Analysis and Mitigation of Co-site Interference Problems on Aircraft

Modern aircraft, especially in military applications, are equipped with dozens of transmitting and receiving antennas for communication and radar systems, altimeters, and more. Even if these systems operate at different frequencies, interference can occur due to out-of-band transmission and reception, the presence of harmonics, intermodulation products and more. The problem is exacerbated when systems can operate over a band of frequencies instead of at one fixed frequency, which is an aspect that often doesn't receive the attention it deserves in publications. This paper will show, for a realistic large aircraft, how electromagnetic simulation can be employed to detect, understand and mitigate co-site interference, including when antennas are capable of operating over a band of frequencies.A full-wave 3D electromagnetic simulation based on the geometries of the aircraft and the installed antennas can determine coupling between the antennas, expressed as S (Scattering) parameters. This needs to be done over a wide range of frequencies. A few methods will be briefly presented. The S-parameters are subsequently imported into a system simulator that includes many properties of the transmit and receive systems: the transmission and reception spectra, transmitter harmonics, receiver noise floor, required system-to-noise-and-interference ratio and other properties. The system simulator will report any potential conflicts and provide the reasons for those. The final step is to investigate how to mitigate the problems, e.g., by the addition of filters, a change in relative position of the antennas, the avoidance of certain frequencies within a band or avoidance of simultaneous operation of certain systems.