  We present new millimetre 43 GHz observations of a sample of radio-bright
Planetary Nebulae. Such observations were carried out to have a good
determination of the high-frequency radio spectra of the sample in order to
evaluate, together with far-IR measurements (IRAS), the fluxes emitted by the
selected source in the millimetre and sub-millimetre band. This spectral range,
even very important to constraint the physics of circumstellar environment, is
still far to be completely exploited. To estimate the millimetre and
sub-millimetre fluxes, we extrapolated and summed together the ionized gas
(free-free radio emission) and dust (thermal emission) contributions at this
frequency range. By comparison of the derived flux densities to the foreseen
sensitivity we investigate the possible detection of such source for all the
channels of the forthcoming ESA's PLANCK mission. We conclude that almost 80%
of our sample will be detected by PLANCK, with the higher detection rate in the
higher frequency channels, where there is a good combination of brighter
intrinsic flux from the sources and reduced extended Galactic foregrounds
contamination despite a worst instrumental sensitivity. From the new 43 GHz,
combined with single-dish 5 GHz observations from the literature, we derive
radio spectral indexes, which are consistent with optically thin free-free
nebula. This result indicates that the high frequency radio spectrum of our
sample sources is dominated by thermal free-free and other emission, if
present, are negligible.
