Curvature radiation is a natural candidate for the emission mechanism of FRB .
However , FRB spectra have structure with \Delta \nu / \nu \sim \text { 0.03 - -0.2 } , inconsistent with the very smooth spectrum of curvature radiation .
Although this spectral structure might be attributed to chromatic scintillation or lensing , in four FRB high spectral resolution data indicate scintillation decorrelation bandwidths much narrower than the observed \sim \text { 30 - -300 } MHz spectral structure .
Some of the observed structure may be intrinsic to the radiation mechanism .
I suggest that the observed spectral structure reflects the spatial structure of a clumpy radiating charge distribution , and that the characteristic curvature radiation frequency may be higher than the observed frequencies .
In this coupled plasma-curvature radiation process the radiated spectra are the product of the spectra of the plasma wave and that of incoherent curvature radiation .
The argument applies to all coherent radiation processes , including those that produce pulsar nanoshots .
The implied FRB ‘ ‘ clump ’ ’ charges are large , and produce electrostatic potentials that suggest electron Lorentz factors \gtrsim 10 ^ { 2 } .
The result applies generally to coherently radiating sources .