We have conceived and realised a microwave tunable transformer. The tunability is obtained by loading a transmission line by reverse biased Schottky diodes eaching as voltage variable capacitances. Only two diode's pairs are necessary to obtain impedance tunability better than ± 100 % around 50 Ω with an operating frequency varying of more than ± 10 %. The length of the impedance transformer lies between 0.32 and 0.38 of the wavelength.