Antenna size of an RFID tag does not have a direct relationship with read range. However, for a good antenna design, the bigger the antenna, the higher the antenna gain, resulting in a longer read range.
By this point you know enough about read range of UHF RFID systems to tell the above title is not entirely correct. To be honest, I’m just trying to connect antenna size to very important characteristics of RFID systems for simplicity’s sake. The (incorrect) assumption here is that both the reader antenna and the tag antenna size are directly linked to their antenna gain.
As you already know, the title should say something like ‘How read range depends on antenna gain’. However, as a general assumption, when the antennas are designed properly, the bigger the antenna, the higher its gain, resulting in a longer read range.
Miniaturization of UHF RFID tags
We’ve all been there: the smaller the electronics, the easier to implement them in most applications. Smaller RFID tags fit in more items, making them a better fit both physically – less space required – and in terms of cost.
However, people assume smaller tags need to work fairly similar to ‘standard’ tags. I’ll call standard tags those tags with antenna size of 90-100 mm approximately. These are the best known tags due to their long read range. Examples include the ALN-9740 ‘Squiggle’ from Alien Technologies or the ‘Dogbone’ from Smartrac.
Due to technical reasons we will not go into here, the length of the antenna has a direct impact on performance – antenna length is linked to 868 MHz wavelength. Standard tags are based on dipole antennas which have a high performance – high antenna gain.
You can design smaller antennas by ‘meandering’ the tag antenna. This is done to maintain the antenna length while reducing the required space. This results in a lower antenna gain, which reduces overall tag performance.
Increasing read range with bigger antennas
Many people show interest on increasing read range of existing tags, especially in RFID sensor solutions.
With a shorter read range due to higher power consumption in the tag, 2 meters read range for a passive UHF RFID sensor may be below customer requirements in some applications. However, remember that the read range of the tag depends on the RFID IC sensitivity, the sensor power consumption and the antenna gain.
You cannot change the RFID IC or the sensor, but you may be able to use different antennas in your application. In case you don’t have limitation in antenna size, completely passive sensors can be designed with longer communication ranges.
We at Farsens have tested battery-free sensor tags with Yagi antennas for example. These showed reliable sensor read ranges of up to 10 meters, the only disadvantage obviously being the size of the passive device.
The key here is to help you understand that read range and tag size are tightly linked. We are not saying a bigger antenna is always better – you may find manufacturers that do not design correctly and thus their tags are bigger and have a worse performance. However, in general, the bigger the antenna, the longer the read range of an RFID tag.
Get the ‘The secrets of read range in UHF RFID sensor systems‘ free eBook to learn more about read range.