Long baseline hardware gradiometer based on HTS rf-SQUIDs with substrate resonators

We have developed an asymmetric thin film first order gradiometer based on an HTS rf-SQUID. A coupling scheme uses an SrTiO₃ substrate resonator as an rf-tank circuit which simplifies the gradiometer balancing. Advantage of the technique is that gradiometers with very long baseline can be fabricated. The performance of 13 mm and 33 mm baseline SQUID gradiometers is presented. The gradiometers have been successfully used in applications for nondestructive evaluation of materials.