This paper presents a 28 GHz to 72 GHz isolation enhanced down-conversion broadband sub-harmonic mixer using the 0.15 μm GaAs pHEMT process with chip dimensions of 1.8 × 0.76 mm2. The configuration of the sub-harmonic mixer consists of one intermediate frequency (IF) amplifier, one compact band pass filter, and one Lange coupler utilizing an anti-parallel diode pair (APDP). To reduce line length and achieve broad operation bandwidth, the proposed grounded capacitor in the band pass filter can also enhance the local oscillation (LO)-radio frequency (RF) isolation of the sub-harmonic mixer. Both high conversion gain and isolation can be achieved between LO and RF-IF in the IF amplifier. The measured results indicate that the conversion gain of the mixer is 5.02 dB to 11.5 dB from RF frequency of 28 GHz to 72 GHz at the LO power of 9 dBm; the isolation of RF-IF, LO-IF, LO-RF, and 2LO-RF is 21 dB to 51.52 dB, 22.13 dB to 30.97 dB, 10.64 dB to 27.9 dB, and 48.9 dB to 72.35 dB, respectively.