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Optically pumped gradiometers have emerged as a promising tool for measuring extremely weak magnetic fields generated by nearby bio-magnetic sources, providing remarkable advantages such as high sensitivity, compact footprint, and the ability to operate in unshielded environments. However, existing gradiometer configurations often employ fixed baseline distances, which hinders the optimization of parameters such as baseline distance when dealing with magnetic sources of different sizes. Moreover, there is a lack of universal conclusions in current research regarding the optimal selection of baseline distance, measurement distance, and other parameters for various magnetic source types. To address this issue, we construct a dual-channel spin-exchange relaxation-free (SERF) gradiometer with an adjustable baseline ranging from 5 mm to 60 mm to investigate the relationships between the baseline distance of the gradiometer, the size of the magnetic field source, and the measurement distance, as well as their impact on the signal-to-noise ratio (SNR). By employing circular coils of different radii to simulate magnetic field sources, we measure the SNR of the gradiometer under the distance ranging from 40 mm to 95 mm and present normalized SNR curves that illustrate the relationship between the baseline distance, field source radius, and measurement distance. To ensure universal applicability, the specific distances are converted into multiples of the source radius. The results demonstrate that positioning the gradiometer closer to the source enhances its SNR, regardless of the source size. However, the optimal baseline distance varies depending on the source size, with smaller sources requiring relatively longer baselines to achieve better performance. We believe these findings can offer reliable evidence for optimizing gradiometer configurations in bio-magnetic measurements and other applications involving sources of different sizes
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