  Interferometric millimeter observations of the cosmic microwave background
and clusters of galaxies with arcmin resolutions require antenna arrays with
short spacings. Having all antennas co-mounted on a single steerable platform
sets limits to the overall weight. A 25 kg lightweight novel carbon-fiber
design for a 1.2 m diameter Cassegrain antenna is presented. The finite element
analysis predicts excellent structural behavior under gravity, wind and thermal
load. The primary and secondary mirror surfaces are aluminum coated with a thin
TiO$_2$ top layer for protection. A low beam sidelobe level is achieved with a
Gaussian feed illumination pattern with edge taper, designed based on feedhorn
antenna simulations and verified in a far field beam pattern measurement. A
shielding baffle reduces inter-antenna coupling to below $\sim$ -135 dB. The
overall antenna efficiency, including a series of efficiency factors, is
estimated to be around 60%, with major losses coming from the feed spillover
and secondary blocking. With this new antenna, a detection rate of about 50
clusters per year is anticipated in a 13-element array operation.
