Learn the basics of how to tackle antenna matching and input impedance to design antennas for UHF RFID sensor tags and standard ‘only ID’ tags.

As an important part of your job as an RF engineer, you know how complex designing an antenna for RFID tags is. Unlike in active wireless systems where wireless device’s antenna is only used for communication, passive RFID tag antenna serves both for communication and energy transmission purposes.

Energy transmission always involves a key parameter which is efficiency. No matter which metrics, algorithms or values apply to different situations, being able to transfer as much energy as possible is your objective.

In passive RFID, there are two key points for the energy transmission efficiency:

  1. Harvesting energy through the antenna and correctly passing it through the electronics – antenna-IC matching
  2. Conversion of the incoming energy into direct current for circuit operation

We will leave the second point for a future post and focus on the antenna-IC matching in this post.

Antenna matching

I’ll need to introduce the concept of electrical impedance to start with. Simply put, electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.

Without going too deep into technical explanations – antenna experts, please bear with this simplification – energy is best transferred between two systems when their electrical impedances are matched.

However, the key with RFID and specifically with RFID sensors is that the electrical impedance of the tag side can vary quite a lot.

Input impedance of an standard RFID tag

Input impedance is given for RFID ICs in their datasheets. It seems to be an exact value but that’s only a simplification the IC manufacturer does for you.

The load of the RFID IC is variable. The electronics in the RFID chip have different power consumptions in different states. When the reader is not transmitting for example the RFID IC is completely off so there is no energy consumption. When the reader is asking the tag to write data on the tag, the power consumption is higher than just reading, and so on.

Measuring input impedance helps matching antennas properly

Measuring input impedance helps matching antennas properly

It is important to note that the input impedance is different as the load changes. In the case of standard, ‘only ID’ tags, the input impedance may not change that much so IC manufacturers select the value for you.

Input impedance of RFID sensor tags

This is not the case for sensor tags. Bear in mind that sensor tags have a load that is comprised of the RFID IC plus the sensor power consumption plus the startup circuit plus potentially a microcontroller.

Input impedance of your system from the antenna design perspective is now very variable. You have multiple states, from all power off – when the reader is not transmitting – to only RFID IC on – gives you the ID but sensor is off – to both RFID IC and sensor circuitry on.

Matching the antenna to your RFID tag

The key takeaway of this post is to understand that it is YOU to decide where to match the antenna.

Obviously, being able to dynamically change the antenna impedance to match that of the RFID circuit’s would be great. It is very difficult to do in a passive system though.

Basically you will have to decide which matching point makes more sense for your applications. Will you match when the tag is completely off to improve startup time, maximizing power transfer in the initial moments but having a mismatch during operation, or will you target a longer communication range by having a mismatch in the initialization – risking it to the system not waking up at all?

Remember to take your own input impedance measurements to make sure the antenna is matched to your needs, not those stated in the datasheet.