By using our improved Optical Electric Field Sensor (OEFS) that incorporates our Log-Periodic Dipole Antenna Array (LPDA-type), it is now possible to measure accurately an EMI. However, detectable bandwidth (1.8-6.0 GHz) of LPDA-type OEFS is partially higher than required bandwidth (1.0-6.0 GHz) of updated EMI standards for information technology equipment. In this paper, lowering detectable frequency of a LPDA-type OEFS is analyzed by using an electromagnetic field simulation that is combined with a newly developed computer program. As a result, we confirmed that a LPDA-type OEFS that length of antenna elements of was magnified by 1.77 times can measured Electric field above 1 GHz.