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A comprehensive characterization and model of the UHF RFID channel is presented for narrowband through ultra-wideband tag localization systems. The analyses are based on ultra-wideband channel measurements in a warehouse portal, centered around 900 MHz. Measured scenarios include an electromagnetically transparent pallet and a pallet containing liquids, each for a portal shielded by metal backplanes and for a portal shielded by absorbing material. The presented analyses cover the individual channels to and from the tag, the feedback channel, and the backscatter channel, for bi- and monostatic reader setups. We find that the direct path is rarely dominant on the backscatter link despite clear line-of-sight conditions and directive reader antennas. The power ratio between the direct and all indirect paths ranges from -20 through 5 dB for the more common metal portal, and RMS delay spreads are in the range of 10-80 ns. Since only the direct (line-of-sight) path carries the correct distance/direction information, tag localization in such portals requires high robustness with respect to weak line-of-sight components. We also show that classical channel models in UHF RFID, despite predicting the incident power level at the tag accurately, produce far too optimistic estimates of channel parameters relevant to ranging and localization.