Formulas for the effective moment of inertia of towers with the linear and the square dependence between the moment of inertia of the tower shaft and the height of the tower are derived and compiled in the paper. Towers with legs of a constant cross-section and constant sloping are also included, assuming that the whole moment of inertia results from the legs. Comparison of a frequency (period) of a first mode of natural vibration and of a tower head deflection calculated and computed using the Robot Millennium program is given for the model of aerial (antenna) steel tower 99.5 m high, with a pyramidal trunk shape, three sections, four legs and with the dead weight of ca. 130 ton.

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