Matson, J. B.Lam, J.Anderson, B. D.O.James, B.2026-01-022026-01-0207803021090743-1619ORCID:/0000-0002-1493-4774/work/174739601https://hdl.handle.net/1885/733803188Control system design specifications are commonly given in terms of log-magnitude quantities (dB). This induces a multiplicative or relative error criterion for plant model reduction. Techniques from the theory of additive Hankel norm approximation and spectral factorization are synthesised to produce a multiplicative approximant of a possibly unstable or nonsquare plant. It is shown that the reduced-order model preserves the right half plane poles and zeros of the original system. Explicit state-space formulae are provided for the construction of the reduced order model and its error properties discussed.5en19920027091886