Enhanced Z-mode radiation from a dipole Original Research Article

An HF transmitter and synchronized receiver both connected to dipoles were operated during the tethered experiment OEDIPUS C. On the flight down leg after the tether had been cut, direct bistatic propagation experiments were made with the transmitter in one subpayload and the receiver in the other subpayload. Medium-frequency Z-mode transmissions were observed to be strong compared to the adjoining wave modes. This Z-mode is the left-hand polarized mode propagating between its cut-off frequency fZ=(−fc+(fc2+4fp2)1/2)/2 and the local plasma frequency fp; fc is the local gyrofrequency. It is of particular theoretical interest because its refractive index surface evolves from a simple ellipsoid to an ellipsoid with inflections as frequency is swept through the band. The Z-mode signal was strongest at frequencies just below fp, at relatively low fp values. When fp increased, the Z-mode signals were strongest at frequencies just above fZ. Details of these characteristics are presented. A possible explanation is that the transmitting dipole impedance in this frequency range provides a better match to the output of the transmitter than in other wave modes swept.