Using data from the University of Minnesota Plasma Wave Experiment aboard the IMP 6 (Explorer 43) satellite, the wavelengths of certain electrostatic waves in Earth's magnetosphere are measured. The antenna response to electrostatic waves having several k vector distributions in space is calculated using the fact that the X and Y dipole antennas on IMP 6 are of unequal length. These results are used to determine the wavelengths (n+1/2)fce gyroharmonic emissions observed just beyond the plasmapause. These wavelengths are then used in conjunction with theoretical models to determine the energy of the electrons driving the waves, and a range of energies between several tens to several hundreds of electron volts is found. This technique is also applied to Langmuir waves observed upstream of Earth's bow shock, and the results are found to be in good agreement with theory. ¿ American Geophysical Union 1988