Demonstration of 124 deg/dB phase tuning at 30 GHz for a bulk ferroelectric beam steering device

We report the microwave characterization of a bulk ferroelectric beam steering device. The device consists of a 1 mm thick BST core tile with ceramic quarter wave transformer plates on both sides. Electrodes and edges of the device were encapsulated in silicone rubber to avoid break down at high DC fields. A horn-to-horn transmission set-up was used to measure the phase tuning and insertion loss. By the application of a DC-field ranging from 0-15 kV/mm, we measured a phase tuning of 0-386 degrees at 30 GHz. The corresponding insertion loss was 5.8 dB at zero DC-field and dropped to 2.1 dB at 15 kV/mm. Using a 8 kV/mm DC-field as a bias point, which is relevant in the center of the antenna in our beam scanning application, we get a figure of merit of 124 degrees/dB. We find that the permittivity is frequency independent in the range 25-40 GHz, and that the loss tangent is proportional to the frequency. When decreasing the field back to zero we found that the permittivity remained 12% lower than initial measurement. The origin of this memory effect is at present not well understood, but among possible explanations we see self-heating, trapping/de-trapping of charges and ferroelectric inclusions.