  We observed the cluster CIZA J2242.8+5301 with the Arcminute Microkelvin
Imager at $16$ GHz and present the first high radio-frequency detection of
diffuse, non-thermal cluster emission. This cluster hosts a variety of bright,
extended, steep-spectrum synchrotron-emitting radio sources, associated with
the intra-cluster medium, called radio relics. Most notably, the northern,
Mpc-wide, narrow relic provides strong evidence for diffusive shock
acceleration in clusters. We detect a puzzling, flat-spectrum, diffuse
extension of the southern relic, which is not visible in the lower
radio-frequency maps. The northern radio relic is unequivocally detected and
measures an integrated flux of $1.2\pm0.3$ mJy. While the low-frequency ($<2$
GHz) spectrum of the northern relic is well represented by a power-law, it
clearly steepens towards $16$ GHz. This result is inconsistent with diffusive
shock acceleration predictions of ageing plasma behind a uniform shock front.
The steepening could be caused by an inhomogeneous medium with
temperature/density gradients or by lower acceleration efficiencies of high
energy electrons. Further modelling is necessary to explain the observed
spectrum.
