Theoretical and numerical results are presented to assure that a tunable, narrow-band, coherent THz radiation source can be based on parametric down-conversion in aphotonic crystal. Our proposal is based on down-conversion mixing and a local-field enhancement mechanism that is available by tuning each of the two driving laserfields either to band-edge or to a defect mode in the band gap. The frequency of the down-converted signal can be tuned by intersecting two non co-linear laser sources. The polarizations are degenerate at normal incidence and have sub-THz down-conversion maximum. For aspecific sample geometry we show that by changing the angle of incidence of one tunable laser to 30 degrees the THz frequency is about11.5 THz for p-polarization and 3.5THz for s-polarization, since the angle-dependent transmission spectrum is different for p- and spolarizations.The peak conversion efficiency for both polarizations is enhanced by over two orders of magnitude. Finally we also introduce some preliminary experimental results which agree with the numerical results we present here.