When a radially-polarized beam passes through a wave plate, the polarization distribution of the beam strongly depends on the retardation angle and the spatial position. When the retardation angle is changed from 0 to π, the polarization of the beam becomes the local elliptic polarization with non-uniform polarization distribution. The rotating direction of polarization in the first and third quadrants is opposite to that in the second and fourth quadrants. In this paper we have numerically simulated the transverse and longitudinal electric field intensity, the transverse energy flux and the longitudinal angular momentum of the tightly focused beam in the focal plane. Our results show that the sum of the longitudinal angular momentum in the focal plane is zero, but it is varying in different quadrants. The liquid crystal variable retarder (LCVR) is adopted as a real-time continuous tunable wave plate with its retardation angle δ changing from 0 to π by varying the applied voltage. In this case the phase retardation angle δ introduced by the LCVR can be used as a control over the longitudinal electric field intensity and the longitudinal angular momentum.