An extended P-matrix model is introduced that includes additional electrical ports to take into account particular electrode polarities. The transduction efficiency and static capacitance of any particular electrode are calculated using the model of reflecting grating and electrostatic charge distribution in the total electrode structure. The electrostatic charge distribution is calculated taking into account singularities at electrode ends and using series expansion in Chebyshev polynomials. Calculated results of imbalance characteristics for longitudinally coupled resonator filters agree well with measured data.