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This paper reports on the design and testing of a new UHF transformer probe sensor which is robust in terms of its mechanical design as well as possessing high electrical sensitivity. Mechanical design and practical safety considerations relating to on-site installation are addressed. The frequency response of the probe has been compared to that of a commercially available sensor. Sensor response has been fully characterised in terms of sensitivity with respect to mounting depth relative to the transformer tank wall. This was achieved using a pulsed GTEM transient calibration cell with a specially designed transformer valve mounting plate. The new UHF probe sensor was found to achieve the minimum recommended sensitivity of 2 mm mean effective height at 0 mm insertion relative to the tank wall. The reference probe achieves the same sensitivity at a depth of 27 mm. This significant improvement in sensitivity should increase the signal-to-noise ratio of any recorded data and improve the detection rate of any incipient failures.