In magnetic resonance mediated radiofrequency ablation (MR-RFA) the RF energy for the ablation is captured by a wire antenna placed in the scanner bore, and channeled to the ablation needle. There are no external wired connections. The effective length of the antenna is adjusted physically or electrically to be resonant with the scanner RF to maximize energy capture. To suppress heating when desired, the antenna must be detuned. An electronic switch to do so reduces antenna efficiency, but a simple wireless hydraulically activated mechanical switch maintains full antenna efficiency and achieves high on-off ratio.