What determines the distance from which sensor data can be transmitted?
The distance from which sensor data can be transmitted depends on the type of sensor used. Broadly speaking, there are three types of RFID sensors: purely passive, battery-assisted and active. A purely passive sensor can be either high-frequency (HF) or ultrahigh-frequency (UHF). The read range is similar to that of a purely passive tag. A passive HF transponder can be read from a distance of about 3 feet, while a UHF tag can be interrogated from 10 to 30 feet away, depending on the tag, environmental conditions and other factors. So this is the range from which you can expect to collect sensor data.
A battery-assisted passive (BAP) UHF tag has a battery to power the sensor, and the chip bit reflects a signal back to the tag like a regular passive UHF tag does. The read range is usually a little longer since the battery can power the chip, leaving more energy to reflect back a signal to the reader—expect a read range of 15 to 30 feet.
An active RFID tag with a sensor can broadcast a signal like a cell phone. The read distance is usually 1,000 feet or more.
—Mark Roberti, Founder and Editor, RFID Journal