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We show that the public experiment held in Venice by F. Tamburini et al andreported in (2012 New J. Phys. 14 033001) can be regarded as a particularimplementation of Multiple-Input Multiple-Output (MIMO) communications, hencebringing no advantages with respect to known techniques. Moreover, we explainthat the use of a 'vortex' mode (orbital angular momentum OAM l = 1) at one ofthe transmit antennas is not necessary to encode different channels since onlydifferent patterns -or similarly different pointing angles- of the transmitantennas are required. Finally, we identify why this MIMO transmission alloweddecoding of two signals despite being line-of-sight. This is due to the largeseparation between the receiving antennas, which places the transmit antennasin the near-field Fresnel region of the receiving 'array'. This strongly limitsthe application of this technique in practice, since, for a fixed separationbetween receiving antennas, the detectable signal power from any additionalvortex mode decays at least as 1/r^4.

Comment on 'Encoding many channels on the same frequency through radio vorticity: first experimental test'