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We quantitatively investigate the possibility of detecting baryonic acousticoscillations (BAO) using single-dish 21cm intensity mapping observations in thepost-reionization era. We show that the telescope beam smears out the isotropicBAO signature and, in the case of the Square Kilometer Array (SKA) instrument,makes it undetectable at redshifts $z\gtrsim1$. We however demonstrate that theBAO peak can still be detected in the radial 21cm power spectrum and describe amethod to make this type of measurements. By means of numerical simulations,containing the 21cm cosmological signal as well as the most relevant Galacticand extra-Galactic foregrounds and basic instrumental effect, we quantify theprecision with which the radial BAO scale can be measured in the 21cm powerspectrum. We systematically investigate the signal-to-noise and the precisionof the recovered BAO signal as a function of cosmic variance, instrumentalnoise, angular resolution and foreground contamination. We find that theexpected noise levels of SKA would degrade the final BAO errors by $\sim5\%$with respect to the cosmic-variance limited case at low redshifts, but that theeffect grows up to $\sim65\%$ at $z\sim2-3$. Furthermore, we find that theradial BAO signature is robust against foreground systematics, and that themain effect is an increase of $\sim20\%$ in the final uncertainty on thestandard ruler caused by the contribution of foreground residuals as well asthe reduction in sky area needed to avoid high-foreground regions. We also findthat it should be possible to detect the radial BAO signature with highsignificance in the full redshift range. We conclude that a 21cm experimentcarried out by the SKA should be able to make direct measurements of theexpansion rate $H(z)$ with competitive per-cent level precision on redshifts$z\lesssim2.5$.

Baryonic acoustic oscillations from 21cm intensity mapping: the Square Kilometre Array case