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We report extensive radio and X-ray observations of SN 2003bg whosespectroscopic evolution shows a transition from a broad-lined Type Ic to ahydrogen-rich Type II and later to a typical hydrogen-poor Type Ibc. We showthat the extraordinarily luminous radio emission is well described by aself-absorption dominated synchrotron spectrum while the observed X-rayemission at t~30 days is adequately fit by Inverse Compton scattering of theoptical photons off of the synchrotron emitting electrons. Our radio modelimplies a sub-relativistic ejecta velocity, v ~ 0.24c, at t_0 ~ 10 days afterthe explosion which emphasizes that broad optical absorption lines do not implyrelativistic ejecta. We find that the total energy of the radio emitting regionevolves as E ~ 7.3 x 10^{48} (t/t_0)^{0.4} erg assuming equipartition of energybetween relativistic electrons and magnetic fields (e_e=e_B=0.1). Thecircumstellar density is well described by a stellar wind profile with modest(factor of ~2) episodic density enhancements which produce abrupt achromaticflux variations. We estimate an average progenitor mass loss rate of M_dot ~ 3x 10^{-4} solar masses per year (assuming a wind velocity of 1000 km/s),consistent with the observed values for Galactic Wolf-Rayet stars. Comparisonwith other events reveals that ~50% of radio supernovae show similar shorttimescale flux variations attributable to circumstellar density irregularities.Specifically, the radio light-curves of SN 2003bg are strikingly similar tothose of the Type IIb SN 2001ig, suggestive of a common progenitor evolution.We conclude that their progenitors experienced quasi-periodic mass lossepisodes just prior to the SN explosion. [ABRIDGED]Comment: Accepted to ApJ, revised version (41 pages, 8 figures, 4 tables

The Radio and X‐Ray Luminous SN 2003bg and the Circumstellar Density Variations around Radio Supernovae