Efficient remote transmission line probe tuning

Abstract

An approach is described which allows remote tuning of a probe via a length of transmission line, while retaining nearly ideal efficiency relative to the equivalent locally tuned coil. This preservation of efficiency is accomplished through the principle of "partial local matching." In comparison to common methods of remote tuning which involve half-wavelength cables, or in which a length of transmission line is itself utilized as a tuning element, the partial matching method results in a substantial increase in the efficiency of power transfer to the coil, and therefore increased B1 field intensity and signal-to-noise ratio. Applications in which the use of local variable tuning capacitors is made difficult by either space, accessibility, or environmental considerations should benefit from this method.

Probe efficiency improvement with remote and transmission line tuning and matching

“…4 In the present case, this range mainly depends on the C 1 capacitor in Fig. 3, and with a 30 pF capacitor we have been able to obtain a 10% margin of the pre-tuned f 0 for the matching condition.…”

“…As is well known, a reduction in the reflectio coefficien signifi cantly decreases the losses. 4 In our examples, lower losses introduced by the higher impedance transmission line inserted have shown significan effect on the evaluated parameters. However, there is a range in which further decreases in SWR values do not translate into critical deterioration of image quality.…”

“…4 By placing fixe local capacitors at the detector coil in the conventional parallel-series configuration the imaginary part of the impedance can be cancelled and its module can be increased close to 50 ⍀ at the junction with the coil. 7 As an alternative, our proposal relies on the partial tuning of the probe by placing a local capacitor and, on the modificatio of the transmission line characteristic impedance, Z 0 , to be as close as possible to the resulting probe impedance (Fig.…”

“…1A), this type of remote tuning carries a cost in efficiency The reduction of power forwarded to the probe itself is caused by the mismatch (and subsequent reflection at the probe/transmission line junction. To reduce this reflectio Rath 4 proposes to introduce a partial matching circuit to the transmission line impedance (Fig. 1B), technique can increase the efficienc up to 90%, three times higher than the value achieved with the half-wave length matching only.…”

“…[1][2][3][4] The relationship between the power transfer from the radio frequency amplifie to the probe during the transmission mode, as well as the power of the signal detected at the probe that reaches the preamplifie during reception (efficienc of the coil) relies in part on the probe matching the characteristic impedance Z 0 of the transmission line used (usually 50 ⍀). 4 Since matching is realized after the firs segment of transmission line placed between the input/output electronics and the probe (Fig. 1A), this type of remote tuning carries a cost in efficiency The reduction of power forwarded to the probe itself is caused by the mismatch (and subsequent reflection at the probe/transmission line junction.…”