  We report ultra-wideband (0.4-4.0 GHz) observation of coherent radio emission
via electron cyclotron maser emission (ECME) produced by the hot magnetic star
HD 142990. With nearly perpendicular rotation and magnetic dipole axes, it
represents an extreme case of oblique rotators. The large obliquity is
predicted to cause complex distribution of stellar wind plasma in the
magnetosphere (Townsend & Owocki 2005). It has been proposed that such a
distribution will give rise to non-trivial frequency dependence of ECME (Das et
al. 2020d). Indeed we discovered strong frequency dependence of different
pulse-properties, such as appearance of secondary pulses, different cut-off
frequencies for pulses observed at different rotational phases etc.. But the
unique feature that we observed is that while at sub-GHz frequencies, the star
appears to produce ECME in the extra-ordinary mode, at GHz frequencies, the
mode indicated by the pulse-property is the ordinary mode. By considering the
physical condition needed by such a scenario, we conclude that the required
transition of the magneto-ionic mode with frequency is unlikely to occur, and
the most promising scenario is refraction caused by the complex plasma
distribution surrounding the star. This suggests that the conventional way to
deduce the magneto-ionic mode based on ECME observed at a given frequency is
not a reliable method for stars with large misalignment between their rotation
and magnetic axes. We also find that ECME exhibits an upper cut-off at
$\lesssim 3.3$ GHz, which is much smaller than the frequency corresponding to
the maximum stellar magnetic field strength.
