Callisto | Consultancy: Antenna Simulation

Callisto has a contract with ESA to provide consultancy in the simulation and analysis of large reflector antennas for ground stations.

The consultancy tasks concerned the design and analysis of large diameter antennas by means of techniques based on geometrical optics (GO), geometrical theory of diffraction (GTD), physical optics (PO), physical theory of diffraction (PTD) and Gaussian beam theory approximations. Commercial software such as TICRA’s GRASP™ and OSU NECREF™ are extensively used to simulate the key parameters of antennas performance such as gain, beam squint and near/far field radiation patterns. Specific in-house codes have been written to optimise performances by shaping the Cassegrain reflectors and appropriately dimensioning the beam waveguide mirrors and dichroics (when present).

Figure 1: diag1 GO rendering of a 35m deep space beam waveguide antenna. Extensive PO analyses of ESA’s Deep Space Antenna 2 (DSA2) Beam Waveguide were undertaken in order to validate contractors’ design in preparation of the design review. Various configurations of the DSA2 main and sub-reflectors have been synthesized in order to increase the performances of the system. The sub-reflector has been reduced and various aperture amplitude distributions have been considered. Several GRASP simulations have shown that a reshaping could improve the performances of the antenna.

Antennas affected by load cases such as gravity, wind and thermal gradient have been also analysed based on mechanical inputs, in order to fully model their behaviour in non-ideal conditions and to select the most appropriate gravity alignment position. The specific case of ESA’s DSA2 antenna has been undertaken to analyse the effect of mechanical and thermal loads on the system phase stability.

Figure 2: 12m VIL-4 antenna far field sampled on a sphere. It is often infeasible to perform far field measurements of electrically large antennas due to the lack of a test source in the far field of the antenna. However, it would be useful to estimate the far field pattern from near field measurements.

The purpose of this activity was to analyse the feasibility and accuracy of estimating the far field pattern from near field data. A specific example of a 12m antenna (VIL4 in Spain) was used. This antenna is being upgraded to work at Ka-Band. The analysis has shown that a slight defocusing of the sub-reflector leads in fact to a near field pattern almost identical to the far field one.

Figure 3: 35m deep space antenna: effects of four sub-reflector struts on the main reflector currents. A holography study is also being carried on with the purpose to derive panel and sub-reflector misplacements of an antenna from its measured radiation characteristics. This could then lead to improvements to system performance by readjusting any eventual misalignment.