Coastal monitoring represents a key element for the support of many sea-related human activities, including ship navigation, national security prevention, and environmental protection. Within these fields, antenna arrays are recognized as viable candidates for the development of wide-area monitoring applications, thanks to the possibility of using multifrequency (MF) radiating systems as reflector feeds or as primary antennas. For this reason, this article proposes a deterministic synthesis method for MF antenna arrays allowing the independent pattern control at different frequencies. Precisely, the optimization of the array excitations is realized to provide a unique set of amplitudes, common to all frequencies, and $S$ sets of phases, corresponding to the $S$ considered frequencies, thus enabling phase-only pattern switching. The method, which is based on the alternating projection approach, allows also to impose an upper bound on the ratio between the maximum and the minimum amplitude, hence providing advantages in terms of feeding network simplification and energy efficiency, while maintaining satisfactory beam and null pointing capabilities. Numerical examples, including electromagnetic simulations, are presented to check the effectiveness of the conceived method for implementation in oceanographic acquisition systems.
