Properties of the single probe potentials. Panel a shows the few-volt potentials on antennas 1 and 2 during a one-millisecond interval. Their difference must be due to the addition of two waves at each antenna, with the Langmuir wave having a variable frequency to avoid the beats produced by two fixed-frequency waves, as illustrated in panel c. The Langmuir-wave frequency varied because of the high-density irregularities illustrated in panel b, to produce a simulation in panel d of the signals on the two antennas that are produced by 1 or 2 percent Langmuir-wave frequency variations.