A fraction of about 10 % of supermassive black holes in the centres of active galactic nuclei (AGN) produce powerful jets emitting across the electromagnetic spectrum. The formation of jets and what collimates them on parsec scales is still poorly understood and debated to date. Many jet launching scenarios predict the existence of helical magnetic fields in the jet, potentially being one of the jet collimation mechanisms and manifesting themselves in polarisation structure and transverse Faraday rotation gradients. Therefore, studying the magnetic fields on the smallest possible scales in AGN jets can give crucial insight into the physics of jet launching, acceleration and collimation. This is one of the main aims of the RadioAstron mission, which operated from 2011 to 2019. The 10-m antenna onboard the Spektr-R spacecraft in a highly elliptical orbit with major axis of 350,000 km was complemented by a ground-array of telescopes, including the EVN, at observing frequencies of 0.32, 1.6, and 22 GHz with full polarisation capabilities. The powerful flat-spectrum radio quasar (FSRQ) 3C 345 is one of those archetypical AGN that underwent several flaring episodes in the optical, 𝛾-rays and at radio wavelengths. Observed with VLBI over several decades, it shows a compact jet closely aligned with the line of sight, with components exhibiting apparent superluminal motion. We observed 3C 345 with RadioAstron at 1.6 GHz on March 30, 2016, resulting in the highestresolution image of this source at this frequency to date (see [1]) with a resolution along the jet direction of ∼300 𝜇as. In addition to the published results we present studies of the spectral index and the rotation measure in the source. These were obtained with the RadioAstron data in conjunction with ground-VLBI data from the same epoch. We test for possible Faraday rotation gradients and study the change of the electric vector position angles.

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