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Elevation Feed Network (EFN) for TerraSAR-L Earth Observation Satellite

For L-band (1.3 GHz) SAR earth observation satellite an Elevation Feed Network (EFN) has been developed by IMST. The module consists of two receive (one for horizontal and one for vertical polarization as power combiner (RxH and RxV) and one transmit network as power divider (Tx). Since a binary tree is utilized, one port has to be terminated to achieve a (1:7) divider or combiner. IMST has designed a stack of 3 networks on top of each other within a multilayer PTFE laminate. Each network consists of 7 Wilkinson couplers which are connected with stripline waveguides. The advantages of this configuration are:

  • One network realized on one substrate layer: a minimum number of transitions ensures excellent stability and balance behaviour,
  • Wilkinson couplers show best divider/combiner properties also in cases of asymmetry within the network or a fault of a Tx/Rx module,
  • high shielding and isolation can be achieved by using shielded stripline waveguides,
  • additional CFRP panels ensure a high stability and a proper thermal match between the multilayer stack and the Tile Structure.

Each network utilizes 50 Ohm stripline waveguides buried into two substrate layers of the PTFE material RT6002 from Rogers [. The thickness of one RT6002 layer is about 0.50 mm and the dielectric constant is 2.92. The ground strips are designed above and below the centre line and are connected by a via chain on the left and right side of the centre line. Ground to ground vias through all three cores connect the different ground levels within the EFN. The figures illustrate the cross view of the multilayer board. A top and button CFRP panel are added for mechanical and thermal stabilisation of the multilayer network.

Wilkinson coupler, inter-layer transitions and transitions to GPO and SMA connectors have been designed and optimised using IMST’s in-house software EMPIRETM, which is based on the 3D Finite Differences Time Domain (FDTD) method. In the simulation the multi-layer stack with the substrate RT6002, the bonding films and prepregs was included. Buried waveguides have been designed in stripline technique with via chains to suppress the excitation of parallel plate modes between the covering ground planes. An other important issue was the proper modelling of the SMD resistors, which have been assembled into cavities of each Wilkinson divider. The electrical behaviour of this element was optimised with respect to return loss at all ports and isolation between the output ports. The following properties could be achieved for the specified frequency band: return losses > 30 dB and isolation of output ports > 35 dB.

The circuit for the inter-layer transition needed for the two receive networks was designed with EMPIRE, too. Small matching circuits with steps in the centre line width were placed next to the transition to improve the electrical behaviour. The complete structure with all layers, the openings in the CFRP plates and the via holes are included in the simulation. This kind of transition is required to route the signal lines from the top divider network to the centre divider network, where the input and output connectors are contacted. Signal and ground strips as well as signal and ground vias are visible in this transparent drawing. Simulations result into return losses better than 40 dB from DC up to 1.6 GHz.

3D FDTD optimisation was also necessary at the transitions from the buried stripline waveguides to the input and output connectors. At the seven output ports GPO connectors with right-angle plugs have been used. Five pins of the connector have been mounted through the substrates and soldered from the backside. The four outer pins are ground connections, while the centre pin is joined with the signal line of the stripline waveguide. The connector footprint is recessed into a top and bottom cavity of the multilayer substrate. The figure illustrate this configuration in a cross view. Return loss is better than 35 dB up to 1.6 GHz.

At the three input ports 5-pins straight SMA connectors are utilized. The mounting technique is the same as described for the GPO connectors. The figures illustrate the cross-section of this configuration. Optimisation does also result in a return loss better than 35 dB. For GPO and SMA connectors ideal coaxial waveguides have been assumed, since the real inner geometry and materials are not known. From the literature the return losses of these connectors are expected to be about 25 dB.

The entire simulation and optimisation effort finally results into a multilayer Elevation Feed Network with a total size of about 120 cm x 20 cm x 0.6 cm. Radiation, susceptibility and thermal simulations (Fraunhofer IZM, Berlin) have been carried out, too. A couple of pre-development demonstrators have been manufactured at Cicorel in Switzerland, assembled and tested at IMST.

S-parameters measurements have been carried out at ambient conditions and in thermal cycling from –15 to 50°C. Moreover, EMC tests have been carried out: radiation and susceptibility measurements. The first networks, which have been delivered to EADS-Astrium Portsmouth, GB, succeeded in further environmental characterisation and have been integrated into its satellite instrumentation to perform system tests. The development of the L-band Elevation Feed Network was funded by the European Space Agency ESA-ESTEC, Noordwijk. The table summarizes the properties of the module, obtained at ambient conditions.

L-Band SAR Elevation Feed Network
Centre Frequency 1.258 GHz
Bandwidth 70 MHz
Return Loss > 25 dB
Insertion Loss (Tx) < -11 dB
Transmission Loss < -2 dB
Combiner Gain (Rx) > 6dB
Amplitude Flatness < 0.09 dB
Gain Balance < 0.16 dB
Phase Balance < 4°
Port Isolation > 32 dB
Cross Talk < -80 dB

Artwork of TerraSAR-L Satellite,
Courtesy and Copyright of EADS-Astrium


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