Efficient design of a C-band aperture-coupled stacked microstrip array using Nexxim and Designer Alberto Di Maria German Aerospace Centre (DLR) – Microwaves and Radar Institute – Oberpfaffenhofen (DE)

Outline Synthetic Aperture Radar (SAR) Antenna specifications range resolution synthetic aperture and azimuth resolution image retrieval Antenna specifications Patch design Array setup Feeding networks Antenna assembly feeding networks optimization Solver-On-Demand configuration refining the model HFSS complete antenna model Conclusions

Synthetic Aperture Radar (SAR) frequency W t phase range range resolution height (z) range resolution cell v range ground-range (y) swath Courtesy of Matteo Nannini azimuth (x)

SAR: synthetic aperture x0 SAR: synthetic aperture azimuth resolution height (z) r(x) Synthetic Aperture x chirp x0 r(x) ground-range (y) Courtesy of Matteo Nannini azimuth (x)

Synthetic Aperture Radar (SAR) image retrieval height (z) A Synthetic Aperture Radar (SAR) system allows the retrieval of reflectivity images of the observed scene with high spatial resolution. SLC Nominal Resolution: 1x1.5 m E-SAR (L-band) 1998 range (y) azimuth (x)

Antenna specifications Carrier frequency 5.3 GHz (C-band) Frequency range 5.05 GHz – 5.55GHz Bandwidth 500 MHz (up to 800 MHz desirable) Polarization Dual linear polarization (h, v) Geometry Planar Power 1.5 kW (peak) Gain 17 dBi min Input adaptation (S11) > 10 dB for 5.05 GHz – 5.55GHz > 12 dB for 5.1 GHz – 5.5GHz Azimuth beam width (θ3dB) 12 deg ± 1 deg Azimuth Side Lobe Level (SLL) > 15 dB Elevation (range) beam width (θ3dB) 34 deg ± 2 deg Elevation Side Lobe Level (SLL) Crosspolarization insulation ≥ 25 dB Critical requirements for SAR are: Bandwidth Azimuth beam width

Patch design W2x W1x A0x Ls A0y Aw Aw

Patch design impedance matching The impedance matching has been optimized for bandwidth. S11 < -20dB over the bandwidth.

Patch design radiation pattern Patch antenna gain = 8.5 dB Front to back ratio = 20 dB

Array setup patch_offset

Array setup radiation pattern Patches distance: optimized to obtain the required beamwidth (12 deg). Feed voltage tapering: using Dolph-Chebyshev to obtain the required SLL (-20dB).

H V Feeding network schematic Special components are created in Designer and inserted in Nexxim. V

Feeding network layout Special components are created in Designer and inserted in Nexxim.

Feeding network A full parameterization and the use of Position Relative function assure the layout consistency over the variables variations.

Antenna assembly schematic The array of six patches and the two feeding networks are combined together in a top level circuit in order to create the entire antenna.

Antenna assembly layout

Antenna assembly Feeding network optimization As the parameterization is retained through the hierarchy, it is possible to set-up optimization of the feeding networks directly in the top level circuit. Ottimizzazioone tiene in considerazione il carico reale (i patches) e usa la velocità di una simulazione circuitale.

Solver-On-Demand configuration The Solver-On-Demand lets you choose the simulation engine. Then each antenna part can be simulated either as circuit or with full wave analysis.

Solver-On-Demand refining the model – step 1 Here a full wave analysis has been done. But the three components (feeding networks H and V and the array) are still independent i.e. no mutual coupling between them is considered.

Solver-On-Demand refining the model – step 2 8dB Simulation shows that, when the entire antenna is simulated at once, the matching is up to 8dB worse than what we obtained with optimization at the circuit level. What can be done? 8dB

Solver-On-Demand Simulation shows that, when the entire antenna is simulated at once, the performance is up to 8dB worse than what we obtained with optimization at the circuit level. What can be done? Through Solver-On-Demand (as the parameterization is retained) it is possible to optimize again the antenna matching, running the simulation with PlanarEM engine. The entire antenna can be exported in one click to HFSS, retaining all the variables and parameters. The final optimization can be run in HFSS. Other elements can be taken in account (connectors, screws, vertical elements)

HFSS complete antenna model The model is electrically large and complex

HFSS complete antenna model Distributes mesh sub-domains to networked processors and memory

HFSS complete antenna model Domain Decomposition Solver Profile 8 Domains

HFSS complete antenna model Field Animation: Antenna, feed and enclosure Vertical Polarization

HFSS complete antenna model Field Animation: Antenna, feed and enclosure Horizontal Polarization

HFSS complete antenna model

Conclusions The design of an array antenna suitable for an airborne SAR system, operating at C-Band has been presented. Every antenna component has been designed separately with a fully parameterized model and has been easily tuned and optimized with Designer, to meet the dimensional and frequency requirements. The individual components are then assembled and interconnected in the Nexxim circuit simulator to form the entire array. The assembly is tuned and optimized using the speed capability of Nexxim. The Solver-On-Demand feature lets us choose which part of the antenna will be solved with a full wave analysis and which part will be solved with a fast circuit simulation. The entire antenna has been then exported in HFSS. The final tuning is hence done in a very detailed model.

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