Preliminary results are presented from an active wave experiment performed with a double‐dipole probe aboard the F4 rocket launched during the PORCUPINE campaign. They show that electrostatic cyclotron waves can be excited linearly around the half‐harmonics of the electron gyrofrequency. The most striking fact is that, under certain plasma conditions, the amplitude of waves excited at these peculiar frequencies can be even stronger than those observed at the characteristic resonances of the magnetoplasma. The fine structure of the signals received is also observed and interpreted. Theoretical numerical calculations of the potential created by a dipole antenna in a Maxwellian plasma are presented; good agreement is obtained with the complicated pattern of the various experimental results.