New target specific absorption rate (SAR) values, calculated using a proposed reference dipole antenna and the reference flat phantom, are presented for an SAR validation test at 150MHz. The reference flat phantom recommended by the International Electrotechnical Commission (IEC) standard for 150MHz requires a significant amount of liquid owing to its large size. We conduct a numerical analysis in order to reduce the size of the flat phantom. The optimum size of the flat phantom is 780 (L₁) × 540 (W) × 200 (H)mm³, which is approximately a 64% reduction in volume compared to the reference flat phantom. The length of the reference dipole antenna required for the optimized flat phantom (extrapolated from the reference values at 300MHz) becomes 760mm. The calculated and measured return losses (S₁₁) of the antenna at 150MHz are 24.1dB and 22dB, respectively. The calculated and measured results for the return loss of the dipole antenna agree well and satisfy the IEC standard (> 20dB). The target SAR values derived from the numerical analysis are 1.08W/kg for 1g of tissue and 0.77W/kg for 10g of tissue for an SAR validation test at 150MHz.