Dispersion fiber sensitivity in optical microwave upconversion links is investigated. Simulations are carried out to calculate output optical spectra and to evaluate mixing power as a function of fiber length for different upconversion systems using dual drive/standard Mach-Zehnder modulators or an unbalanced Mach-Zehnder interferometer (UMZ). The latter solution prevents complete cancellation of mixing power because of the asymmetric double sideband optical spectrum detected at the output of the UMZ. It is shown that an optimum value of the linewidth enhancement factor of the laser diode can be found to minimize degradations of the mixing power along the fiber.