  In this paper we present observations of eta Carinae in the 1.3 mm and 7 mm
radio continuum, during the 2003.5 low excitation phase. The expected minimum
in the light curves was confirmed at both wavelengths and was probably due to a
decrease in the number of UV photons available to ionize the gas surrounding
the binary system. At 7 mm a very well defined peak was superimposed on the
declining flux density. It presented maximum amplitude in 29 June 2003 and
lasted for about 10 days. We show that its origin can be free-free emission
from the gas at the shock formed by wind-wind collision, which is also
responsible for the observed X-ray emission. Even though the shock strength is
strongly enhanced as the two stars in the binary system approach each other,
during periastron passage the X-ray emission is strongly absorbed and the 7 mm
observations represent the only direct evidence of this event.
