Impedance of an unknown load is determined by applying a reference signal from a signal generator through a multi-position switch to the unknown load, a first known calibration impedance, and second known impedance, which can be an open circuit, while measuring the complex voltage developed across each load. The retaining strip extends in an essentially parallel manner in relation to the frame part of the roof stand when in a connecting position and is retained thereon. The adjustment device includes nesting members adjustable in height by sliding and distributed over at least two opposite regions of the sidewall of the well and of the periphery of the support.