The theoretical limits of the sensitivity of a pair of back-to-back electron retarding potential analyzers for the detection of the density fluctuations and currents of plasma waves, particularly in the solar wind, are considered. Such an instrument might replace or augment electric antennas for the detection of waves. It is shown that such an instrument, of sufficiently small size to be hidden behind a heat shield, can have sufficient sensitivity to detect most known phenomena, and to detect all waves whose power would be of interest in energizing the solar wind. It is capable of measuring electrostatic waves in a frequency range which has been little studied. An added advantage is that in certain cases, possibly in conjunction with electric antennas, it can provide a measure of the wave vector k. ¿ 1999 American Geophysical Union