  We present the results of a series of short radio observations of six
ultracool dwarfs made using the upgraded VLA in S (2--4GHz) and C (4--7GHz)
bands. LSR J1835+3259 exhibits a 100 percent right-hand circularly polarised
burst which shows intense narrowband features with a fast negative frequency
drift of about $-30$ MHz $\textrm{s}^{-1}$. They are superimposed on a fainter
broadband emission feature with a total duration of about 20 minutes, bandwidth
of about 1 GHz, centred at about 3.5 GHz, and a slow positive frequency drift
of about 1 MHz $\textrm{s}^{-1}$. This makes it the first such event detected
below 4 GHz and the first one exhibiting both positive and negative frequency
drifts. Polarised radio emission is also seen in 2MASS J00361617+1821104 and
NLTT 33370, while LP 349-25 and TVLM 513-46546 have unpolarised emission and
BRI B0021-0214 was not detected. We can reproduce the main characteristics of
the burst from LSR J1835+3259 using a model describing the magnetic field of
the dwarf as a tilted dipole. We also analyse the origins of the quiescent
radio emission and estimate the required parameters of the magnetic field and
energetic electrons. Although our results are non-unique, we find a set of
models which agree well with the observations.
