Frequency-Tunable Slot-Loop Antenna Based on KNN Ferroelectric Interdigitated Varactors

A miniature frequency-tunable slot-loop antenna (0.128λ ₀ × 0.077λ ₀ ) based on ferroelectric K _0.5 Na _0.5 NbO ₃ (KNN) interdigitated varactors is investigated at Ku-band. KNN material has been selected among the ferroelectric oxide family because of its competitive properties for microwave applications. Its high permittivity value (ϵ _r ≈ 360 at 10 GHz) and its wide frequency tunability under an external static electric field E _bias make it a promising candidate for frequency-agile antennas and circuits at microwaves. The antenna prototype has been designed and elaborated on MgO substrate (ϵ _r = 9.8 and tanδ ≈ 10 ^-4 at 10 GHz). Fabrication details including laser microetching process of the ferroelectric layer are also described. The center frequency of the slot-loop antenna can therefore be tuned monotonously from 15.22 to 15.97 GHz under E _bias = 88 kV/cm, leading to a frequency tunability rate equal to 4.6%. These results demonstrate for the first time the ability of the KNN material to be used for miniature tunable antennas, in comparison with Ba _x Sr _1-x TiO ₃ , currently considered as the most relevant material for such applications.