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 are 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 &pgr;/3 radians between the two probes. This implies that the wavelength was 6 meters compared to an 0+ 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 beam.