An impedance detector, in particular for radiosonde operation. The detector comprises a substrate (10 . . . 10E) made of an insulating material, onto which substrate the electrode and contact patterns (11, 11a, 13) necessary for the formation and connecting of the detector impedance have been applied. Between the detector impedance electrodes (11, 13), there is an active film (12), whose impedance values are a function of the physical quantity measured by means of the detector. The substrate of the detector is an oblong core filament (10; 10B; 10C; 10D; 10E) of an insulating material, onto and around which core filament said electrodes (11, 13) and said active insulating film (12) have been applied. Also, a novel process is described for the manufacture of said impedance detector.