How to design Frequency Selective Surfaces (FSS) in Altair Feko

ChristophM
ChristophM
Altair Employee
edited July 2023 in Altair HyperWorks

Frequency selected surfaces (FSS) are periodic surfaces with identical two-dimensional arrays of elements arranged in a multilayer dielectric substrate. They form a 3D-filter for incident electromagnetic plane waves and transmit or reflect them for different frequency ranges.

A typical example is the sub-reflector design of a Cassegrain dual-reflector system.

 

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The Figure above shows the gain pattern of such a dual antenna system: The horn at the top is the feeding antenna at 9.5 GHz (X-band) and the smaller horn near the main reflector  works at 35.75 GHz (Ka-band). Therefore, the sub-reflector of the antenna system is designed for transmission in the X-band and reflection in the Ka-band.

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The design is built from a four-legged shape that is embedded in a dielectric. The cartesian graph of the transmission (blue curve) and reflection coefficients (red curve) show the desired behavior.

There are many shapes and multilayer options to design a FSS. The key to FSS-design and optimization is using parameterized models for simulation with periodic boundary conditions (PBC). With the compact PBC models in Feko [1] the computation times are low and therefore the frequency-dependent behavior can be computed efficiently. This is also a door-opener for the application of optimization techniques, that are available in OPTFEKO or in HyperStudy [2] .

If you want to learn more about FSS design strategies you may look into Munk’s well-known textbook [3], which gives a nice overview on the topic.

Workflow for FSS design 

The workflow to evaluate and optimize frequency selective surfaces has improved in recent CADFEKO versions:

A new Periodic Structures extension has been added to CADFEKO2022.2. This extension supports the preparation of frequency selective surfaces (FSS), radome’s and  radar absorbing material (RAM). The new shapes, geometry and unit-cell tool can be used to quickly define simple or complex parametric unit-cell representations of multi-layer FSS structures for material characterization and transmission analysis using a periodic boundary condition approach.

The TR-coefficients of the PBC-simulation can be exported and applied directly as characterized surfaces to faces of complex 3D models. To solve such large problems efficiently (like the Cassegrain dual reflector) the characterised surface approach has been extended in Feko2022.2 from RL-GO to MoM/MLFMM. This provides an extremely powerful, flexible, and efficient approach for FSS-, radome- or RAM-analysis.  

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A detailed step-by-step workflow for the setup of characterized surfaces for MoM/MLFMM in CADFEKO is described in [4].

The Cassegrain dual reflector antenna model of this example has 159.385 triangle elements in the X-band and is solved with MLFMM in 103 seconds using 10.8 GB memory. In the Ka-band the model is electrically much larger with 2.189.807 triangle elements solved in 2387 seconds using 130 GB memory. Both simulations use 24 parallel processes on 2 CPUs.

The near-field results of the Feko simulations show the principal behavior of wave propagation in the two frequency bands. In the X-band, the FSS-subreflector is transparent, while in the Ka-band it is fully reflective.

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