The effects of the propagation angle of electromagnetic waves and the pitch angle of non-thermal electrons on the electron cyclotron maser instability and beam–plasma instability are compared. It is pointed out if the direction of propagation of electromagnetic waves is opposite to the direction of the injected electrons, the maser instability will be suppressed, and hence it cannot explain the radiation with narrow bandwidth excited by energetic electrons propagating downwards in solar flares, such as millisecond spikes, blips and the type III bursts with positive-frequency drift. This discrepancy may be solved by the non-resonant wave–particle interaction in the beam–plasma instability, in which the electromagnetic waves propagating in the same direction as the injected electron beam are mainly composed of the ordinary modes (left-circular polarization), while the electromagnetic waves propagating in the opposite direction to the beam are mainly composed of the extraordinary modes (right-circular polarization). This result is compared with a typical model of a magnetic tube in solar flares, in which the non-thermal electrons propagating upwards excite spikes, blips and type III bursts in the metre and decimetre bands, while the electron beams propagating downwards excite these temporal and spectral structures in the microwave bands.