  This work aims at online calibrating the signal attenuation of the long cable
used in Tibet Water Cherenkov Muon detector array-A (Tibet MD-A) under the 37
000 m2 Tibet air shower array. Based on the waterproof connection of the PMT
(R3600_06) with the signal cable, and the characters of the high voltage
divider, terminal reflection method is explored and used for measuring the
signal attenuation with a practical way to eliminate the contribution of the
pulse baseline (pedestal). Comparison measurement between it and QDC
(Charge-to-Digital Conversion) data taking method are carried out by using
open-ended cables, confirming that terminal reflection method is a fast and
convenient method, and suitable to online calibrate the signal attenuation for
Tibet MD-A. At 26\degree C, the measured attenuation coefficient with the 250-m
cable, permanently connected to the PMT, is ~13.9%. Also, the cable frequency
response is measured by using the sinusoidal signals, which could be used to
study the time dispersion of the real signal produced with Tibet MD-A detector
via Fourier analysis.
