  Generalised Faraday rotation can induce frequency-dependent conversion
between the linear and circular polarisation spectra of compact radio sources
such as pulsars, fast radio bursts and active galactic nuclei. I devise a
simple phenomenological model that can be used to measure the effects of
generalised Faraday rotation on the linearly and circularly polarised spectra
of these sources. The model is theory-agnostic, with an arbitrary wavelength
dependence, and hence can accommodate for a variety of potential generalised
Faraday rotation inducing media. It can also be combined with additional
observables to infer the physical properties of the intervening medium.
