A dual frequency dual circularly polarized cross-slot waveguide array working at 4.9 GHz and 5.8 GHz is proposed for wireless communication/airborne weather radar applications. Different from the traditional cross-slotted waveguide antenna, to improve space utilization, two sets of cross-slots are slit on both sides of the longitudinal axis of the waveguide's E-plane to realize dual-frequency operation. When the antenna operates in the TE10 mode, the cross-slots on each side radiate left-handed and right-handed circularly polarized electromagnetic waves at two different frequencies, respectively. To suppress grating lobes, phase perturbation structures are periodically loaded in the waveguide to tune the propagation phase constant, thereby changing the effective electric spacing between radiating elements while keeping the antenna a compact physical aperture. The proposed grating lobe suppression method avoids the dielectric loss caused by dielectric loading, eliminates the need for complex array arrangement, and achieves the grating lobe suppression at dual frequencies simultaneously. The metallic 3D printing technology, selective laser melting (SLM), is used to fabricate the antenna in one piece in one run using aluminum alloy. The proposed antenna has gains of 10 dBic and 14.5 dBic with 47% and 69% aperture efficiencies at 4.9 GHz and 5.8 GHz, respectively. It is a capable candidate for air-to-ground (ATG) communication applications.