A wideband, circularly polarized (CP), artificial magnetic conductor (AMC) reflector backed octagonal-shaped aperture (OSA) antenna is proposed for unidirectional radiation. The proposed antenna consists of an OSA fed by microstrip along with L-shaped stub and an AMC reflector. Bidirectional radiation of OSA antenna is changed to unidirectional radiation using an AMC reflector. The antenna height measured from the upper surface of the AMC reflector to OSA radiator is chosen to be small to realize a low-profile antenna: at the lowest analysis frequency of 4.5 GHz. Three different surfaces are studied and compared as back reflectors for OSA: perfect electric conductor (PEC), single layered AMC surface, and double layered AMC surface. The OSA with double-layered AMC surface demonstrates the largest measured 3-dB axial ratio bandwidth of 33.2% (5.20-7.19 GHz) with impedance bandwidth of 36.2% (5.04-7.21 GHz) for VSWR of 2. Almost constant gain of around 7 dBic is achieved over the band for the overall antenna volume of 0.72λo × 0.60λo × 0.19λo at the center frequency of 6.0 GHz.