Thanks to their versatility, smart antenna arrays are used in a wide variety of contexts, including, in particular, coastal monitoring. Within this framework, operating in multi-frequency modality might reveal a fundamental requirement. So, in this paper the problem of synthesis of multi-frequency antenna arrays is addressed. Precisely, a fixed grid array is considered, having an arbitrary but known geometry. The excitation vectors are optimized in such a way that the radiation patterns belong to a prescribed mask at certain assigned frequencies. Moreover, in order to have a simpler, cheaper and more efficient antenna, the phase-only synthesis is performed. In fact, thanks to this requirement, variable attenuators are not necessary and only phase-shifters are used in the feeding network. Numerical examples are provided, which validate the effectiveness of the method.
