
Therefore, alternative topologies are under investigation
at IMST that are combinations of DBF-technology and
special IF-feeding architectures. The concept behind
such topologies is to reduce the number of
interconnections between the RF-part and digital logic,
thus reducing the number of antenna element signals to
be processed (reduction of the computational load).
3.
Converter (BUC) with a true time delay in the L-band.
A time delay range of 15 ps for Tx and 25 ps for Rx can
be controlled in discrete steps with the antenna control
unit board shown in Fig. 6, where the time delays are
composed of switched transmission lines with different
lengths.
KASANOVA
Within the frame of the KASANOVA project, funded
by the German “Bundesministerium für Wirtschaft und
Technologie” BMWi, under the research contract
50YB1024, Astrium is developing the infrastructure and
corresponding satcom services for disaster relief
management. For these scenarios, IMST is developing a
nomadic Ka-Band planar antenna terminal (Tx: 29.5-30
GHz & Rx: 19.7-20.2 GHz) to be used for disaster relief
vehicles looking towards the future exploitation of high
data rates in the Ka-band.
The KASANOVA antenna aims at a hybrid solution, i.e.
a combined mechanical/electrical beam steering. The
mechanical positioning shall be achieved with an
accuracy of approximately ±1° in azimuth and
elevation, hence providing a coarse beam pointing
towards the satellite. The fine positioning of the beam
between -1° and 1° from boresight will be achieved
electronically by means of four individual sub-arrays
via true time delays. The Tx antenna aperture is
composed of circular horn antenna elements that are
uniformly fed by two waveguide distribution networks,
one for horizontal and the other for vertical polarization,
respectively. Arbitrary polarizations including circular
polarization can be achieved by properly combining
these two. The Rx aperture is formed by a separate
antenna, with the Rx horn element and the Rx
waveguide networks being a scaled version (factor 1.5)
of the Tx geometry.
Here, the concept and first measurement results of part
of the antenna aperture under development are
presented. Fig. 4 and Fig. 5 show the Rx and Tx
architecture of the hybrid beam steering system,
respectively. A mechanical positioning unit provided by
the German company ten Haaft is employed having a
pointing accuracy of ±1°. The antenna aperture is
divided into three sub-arrays in the Rx case and four
sub-arrays in the Tx case. Each sub-array is connected
to a separate Low Noise Block (LNB) or Block Up
Figure 4. Rx-architecture of the hybrid
mechanical/electrical beam steering antenna
Figure 5. Tx-architecture of the hybrid
mechanical/electrical beam steering antenna
Figure 6. Antenna control unit board with true time
delays in the L-band for Rx and Tx
Fig. 7 shows the fabricated prototype of an 8x8 element
Tx antenna, which is used to validate the simulations
performed with the FDTD-based (Finite-Difference
Time-Domain) field solver EMPIRE XCcel.
Furthermore, the prototype makes it possible to
investigate the influence of manufacturing tolerances on
the antenna performance including the antenna
efficiency. The element spacing is approximately one
free space wavelength λ. This is necessary in order to
incorporate two waveguide networks with a reasonable
waveguide/channel spacing of at least 1mm. This
ensures a proper mechanical support and paves the way
for plastic injected molding technologies. The two
networks are necessary in order to generate circular
polarization. The grating lobes induced by the element
spacing can be minimized by adjusting the horn taper of
the radiator element, so to superimpose the grating lobe
in the array factor with a null at the element factor.
Figure 8. Nearfield measurement setup for the 8x8 proof
of concept
Figure 9. Simulated and measured gain (circular
polarization) of the 8x8 sub-array prototype in the 45°
azimuth plane
Figure 7. Proof of concept: 8x8 elements aperture (1)
and two stacked waveguide networks for horizontal (2)
and vertical polarization (3) for achieving arbitrary
polarization control
This 8x8 antenna was measured employing the IMST
planar nearfield range, shown in Fig. 8. Excellent
agreement with the synthesized patterns were obtained,
showing that the measured gain in the Φ=45° azimuth
cut perfectly matches the predicted pattern from the
simulation as one can observe in Fig. 9. Furthermore,
the losses of the waveguide distribution network were
measured to be around 0.4 dB. The side lobe level is
approximately 27 dB due to the geometrical taper in the
45° cut. Finally, at this point we can conclude for the
8x8 proof of concept prototype that the measured
characteristic gain patterns as well as S-parameters (not
shown here) are in excellent agreement with
simulations, hence clearing the way for the full scale
demonstrator containing the Rx and Tx antenna
apertures and networks.
Here, for the proof of concept, only the more critical
8x8 Tx antenna operating was examined, since the Rx
design operates at a much lower frequency, and is
basically obtained through a down scaling by a factor of
1.5 which results in more relaxed fabrication tolerance
requirements. Currently, the overall Rx/Tx antennas are
being fabricated.
Fig. 10 shows the EMPIRE XCcel simulation model of
the overall 32x32 Tx antenna. Two separate simulations
were performed with one port exciting the horizontal
polarization and another port exciting the vertical
polarization. The radiation pattern in Fig. 11 shows the
resulting directivity (or gain, lossless model), where
circular polarization was achieved by the proper
combination of the two excitation ports. A maximum
directivity of 42 dB is predicted from this simulation.
This agrees with the results of the 8x8 prototype gain in
Fig. 9, when the aperture area is enlarged by a factor of
16 corresponding to an additional 12 dB. However, the
network losses will increase and a final gain of
approximately 41 dB in the measurement is expected.
realization, hopefully resulting in a successful market
introduction of such terminals.
5.
ACKNOWLEDGEMENTS
The authors wish to thank the German Ministry of
Education and Research (BMBF/BMWi/DLR) for
funding the projects SANTANA (I-II-III) and
KASANOVA under research contracts 50YB0101,
50YB0311, 50YB0710 and 50YB1024, respectively.
6.
Figure 10. Simulation model, including the two
waveguide distribution networks of the 32x32 elements
Tx antenna aperture
Figure 11. Simulated gain of the 32x32 elements Tx
antenna aperture
4.
CONCLUSIONS
In this paper two Ka-Band satcom projects from the past
and present have been presented. The project from the
past, SANTANA focused on highly complex DBFarchitectures. The overall realization has been presented
and details of the RF-part have been discussed. It has
been technologically one of the most challenging
antenna projects conducted at IMST. The other project,
KASANOVA, that is still on-going, aims at planar
hybrid solution with a medium complexity for nomadic
satcom applications. This antenna aims at the
combination of mechanically and electronically steering
using true time delays. Innovative waveguide
technology is applied here to arrive at a cost-effective
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