The use of an asymmetric Mach-Zender interferometric amplitude modulator to measure a relatively low frequency electric field strength is described. The sensitiviy of an electric field sensor using a Ti: LiNbO3 optical modulator is strongly affected by the shape of a electrode (probe antenna). To measure the low frequency electric field, a probe antenna of wide effective area is more useful than the usual dipole antenna. As proof of this, the optical modulator was fabricated with a plate-type probe antenna and the usefulness of this antenna tested for measuring low frequency electric field strength.