  We present the numerical simulations for an electron-beam-driven and
loss-cone-driven electron-cyclotron maser (ECM) with different plasma
parameters and different magnetic field strengths for a relatively small region
and short time-scale in an attempt to interpret the recent discovered intense
radio emission from ultracool dwarfs. We find that a large amount of
electromagnetic field energy can be effectively released from the beam-driven
ECM, which rapidly heats the surrounding plasma. A rapidly developed
high-energy tail of electrons in velocity space (resulting from the heating
process of the ECM) may produce the radio continuum depending on the initial
strength of the external magnetic field and the electron beam current. Both
significant linear polarization and circular polarization of electromagnetic
waves can be obtained from the simulations. The spectral energy distributions
of the simulated radio waves show that harmonics may appear from 10 to
70$\nu_{\rm pe}$ ($\nu_{\rm pe}$ is the electron plasma frequency) in the
non-relativistic case and from 10 to 600$\nu_{\rm pe}$ in the relativistic
case, which makes it difficult to find the fundamental cyclotron frequency in
the observed radio frequencies. A wide frequency band should therefore be
covered by future radio observations.
