Unmatched delay line NMR probes for magnetic resonance

In magnetic resonance experiments the delay line probe is useful for its broadband impedance match and short ring down times. However, it is usually difficult to construct, and less sensitive than resonant probes. Additionally, the delay line has been widely misused in a manner that further degrades its performance due to an interference effect. We report that significant improvements in signal-to-noise ratio (∼2x) are available when the probe is used with an unmatched termination. This improvement is experimentally demonstrated using both an ¹H NMR signal in a variable electromagnet and with typical "sniffer" coil measurements that characterize B₁. A quantitative comparison is made with both resonant and bare-coil ("no-tune-no-match") probes. A novel "echo" type signal is observed in the delay line free induction decay under certain conditions. To alleviate the difficulty of construction we describe an approach using printed circuit board fabrication methods. A PCB delay line probe operating from 4-400 MHz is realized for a magnetometry application. The probe is observed to be robust in cryogenic environments. Finally, we show how to better characterize and optimize a delay line style probe with ordinary vector network analyzer measurements. These practical details are missing from the literature, and relevant to several branches of physics in which broadband spin control is an important technique.