We present a contribution to a new mode of scanning thermal microscopy (SThM) based on the use of thermoelectric junction operating in ac active mode. This is the first alternative to 3omega operating mode of a resistive SThM probe for measuring thermophysical parameters of materials at micro- and nanoscale. Whereas a current at omega frequency generates by Joule effect a 2omega thermal oscillation along the wires, the junction thermoelectric voltage can be measured by means of a differential bridge scheme associated to a lock-in amplifier. A thermal model is presented that confirms measurements performed in different situations with different wire probes. Values of thermal contact conductance of different materials have been extracted and a comparison has been performed between this technique and the resistive 3omega mode.