Automatic coupling control of a loop-gap resonator by a variable capacitorattached coupling coil for EPR measurements at 650 MHz

A coupling coil was fabricated that can electrically change the magnetic co upling with a loop-gap resonator (LGR) for EPR studies at 650 MHz. It is co mposed 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 mec hanically changing the distance between the LGR and single-turn coil. The f ine control is obtained by changing the capacitance of the varactor diode t hat 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 cou pling of the LGR is possible. Automatic coupling control (ACC) was accompli shed by negative feedback of the DC component in the radiowaves reflected f rom the LCR to the coupling control circuit when the LCR was irradiated pre cisely at its resonant frequency. To accomplish this, automatic frequency c ontrol (AFC) is used. In EPR measurements of a phantom that included a phys iological saline solution containing a nitroxide radical, it was confirmed that the drifts in the coupling and resonant frequency caused by the pertur bation 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 deviatio n 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. (C) 2001 Academic Press.