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In this study, we measured transmission loss characteristics of the power line and physical layer (PHY) rate through the power line communication (PLC) modem in order to examine how branching influences the PHY rate of the orthogonal frequency-division multiplexing (OFDM) signal. The branch of the power line was lengthened so that the attenuation peak was created in the frequency band used in the PLC modem, and the regular attenuation of 50 dB also was inserted so that the PHY rate could be measured near the state of the transmission limit. Moreover, the transmission loss characteristic of the power line was calculated by using the 4 port pair network method, and the PHY rate of the PLC modem by MATLAB/Simulink software, and these calculation results were compared with the measurement ones. From these results, we found the following: (1) A sharp attenuation peak of -20 dB or more was created in the frequency region of 2 MHz to 30 MHz for the power line, and this attenuation peak moved to the low frequency region as the branch lengthened; (2) the PHY rate of about 190 Mbps to 200 Mbps was achieved and almost totally corresponded to the ideal state of a PLC modem, even when inserting the attenuation of up to 30 dB, but the PHY rates decreased about 20 Mbps to 30 Mbps when inserting the attenuation up to 50 dB; and (3) results for the PHY rate for the PLC modem calculated by using the calculation software of MATLAB/Simulink agreed with the measurement ones, and so the method for calculating the PHY rate for the OFDM signal through PLC modem was confirmed to be effective.