Active phased array antennas must comply with stringent requirements in terms of sensitivity to interference caused by other nearby radiating systems, especially in complex platforms, which are populated by an ever-increasing number of sensors and communication systems. Radio frequency interference can cause a number of issues, e.g., saturation of the array receiver, with consequent loss of sensitivity and missed detections, increased false alarm rates, or a reduced channel capacity for telecommunications. This performance degradation can be prevented by implementing frequency selective functions in the antenna panel and RF front end. For this purpose, filters can be inserted in the transmit/receive module of the individual antenna elements. Their function is to isolate the frequency band of the system thus making the antenna unable to transmit/receive a signal that can be received/transmitted by a neighbouring device. The main challenge in designing filters for wide scanning phased array resides in the limited available space of a radiating element which usually occupies an area of only λ/2 × λ/2 (half-wavelength-squared). Implementing miniaturized filters with acceptable performance (low loss and high rejection) is an open research problem.