  Extremely large-scale antenna array (ELAA) technologies consisting of
ultra-massive multiple-input-multiple-output (UM-MIMO) or reconfigurable
intelligent surfaces (RISs), are emerging to meet the demand of wireless
systems in sixth-generation and beyond communications for enhanced coverage and
extreme data rates up to Terabits per second. For ELAA operating at Terahertz
(THz) frequencies, the Rayleigh distance expands, and users are likely to be
located in both far-field (FF) and near-field (NF) regions. On one hand, new
features like NF propagation and spatial non-stationarity need to be
characterized. On the other hand, the transition of properties near the FF and
NF boundary is worth exploring. In this paper, a complete experimental analysis
of far- and near-field channel characteristics using a THz virtual antenna
array is provided based on measurement of the multi-input-single-output channel
with the virtual uniform planar array (UPA) structure of at most 4096 elements.
In particular, non-linear phase change is observed in the NF, and the Rayleigh
criterion regarding the maximum phase error is verified. Then, a new
cross-field path loss model is proposed, which characterizes the power change
at antenna elements in the UPA and is compatible with both FF and NF cases.
