Phase velocities of plasma waves near the lower hybrid frequency were measured with an interferometer composed of two spatially separated electron density probes. The plasma waves were produced in the F‐region ionosphere by an argon ion beam. By calculating the normalized cross spectrum of the plasma waves a coherency of .98 was estimated along with a maximum phase difference of π/3 radians between the two probes. This implies that the wavelength was 6 meters compared to an O+gyroradius of 3.8 meters and that the phase velocity was 45 km/sec compared to an ion beam velocity of 12.4 km/sec. These numbers compare favorably with recent predictions of a nonresonant mode produced by a dense ion be