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In this paper we investigate the use of RF signals to measure dielectric loss and the change of permittivity of an energized cable's insulation. One common degradation mechanism is growth of water trees in the presence of water. We present theoretical analysis of a novel on-line water tree detection technique based on using RF TEM wave propagation via test points on an elbow. Our method is based on injecting a high frequency RF signal into an energized cable at different points of the line voltage values and measure the nonlinearity of permittivity and dielectric loss.