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The design, manufacture and characterization of a sub-harmonic mixer operating in the millimeter wavelength range is described. The mixer combines for the first time electromagnetic band gap (EBG) technology (which improves the radiation features of the dipole antenna used to couple radiation to the mixer), with conventional local oscillator (LO) waveguide circuitry. The mixer is designed to operate in an RF frequency band around 250 GHz when supplied with a LO frequency between 115 and 135 GHz. The fixed IF frequency spans 2.5-3.5 GHz. Performance predictions were made using a combination of the finite elements (FE) method to compute the embedding impedance of the diodes and the harmonic balance analysis (HBA) to predict the noise temperature. A prototype mixer has been fabricated and tested. Best mixer performance (double side band (DSB) mixer noise temperature and conversion loss of 3000 K and 11.5 dB, respectively) was measured with a LO power level of about 5 mW. Good agreement is observed with the predicted performance.