  We propose an alternative scheme for a tunable THz radiation source generated
by relativistic electron bunches. This technique relies on the combination of
dispersive selection and flat electron bunch. The dispersive selection uses a
slit mask inside a bunch compressor to transform the energy-chirped electron
beam into a bunch train. The flat beam transformation boosts the frequency
range of the THz source by reducing the beam emittance in one plane. This
technique generates narrow-band THz radiation with a tuning range between 0.2 -
4 THz. Single frequency THz spectrum can also be generated by properly choosing
the slit spacing, slit width, and the energy chirp.
