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A compact integrated antenna with direct quadrature conversion circuitry for broad-band millimeter-wave communications is proposed. The conversion circuits include two even-harmonic mixers based on antiparallel diode pairs (APDPs). The equivalent circuit of the APDP derived here provides good agreement with the measured data from 17 to 23 GHz. Overall phase and amplitude imbalance between the in-phase/quadrature (I/Q) output channels are less than 1.2° and 1 dB at IFs of 10 and 100 MHz, respectively. An overall RF power conversion loss of 14.6 dB at the quadrature I/Q channels including the antenna is achieved in the frequency range from 39.75 to 40.25 GHz with a local oscillator (LO) power level of 11.8 dBm. LO leakages at 20 and 40 GHz are -31.5 and -44.8 dBm, respectively. In order to demonstrate the system capabilities for broad-band digital communication, a communication link is built with a pair of the proposed front-ends. Data transmission up to 1 Gb/s data rate for quadrature phase-shift keying modulation is demonstrated.