  The spherical harmonics $m$-mode decomposition is a powerful sky map
reconstruction method suitable for radio interferometers operating in transit
mode. It can be applied to various configurations, including dish arrays and
cylinders. We describe the computation of the instrument response function, the
point spread function (PSF), transfer function, the noise covariance matrix and
noise power spectrum. The analysis in this paper is focused on dish arrays
operating in transit mode. We show that arrays with regular spacing have more
pronounced side lobes as well as structures in their noise power spectrum,
compared to arrays with irregular spacing, specially in the north-south
direction. A good knowledge of the noise power spectrum
$C^{\mathrm{noise}}(\ell)$ is essential for intensity mapping experiments as
non uniform $C^{\mathrm{noise}}(\ell)$ is a potential problem for the
measurement of the HI power spectrum. Different configurations have been
studied to optimise the PAON-4 and Tianlai dish array layouts. We present their
expected performance and their sensitivities to the 21-cm emission of the Milky
Way and local extragalactic HI clumps
