  Ultracool dwarfs, the least-massive contributors to the stellar mass
function, exhibit striking magnetic properties that are inconsistent with
trends for more massive stars. Here, we present the widest-band radio
observations to date of an ultracool dwarf, DENIS-P J104814.9-395604, in four 2
GHz bandwidths between wavelengths of 1 cm and 10 cm. These data were obtained
with the Australia Telescope Compact Array using the new Compact Array
Broadband Backend instrument. We detected a stable negatively-sloped power-law
spectrum in total intensity, with spectral index alpha=1.71+-0.09. Circular
polarization fractions between 0.25 and 0.4 were found at the low-frequency end
of our detection band. We interpret these results as indicative of
gyrosynchrotron emission. We suggest that the radio emission originates from
beyond the co-rotation radius, R_C, of the star. Adopting this model, we find
R_C between 1.2-2.9 R_*, and a non-thermal electron density and magnetic field
strength between 10^(5)-10^(7.2) cm^(-3) and 70-260 G respectively at R_C. The
model accounts for the violation of the Guedel-Benz relation between X-ray and
radio luminosities of low-mass stars by DENIS-P J104814.9-395604.
