An antenna system for sharing of operation employs contiguous transmit frequencies. Transmit frequencies are separated into non-contiguous sub-groups isolated from one another by filters 158(+) and 160(-) associated with positive and negative polarisation. Received frequencies are filtered and split into five signals for input to base station receive ports. Non-contiguous transmit frequency sub-groups are combined by a quadrature hybrid 110 and pass with 90 degree relative phase shift to mutually orthogonal antenna stack ports P(+) and P(-) associated with orthogonally polarised sets of antenna elements AS(+) and AS(-): the ports P(+) and P(-) are isolated from one another by the hybrid 110. The 90 degree phase shift results in one transmit subgroup being radiated with left hand circular polarisation and the other transmit subgroup being radiated with right hand circular polarisation. Changing the relative phase shift changes the radiated polarisation to linear or elliptical, and signal amplitude weighting provides control of antenna beam polarisation direction.