We present a polarisation rotator based on a dielectrically embedded metal Mesh Half Wave Plate (MHWP) working in the W-band frequency range (75-110 GHz). The device was realised using metallic grids with sub-wavelength anisotropic geometries able to mimic the behaviour of natural birefringent materials. The device was designed using a combination of transmission line codes and finite-element analysis able to achieve phase accuracy down to a fraction of degree. Very accurate intensity and phase measurements were carried out using coherent radiation from a Vector Network Analyser (VNA). The presented device performs better and it is much thinner than previous devices having reduced the number of grids by a factor two and minimised their inductive losses. The new mesh HWP has excellent performances in terms of differential phase-shift flatness and cross-polarisation, respectively 180.4±2.9° and -28 dB across a 25% bandwidth.
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