A device is provided for use of an antennal base formed of two antennas which pick up the emissions present and produce two radioelectric signals S1 and S2. These two signals are used to produce at least one intermediate-frequency signal Fl by demodulation of one of the two signals by the other (autotransposition). The demodulation is carried out by firstly transposing one of the signals, S1 for example, around a given frequency F1, the signal S2 being preserved around its initial central frequency F0. Thus, whatever the central frequency F0 of the emission picked up by the antennas, the demodulation produces a signal of central frequency F1, thereafter demodulated into a given intermediate frequency Fl by a local oscillator of constant frequency F2=F1+Fl. The device is applied to the production of a device for detecting emissions and for characterizing the emissions picked up.