A tunable filter element comprises a resonator unit that defines a longitudinal axis, the resonator unit includes an inner conducting portion defining an inner shorting end along the longitudinal axis and an inner capacitive end opposing the inner shorting end, an outer conducting portion arranged around the inner conductor defining an outer shorting end along the longitudinal axis and an outer capacitive end opposite to the outer shorting end. The inner and the outer conductors maintain an annular gap there-between, and are coupled to form a shorting end at one end and a capacitive end at the other. The filter element further comprises a ferrite insert disposed between the inner and the outer conducting portions and substantially filling the annular gap, the ferrite insert being configured to receive a bias magnetic field in a direction substantially parallel to the longitudinal axis.