ELECTROMAGNETIC INVESTIGATION OF ANTENNAS ON PLATFORMS
When designing naval vessels, numerous parameters have to be considered already in the planning phase to ensure that the planned radio, radar and navigation systems do not interfere with each other when in operation. The simulation tools developed at FHR play an important role in the investigation of the proper functionality of each antenna system prior to installation to ensure that mutual interference can be eliminated to the greatest extent possible.
Modern vehicles, aircraft and ships are equipped with a growing number of antennas and sensors for a variety of navigation, localization and communication tasks. Planning the positions of antennas on maritime vessels is a difficult task as numerous antenna systems compete for the best position (the higher the better) in the restricted space available. The positioning of an antenna at a specific location can also have an influence on the design of the vessel.
The frigate F125 of the Federal German Navy, which is currently in the design phase and is scheduled to be put into service in 2016, serves as a good example in this respect. Due to the changed operating conditions, a new design, which is optimized for the special requirements of peace-stabilizing missions, is now being realized. The integration of two IFF subsystems (Identification Friend-Foe) with rotating antennas located on the front mast and on one of the rear decks is just one of the special features. To ensure that these IFF antennas function correctly, it is essential that the antenna patterns in the assigned operational areas are not unduly influenced by the superstructures as detected targets, for example, might otherwise be assigned incorrect positions. For this reason, influences the ship's geometry may have on the antenna pattern must be investigated and, if necessary, the position of the antenna must be optimized.
In the planning phase of a procurement project, the industry commissioned FHR to carry out an investigation. FHR was provided with a simplified geometry model of the frigate (Fig. 2) together with an IFF antenna pattern which was measured by the manufacturer. An equivalent antenna model with 84 dipoles was created for the simulation using a so-called numerical exact method. As the excitation coefficients of these dipoles could not be obtained from the manufacturer, these were determined through a special optimization process in such a way that the measured antenna patterns could be reproduced as accurately as possible in free space. This geometry model was subsequently integrated into the CAD model of the frigate. The mutual interference between the antenna and the ship's geometry is clearly visible in the calculated surface currents (illustration).
To establish clarity on the extent to which the antenna patterns are influenced by the ship's geometry, consideration must be given to the fact that this is a system which, in part, comprises rotating components. To retain the computation times within reasonable limits, FHR and the project team F125 selected a number of meaningful antenna positions that should be investigated for the front and rear IFF system. The radiated antenna pattern for each of these antenna positions was simulated taking account of the ship's geometry and the results were evaluated in three-dimensional space (Fig. 1). To guarantee the proper functioning of the antenna, the difference between the two antenna patterns (sum and side lobe suppression pattern) radiated by the IFF antenna must, with the exception of the main lobe (in Fig. 1 at azimuth angle 240°), be less than zero, i.e. outside the main lobe the sum pattern must lie below the SLS pattern. Individual angles that do not meet this requirement may be tolerated. It should also be noted that only a specific elevation range is relevant during the evaluation as the water surface is hit when using small (negative) elevation angles and very large elevation angles are not relevant because possible targets can be detected and identified in advance, i.e. at a large distance (corresponding to smaller elevation angles) while they are approaching.
Within the framework of the study, evaluations – as shown in Fig. 1 – were carried out systematically for the front and rear IFF system. Using this data and the determined system incompatibilities, the position of the antennas, and, in particular, the position of the rear antenna, was optimized to such an extent that interference with other systems was suppressed to the greatest possible extent. Furthermore, angular ranges which meet the primary IFF system requirements in accordance with the relevant STANAGs were also derived or verified in the course of the study. It was clearly demonstrated that investigations of this kind are meaningful and purposeful measures in the areas of vehicle design and antenna integration, and that the findings of the study can be utilized, above all, in the evaluation of difficult integration aspects.