Direction Finding Antenna Systems
Radio direction finding systems are among the most established technical applications of antenna systems. By moving the signal processing from the analogue to the digital domain in the last decades, the requirements for the signal processor were drastically reduced in size and weight. With these new opportunities, novel applications arise for cooperative radio direction finding. These range from indoor localization scenarios using WiFi, over cellular communication systems, to small aircrafts like unmanned aerial vehicles (UAVs), for which established systems for passenger aircraft are too large and expensive. At the Institute of Microwave and Wireless Systems, these novel scenarios are addressed by using MultiModeMultiPort Antennas (M³PA).

Direction Finding Using Antenna Arrays
Antenna Arrays are groups of (in this case usually identical) antennas, whose outputs are combined in the analogue or digital domain. In radio direction finding, it is assumed that a plane wave is incident on the array under a certain angle of arrival (AoA) f. As illustrated below, the wave will arrive slightly earlier at some antennas of the array and slightly later at others. This time difference depends on the angle of arrival and can be measured as a phase difference β between the ports of the antennas. The AoA can then be calculated from the measured results by the formula given below.
By using more sophisticated algorithms like MUSIC [1], multiple plane waves incident on the array from different angles of arrival can be tracked simultaneously. Also, by using e.g. circular arrays, ambiguities existing in the example above can be resolved. Moreover, the angle of arrival can be extended to the direction of arrival (DoA), which also provides the elevation angle of the incident plane wave.
Direction finding using antenna arrays is a wellestablished method employed in many modern systems. However, this approach imposes specific geometry and size requirements on the antenna system, in particular with respect to the distance of the array elements d.

Direction Finding Using MultiModeMultiPort Antennas
With a MultiModeMultiPortAntenna (M³PA), the array consisting of multiple separate antennas is replaced by a single conducting structure, which can support several independent antenna ports. The application of this antenna concept allows for more flexible direction finding antenna systems. In principle, this approach allows arbitrary antenna structures. However, similar to communication applications, the utilization of symmetric structures allows orthogonal ports, which can significantly improve the performance [2]. As illustrated below, the idea is as follows [3]: The information on the direction of arrival is contained in the plane wave incident on the (say cubic) antenna structure. Once it reaches the antenna structure, the information is now in the current I induced by the wave on the surface of the structure. The theory of characteristic modes provides a way to decompose this current into modal currents I_{n}, each with a specific modal weight b_{n}. Therefore, the information on the direction of arrival is now contained in the modal weights. If a set of ports is defined in such a way that these modal weights can be determined, the DoA can be determined in turn.
Also, if the modal weights are known, these can be used to obtain an estimate of the original incident field [4]. Since the technical complexity of any receiving system is finite, the maximum number of ports and therefore also the maximum number of characteristic modes to be utilized for the direction finding is limited. Determining the set of modes which is best suited for a specific application is therefore a vital step in the design of a direction finding M^{3}PA [5].

Project Master360
The cooperative project Master360 aims at developing a system that allows the safe integration of autonomous/ unmanned aerial vehicles (UAV) into the airspace. In particular, detecting other aircraft is important so a safe distance can be maintained. As one component of this system, a cooperative communication unit is developed that also supports the determination of the direction of arrival of a signal send by another aircraft. The required antenna system is developed by the Institute of Microwave and Wireless Systems in close coordination with the project partners from industry and academia.
The subproject MAFuKo is funded by the Federal Ministry for Economic Affairs and Climate Protection under grant 20D1905C within the Luftfahrtforschungsprogramm (LuFo).

References
[1] R. Schmidt, “Multiple emitter location and signal parameter estimation,” IEEE Transactions on Antennas and Propagation, vol. 34, no. 3, Art. no. 3, Mar. 1986, doi: 10.1109/TAP.1986.1143830.
[2] N. Peitzmeier, T. Hahn and D. Manteuffel, "Systematic Design of Multimode Antennas for MIMO Applications by Leveraging Symmetry," in IEEE Transactions on Antennas and Propagation, doi: 10.1109/TAP.2021.3098610.
[3] L. Grundmann, N. Peitzmeier, and D. Manteuffel, “Investigation of Direction of Arrival Estimation Using Characteristic Modes,” in 2021 15th European Conference on Antennas and Propagation (EuCAP), Mar. 2021, pp. 1–5. doi: 10.23919/EuCAP51087.2021.9410924.
[4] L. Grundmann and D. Manteuffel, “Using Characteristic Modes for Determining the Incident Field in a Scattering Problem,” in 2021 IEEE International Symposium on Antennas and Propagation and USNCURSI Radio Science Meeting, Singapore, Dec. 2021, pp. 855–856.
[5] L. Grundmann and D. Manteuffel, “Selecting Characteristic Modes in MultiMode Direction Finding Antenna Design by Using Reconstructed Incident Fields,” in 2022 16th European Conference on Antennas and Propagation (EuCAP), Mar. 2022, pp. 15.
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