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Active SAR Antenna for Airborne 56-Channel Operation
The Mini-Circuits Student Fellowship was awarded to Y. Venot,
M. Younis and W. Wiesbeck at the 30th European Microwave Conference
for this paper, which describes a new system implementation of an
active antenna for a forward looking airborne synthetic aperture
array radar (SAR) with digital beamforming on-receive
only. The
system uses 56 receive antennas and one transmit antenna, and a
simplified hardware configuration is achieved by switching through
the receive array, thus sequentially recording return signals from
each receive antenna.
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The principle of SAR is to use the aircrafts' motion to get the
synthetic aperture resolution in the azimuth. Multifrequency operation,
wide bandwidth, polarisation agility and multiple operation modes
are the common techniques. To control the amplitude and the phase
of the various antenna
elements, electronic T/R modules are employed.
For scanmode operation i.e. analogue beamforming is applied to
the antenna to sharpen the beam. However there is a significant
loss of information due to the reduced spot beam coverage. Other
disadvantages of the complex T/R modules are the high weight, the
high price, the high power consumption and the noise figure [1].
These main disadvantages are
eliminated in the design of the active
antenna presented in this paper. Using digital beamforming (DBF)
on receive allows focusing on the stored data without any loss of
information within the illuminated area [3,4].
The interface of the new antenna system allows operation with the
conventional onboard radar equipment of most aircraft with only
small modifications. The patent is held by the DLR, Oberpfaffenhofen,
Germany [2].
Innovative future operation
The most significant improvement is to shift the conventional analogue
beam- forming using T/R modules to the digital processing of the
received data. By this the side looking SAR capability will be of
higher performance compared to common systems. The active antenna
even allows forward looking modes by switching the receiving elements
sequentially.
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Figure 1: SAR Image of the "Lechschleife"
near Munich.
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This antenna will be particularly
suitable for helicopters in hovering
flight in poor visibillity. Also it is possible for the antenna
array to be integrated into the airfoil of all aircraft and therefore
will be suitable to equip all aircraft in future. The first measurements
with this new SAR antenna system have been performed on a helicopter
of the DLR, Oberpfaffenhofen, Germany. Figure 1 shows the first
forward looking imaging result. It is a
famous part of the"Lech"-river
near Munich called "Lechschleife". The white point in the middle
of the image is a corner reflector.
The antenna concept
For transmit a high power horn antenna for illumination with the
radar pulse is mounted in the middle (see Figure 2) of the antenna
system. For receive, a linear array of 56 horizontally polarised
aperture coupled
microstrip 2x2 subarrays are built. The centre
frequency and bandwidth are 9.55GHz and 400MHz, respectively. The
56 receiving subarrays are connected to one common output via a
switch matrix. This digitally controlled matrix is driven by differential
TTL-signals. The switching matrix is realised by cascaded single-pole-dual-throw-switches
(SPDT). To reduce the noise figure (NF) and to achieve high efficiency,
low noise
amplifiers (LNAs) are placed after the first SPDTs to
amplify the received signal. In this way one amplifier is used for
two subarrays. The matrix driving signals, selecting one of the
56 channels, are available in form of six bits at one bus-connector.
The total length of the whole antenna is 3m and the RF and control
units are placed on a mechanically stable aluminium carrier. To
provide
functionality even at very low temperature values i.e. at
high altitude, a heating system is implemented on the aluminium
carrier. For protection the antenna is covered with a PVC pipe forming
the radome of the antenna.
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Figure 2: The antenna system (pipe on bottom)
fixed at the skids of the DLR-helicopter.
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Figure 2 shows the antenna fixed at the bottom of the DLR helicopter.
The high power transmitting horn antenna is placed in the centre
above the pipe and is covered with a separate radome. Its thickness
is optimised for
an optimum match. The radiation is inclined 45ý
from the horizontal to the ground and the antenna is looking in
the forward direction.
Principle of switching the receiving patch antennas
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Figure 3: Block diagram of one RF unit
representing an 8:1 matrix.
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The antenna system consists of 56 planar patch subarrays, eight
of which are placed on a single unit (see figure 3). The switching
is realised in four levels. The first three levels are realised
by seven
equal RF units. Every unit is switching eight receiving
channels to one common output. Finally the output signals of the
seven 8:1 receiving subsystems are collected with one central SP7T
forming the fourth switching level to perform the 56 channel decoding.
The received data is then amplified in a final stage.
As mentioned above the 56:1 matrix is made of seven 8:1 RF units.
Figure 3 shows the concept of such a RF
unit-made of seven switches,
four amplifiers and the patches. Three levels of SPDTs are required
to switch one of the eight 2x2 receiving patches to the RF output.
The first level of SPDTs is followed by the LNAs (15dB gain, 2.5dB
noise figure) to minimise the total noise figure. By this way a
total noise figure of 5dB is obtained considering the degradation
due to the losses of the switches (2.5dB) and the microstrip feed
network.
The transmitting horn antenna
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Figure 4: Azimuth painting of the transmittig
horn antenna
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The transmitting horn antenna is centred along the antenna and
placed out-side the radome. The horn antenna was designed to yield
the required beamwidth of 40ý in elevation and 38ý in azimuth. The
horn is protected by a radome placed in front of it: the thickness
of the radome is optimized to yield an optimum match. The radiation
pattern of the antenna is shown in Figure 4, where it can be seen
that the radome has negligible effect on the pattern within the
main beam but causes a degradation of the sidelobe level of about
4dB.
RF receiving components
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Figure 5: RF unit (single sub-element),
aluminium carrier, active layer with feed network SPDTs and
LNAs, passive layer with patches.
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Figure 5 shows the practical realisation of one RF unit. A RF unit
is a sandwich construction consisting of different layers fixed
on a 3m long aluminium carrier. First there is an aluminium base
plane to support
the following layers. The active layer is built
on a 0.504mm RO4003 substrate with the antenna feed network and
the active components (SPDTs and LNAs). Also the control and power
supply lines are printed on this side. The layer is placed upside
down on the aluminium base plane, the coupling slots being in the
ground plane. The next layer is the passive one with the 8 2x2 patch
subarrays printed on kapton foil. These
three elements are stuck
together to form one block. The seven RF units are fixed on one
side of the carrier and on both ends of this carrier one passive
2x2 patch subarray is placed to generate equal conditions for all
elements.
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Figure 6: Detailed view of the RF unit
active layer, feed network with SPDTs and LNA.
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Figure 6 shows a detailed view of the practical realisation of
the active layer. The space for the SPDTs and the LNAs is milled
into the substrate and the components are stuck into this place
in a flipchip technology. The digital signal lines are also visible
in this figure. To drive the RF unit a corresponding control unit
is placed on the other side of the carrier and connected to the
RF unit via flat cables to achieve a compact design.
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Figure 7: Central SP71 collecting the seven
output signals of the RF units.
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To select one of the seven RF units one central single pole seven
through switch (SP7T) is placed on the bottom side of the carrier
in a centre position. The SP7T is a pin diode switch including the
channel drivers, thus the switch can be directly controlled by TTL
signals. The required interface to decode the seven channels out
of the three incoming bits is placed next to the SP7T. To amplify
the RF output signal, a final amplifier of 15dB is connected to
the output of the switch. Figure 7 shows the SP7T and the coaxial
SMA connections of the seven RF units and the RF output.
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Figure 8: Azimuth pattern of 4 receiving
subarrays at 9.55 GHz.
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To evaluate the performance of the receiving array without the
contribution of the ctive components one unit was built consisting
of 8 passive subarrays, where each subarray is formed by combining
four patch elements through a 2x2 feed network. The normalized azimuth
and elevation radiation patterns of the subarrays placed on the
carrier and inside the radome are shown in Figure. 8. It can be
seen that the azimuth pattern shows a ripple over the scan angle;
further measurements show
that the ripple is caused by reflections
on the metal carrier used for mounting the antenna and not caused
by the radome. As long as the peaks of the ripple do not occur at
the same angle for all elements its contribution is insignificant
for practical purposes.
The transmitting power of 63dBm requires a high decoupling S
Re,Tr
between the transmitting antenna and the receiving
elements
in order to avoid damage of the active components.
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Figure 9: Decoupling receive-tramsmit.
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The
maximum input power overdrive for the active components is
specified to be 15dBm. Allowing a 3dB safety margin the decoupling
must be in the order of 51dB
Figure 9 shows the measured value of S
Re,Tr
for the optimum position of the horn antenna. It can be seen that
a value of 55dB is achieved which lies within the calculated margin.
The control unit
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Fig. 10: control unit with power supply,
switching signal generation and heating system on one compact
board
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Each RF unit has
its corresponding control unit to generate the
required power levels and the SPDT driving signals. The control
units are buffered and connected to each other to form a compact
and fast driving system. The subarrays can be switched up to 50kHz
without anydegradation of the driving signals over the whole antenna
length. For stabilisation a heating system is integrated in the
control unit. Figure 10 shows a control unit
placed on the bottom
side of the aluminium carrier.
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Fig. 11: Antenna system without the PVC
pipe, power-, RF-and data connector
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The driving unit can be divided into different parts. The first
part is an analogue generation of very precise negative and positive
voltage to drive the transistor gates of the SPDTs. The second part
is the decoding of the switching matrix. This part is realised in
digital CMOS technology. The circuitry is required for the multiplexing
of the switches. The third part of the
control unit is the heating
system. The heating system ensures functionality even in important
heights of flight making the system environmental stable. The power
module inside the pipe is able to handle the common supply voltages
in airplanes typically 28V DC and the current does not exceed 1A
without heating.
Figure 11 shows the complete antenna system without the PVC protection
pipe. A view on the RF side
with the 56 2x2 patch subarrays without
the radome is presented.
Conclusion
In this paper a new active antenna concept in X-band has been presented.
The system is based on a highly modular concept, making it suitable
for different kinds of applications. It can be easily translated
to other standard SAR frequencies. The performance of the sub-arrays
out-perform state-of-the-art SAR antennas.
Compared to RF beamforming
(T/R modules) a significant improvement in the cost, weight, power
consumption, noise figure and coverage is achieved. The effort in
the hardware is shifted to digital processing of the received data
and of course it is envisaged that it will ultimately interface
with the A/D converter closer to the antenna. The forward looking
capability makes the system suitable for integration in airborne
or spaceborne systems, especially for hovering flight in helicopters.
Acknowledgments
We acknowledge the support and cooperation with Dr. Alberto Moreira
and Dr. Wolfgang Keydel, DLR
References
[1] R.Zahn: "Hardware Developments
for the German Smart SAR Program", 1998 IEEE International Geoscience
and Remote Sensing
Symposium Proceedings, Seattle, Vol. 3, pp. 1701-1703
[2] DLR, "Vorwýrtssicht-Radar", Patentschrift: DE 40 07 611 C1,
Deutsches Patentamt, March 1990
[3] M. Younis, W. Wiesbeck, "SAR with digital beam-forming on receive
only", Proc. International Geoscience and Remote Sensing Symposium
IGARSS 99, Hamburg, Germany, June1999, pp. 1773-1775
[4] T. Sutor et al., "SIREV: Sector imaging radar for enhanced vision",
Proceedings Eusar 2000, Munich, Germany, May 2000, pp. 357-359
The system was built under contract from the
DLR, Oberpfaffenhofen, Germany.
Y. Venot, M. Younis, W. Wiesbeck.
Institut fýr Hýchstfrequenztechnik und Elektronik, Kaiserstr. 12,
76128 Karlsruhe, Germany
Tel: +49 (0)721 608 6252, Fax: +49 (0)721 691865, E-mail:
yan.venot@etec.uni-karlsruhe.de
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