  There is much evidence that planet formation is occurring in the disk around
the Herbig Be star HD100546. To learn more about the processes occurring in
this disk we conducted high resolution imaging at 43/45 GHz with the Australia
Telescope Compact Array (ATCA). Multiple array configurations were used,
providing a best spatial resolution of $\sim$ 0.15 arcsec, or 15 AU at
HD100546's distance of $\sim$ 100 pc. Significant structure is revealed, but
its precise form is dependent on the $u-v$ plane sampling used for the image
reconstruction. At a resolution of $\leq$ 30 AU we detected an inner gap in the
disk with a radius of $\sim$ 25 AU and a position angle approximately along the
known disk major axis. With different weighting, and an achieved resolution of
$\sim$ 15 AU, emission appears at the centre and the disk takes on the shape of
an incomplete ring, much like a horseshoe, again with a gap radius of $\sim$ 25
AU. The position angle of the disk major axis and its inclination from face-on
are determined to be $140^{\circ}\pm5^{\circ}$ and $40^{\circ}\pm5^{\circ}$
respectively. The $\sim$ 25 AU gap radius is confirmed by a null in the real
part of the binned visibilities at 320$\pm$10 k$\lambda$, whilst the
non-axisymmetric nature is also confirmed through significant structure in the
imaginary component. The emission mechanism at the central peak is most likely
to be free-free emission from a stellar or disk wind. Overall our data support
the picture of at least one, but probably several, giant planets orbiting
HD100546 within 25 AU.
