By using the second bandgap in mushroom electromagnetic bandgap (EBG) structures, a high isolation can be achieved between transmitting (TX) and receiving (RX) microstrip array antennas integrated with a millimeter-wave single-chip frequency-modulated continuous-wave (FMCW) radar. These EBG structures are much easier to manufacture at millimeter waves as compared to the first bandgap mushroom EBG structures. In addition to improving isolation, the EBG structures are also used to reduce radiation pattern ripples caused by edge diffractions due to surface waves. Experimental validation is performed on antenna level and on the radar system level. For the system-level validation, the antenna arrays have been integrated with a single-chip mm-wave FMCW radar using the bond-wire interconnect technology. Measurement results in the 57-64 GHz band show the TX-RX isolation of 40 dB, which is 15-20 dB better than the isolation between TX and RX arrays without EBG structures. In addition, the radiation pattern ripple caused by edge diffractions is reduced below 3 dB.