Matching using a coaxial double slug transformer particularly in relation to measurements on microwave transistors at 0.5–2 GHz, is described. The limits arising from tuner loss and the correction to transistor gain which needs to be applied are determined analytically. The correction is compared with that deduced from reflection measurements, good agreement being obtained. The use of parallel plate line is investigated and from itsQ-factor is shown to be a possible lower loss alternative to coaxial line in this application, provided that small radiation effects can be suppressed.