Coupling prediction of HF antennas mounted on helicopter structures using the NEC code

The moment method-based NEC code is a well-known and widely-used low frequency code. We apply the NEC code to complex structures at high frequencies. At HF, the electrical size of the structural elements become very small, compared to the wavelength. Moreover, these small elements are necessary to represent the geometrical complexity of a structure so that they cannot be ignored in geometry modeling. Thus, at HF, the NEC code has to deal with electrically small elements near its lower limits, which requires careful geometry modeling to minimize numerical inaccuracy. In addition, the prediction of antenna coupling involves near-field calculations that need extra care during the computation process. We discuss geometry modeling guidelines for complex structures. Using these rules, model two HF antennas mounted on an Apache helicopter airframe. We then analyze the coupling effects between the two antennas in terms of radiation patterns, maximum coupling factors and input impedance.