1. 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)

2. 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

3. 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)

4. SAR: synthetic aperturex0
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)

5. 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)

6. 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

7. Patch design
W2x
W1x
A0x Ls
A0y Aw Aw

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

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

10. Array setup
patch_offset

11. 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).

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

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

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

15. 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.

16. Antenna assembly
layout

17. 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.

18. 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.

19. 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.

20. 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

21. 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)

22. HFSS complete antenna model
The model is electrically large and complex

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

24. HFSS complete antenna model
Domain Decomposition Solver Profile
8 Domains

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

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

27. HFSS complete antenna model

28. 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.

29. Thank you