In large Cassegrain antennas careful design is required to minimise the detrimental effects on the antenna performance caused by blocking of the radiation by the subreflector support. A study of the aperture blocking by some possible support structures has been carried out for the United Kingdom/Netherlands millimetrewave telescope antenna which has very stringent mechanical specifications. By examining the dependence of the blockage on the structural geometry and analysing the mechanical performance, an optimum structure has been achieved. Although the structure is required to support a secondary, which oscillates at up to 15 Hz without causing any spurious movements which are significant in comparison to the 0.4 mm shortest operating wavelength, the loss of gain has been kept down to 7.4%.