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Ubiquitous temperature monitoring is important to boost visibility in applications such as cold supply chain management. Current sensors monitor and log a time history of temperature data, but their cost limits the scale of deployment. In this paper, we propose an ultra-low cost temperature threshold sensor using the UHF RFID tag antenna as a sensing mechanism. Permanent changes are induced in the tag antenna electrical properties upon violation of a temperature threshold. This manifests itself in a change in backscatter power detected at the reader. We demonstrate how these changes are effected via shape memory polymer actuation. Experiments demonstrate that cheap, reliable temperature threshold sensors can be developed which are independent of the material of deployment, orientation of the sensor, which have a read range of over 3 m and which have a customizable critical temperature threshold.