Learn how passive UHF sensor tags can be read to over two meters while market solutions in LF and HF sensors can only communicate to some centimeters.

Passive UHF sensor tag with integrated relay

Passive UHF sensor tag with embedded relay

If you are shopping for wireless sensor solutions you will typically find two specific groups of solutions:

  • Active wireless sensors. These are generally in the UHF band, ranging from 433MHz to 2.4GHz, different communication protocols (WiFi, Bluetooth, Zigbee, proprietary, etc.) and long communication ranges of over a kilometer in some cases. These include batteries or other external power sources.
  • Passive wireless sensors. You will find more solutions on the LF and HF bands in this case and their communication ranges vary from some millimeters to some centimeters. However, these do not require batteries for performing.

Why using different bands for apparently similar functionalities?

As discussed in this previous post, working in the near field allows you to harvest more power from the RF field. This is why most passive sensor solutions are designed as LF and HF solutions – you need the energy to power up the sensors that you need for your application.

However, you are also looking for a form factor that is acceptable. LF and HF solutions require very long antennas to work properly due to their long wavelengths. In contrast, UHF sensor solutions require much shorter antennas, which makes them easier to implement in real life environments. For this reason, in active wireless sensor networks – where the energy is not a limitation since you have a battery – UHF is much more popular.

So how do I get a longer read range for passive sensor tags?

There are two options: you either design bigger components (both at the reader side and the tag side. Consider antenna lengths shown in this picture) or try to work in the far field.

RFID UHF sensor tags take this second approach. Working in the far field means reducing the power harvested BUT being able to maintain it at longer ranges (remember that power received is not reduced as dramatically as in near-field).

The key with UHF sensor tags is being smart when managing the power you harvest. Starting from an optimized RF IC for battery-free sensor tags to selecting and designing the electronics for low power consumption.

Analog and digital sensors as well as actuators can be implemented in passive UHF sensor tags to work battery-free and under the EPC C1G2 standard with read ranges of over 2 meters.

Got a better understanding about LF, HF and UHF sensor solutions?