One of the most accurate techniques for in situ measuring the electron density and temperature in space plasmas is the quasi-thermal noise spectroscopy, which uses the voltage fluctuation spectrum on an electric antenna. This technique has been used successfully on the WIND and ULYSSES spacecraft; however, on STEREO this technique may only work in high-density filamentary structures, where the Debye length is small, because the STEREO/WAVES antennas have a large surface area, so that the resulting shot noise spectrum in the solar wind dominates the power at lower frequencies. In the unperturbed solar wind, we can use instead the electron shot noise to infer the plasma density. For doing so, we use well calibrated WIND particle data to deduce the stray capacitance of the STEREO/WAVES antenna system in a special configuration when the STEREO-B spacecraft was just downstream of WIND. This stray capacitance is also compared to ground experiments done on the flight spare equipment and independent calibrations performed using the galactic radio background.