Automatic Coupling Control of a Loop–Gap Resonator by a Variable Capacitor Attached Coupling Coil for EPR Measurements at 650 MHz

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Abstract:

A coupling coil was fabricated that can electrically change the magnetic coupling with a loop–gap resonator (LGR) for EPR studies at 650 MHz. It is composed of a single-turn coil and a coupling control circuit that includes a varactor diode. The coarse control of the magnetic coupling is made by mechanically changing the distance between the LGR and single-turn coil. The fine control is obtained by changing the capacitance of the varactor diode that is connected in parallel with the single-turn coil. This capacitance is controlled by changing reverse voltage from a variable bias voltage source. Because this can be located far from the resonator, remote control of coupling of the LGR is possible. Automatic coupling control (ACC) was accomplished by negative feedback of the DC component in the radiowaves reflected from the LGR to the coupling control circuit when the LGR was irradiated precisely at its resonant frequency. To accomplish this, automatic frequency control (AFC) is used. In EPR measurements of a phantom that included a physiological saline solution containing a nitroxide radical, it was confirmed that the drifts in the coupling and resonant frequency caused by the perturbation of the resonant nature could be sufficiently compensated by the ACC and AFC systems. In the in vivo EPR studies, it was found that the deviation of coupling at the chest of a mouse is greater than that at the head of a rat, but the ACC system could compensate for the respiratory motions of a living animal.

Document Type: Research Article

Affiliations: 1: Institute for Life Support Technology, 2-2-1 Matsuei, Yamagata, 990-2473, Japan 2: Yamagata Research Institute of Technology, 2-2-1 Matsuei, Yamagata, 990-2473, Japan 3: Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, 992-8510, Japan

Publication date: March 1, 2001

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