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The mutual coupling between very low frequency (VLF) antenna elements is an important factor affecting the radiation performance of umbrella antenna arrays. This study evaluates the factors influencing the mutual coupling between the elements of an umbrella antenna array. We develop a mutual coupling analysis method for calculating the input impedances of a VLF antenna based on the impedance effect of mutual coupling. The radiation resistance of the VLF umbrella antenna can be obtained using numeric integral from Method of Moments (MoM) solution. Using the FEKO simulation software, a model of a trideco-tower umbrella antenna array is established. The electrical parameters of the VLF umbrella antenna array on inhomogeneous ground are calculated for both single and dual feeding modes. The impedance characteristics of the umbrella antenna arrays are also simulated for different array inter-element spacings on homogeneous ground. Representative numerical results are reported and discussed to assess the mutual coupling effect of the proposed method in comparison with full-wave simulations.

A MOMENT-BASED STUDY ON THE IMPEDANCE EFFECT OF MUTUAL COUPLING FOR VLF UMBRELLA ANTENNA ARRAYS