When it is required to make a measurement at a node of an RF circuit, connecting to the circuit using a normal oscilloscope probe, even on the x10 setting can change the behaviour of the circuit. For those difficult cases, you need a special probe like the one described here.Any probe presents extra impedance for the circuit to drive, usually consisting of some resistance and stray capacitance, resulting in reduction in gain, or in extreme cases, causing instability. The loading effect of the resistance and stray capacitance can be largely removed by using an active probe. The major manufacturers in the oscilloscope market all offer suitable models (see also ‘Scope for Scopes’ elsewhere in this issue), but costing over a £1000 they are too expensive for amateur use. This article describes a probe that can be constructed at home, for very little money and has useful performance.