An efficient design for a quarter-wave (λ/4) retardation plate (QWP) operating at microwave frequencies has been designed and manufactured using dual head fused deposition modelling (FDM) 3D printing. Exploiting a bespoke composite material feedstock filament with high dielectric permittivity ϵr = 10.8, the resulting 3D-printed QWP comprising alternative layers of high and low permittivity had a high artificial double refraction of Δϵ = 2.9. The QWP provided broadband conversion of linear to circular polarization and phase modulation of an incident plane electromagnetic wave at 12-18 GHz, and demonstrated the potential for optical devices via additive manufacture for use in the microwave frequency range.