Generally speaking, three main parts make up in a passive RFID system – an RFID reader or interrogator, an RFID antenna, and RFID tags. Unlike active RFID tags, passive RFID tags only have two main components – the tag’s antenna and the microchip or integrated circuit (IC).
As the name implies, passive tags wait for a signal from an RFID reader. The reader sends energy to an antenna which converts that energy into an RF wave that is sent into the read zone. Once the tag is read within the read zone, the RFID tag’s internal antenna draws in energy from the RF waves. The energy moves from the tag’s antenna to the IC and powers the chip which generates a signal back to the RF system. This is called backscatter. The backscatter, or change in the electromagnetic or RF wave, is detected by the reader (via the antenna), which interprets the information.
As mentioned above, passive RFID tags have no internal power source, and a standard passive RFID tag consists only of an IC and internal antenna; this basic structure is commonly referred to as an RFID inlay. Countless other types of passive RFID tags exist on the market, but all tags generally fall into two categories – inlays or hard tags. Hard RFID tags are durable and made of plastic, metal, ceramic and even rubber. They come in all shapes and sizes and are typically designed for a unique function, material, or application.
A few different groups work to further divide passive hard tags; however, some tags will exist within two or more groups.
High Temperature – Certain industries, like healthcare, track the number of cycles that instruments undergo in punishing autoclaves. Specific passive RFID tags are designed to withstand extreme temperatures and accommodate for those types of applications, among others.
Rugged – Applications in outdoor environments or tough warehouses need a tag that can withstand snow and ice, dust and debris, or even the crushing forces felt under a tractor wheel. For these applications, a highly rugged passive tag is needed to make the application successful.
Size – Some applications have specific size constraints when tracking small or large items. Size is one of the more important questions to answer when choosing an RFID tag because there are many different sizes available.
Materials – If an application requires tracking metal assets, UHF metal-mount tags may be the only option. These tags are specifically designed to mitigate the problems UHF RFID faces around metal.
Embeddable – If tagging an item becomes a problem for specific applications due to significant wear and tear, embeddable tags can fit in small crevices and be covered in epoxy so the RFID tag is out of harm’s way.
Inlays are usually the cheapest RFID tags costing as low as $0.12 per tag in high volumes, but the price does not affect the performance. These inlays are grouped into three main types:
Dry Inlays – An RFID microchip (IC) and antenna attached to a material or substrate called a web. These inlays look like they have been laminated and come standard with no adhesive.
Wet Inlays – An RFID microchip (IC) and antenna attached to a material, usually PET or PVT, with an adhesive backing. Most of the time these inlays are clear and can be peeled off their roll and immediately stuck on an item.
Paper Face Tags – These are essentially wet inlays with a white paper or poly face. These are ideal for applications that need printed numbers or logos on the front for identification.
Passive RFID tags do not all operate at the same frequency. There are three main frequencies within which passive RFID tags operate. The frequency range, along with other factors, strongly determines the read range, attachment materials, and application options.
125 – 134 KHz – Low Frequency (LF) – An extremely long wavelength with usually a short read range of about 1 – 10 centimeters. This frequency is typically used with animal tracking because it is not affected much by water or metal.
13.56 MHz – High Frequency (HF) & Near-Field Communication (NFC) – A medium wavelength with a typical read range of about 1 centimeter up to 1 meter. This frequency is used with data transmissions, access control applications, DVD kiosks, and passport security – applications that do not require a long read range.
865 – 960 MHz – Ultra High Frequency (UHF) – A short, high-energy wavelength of about a one meter which translates to long read range. Passive UHF tags can be read from an average distance of about 5 – 6 meters, but larger UHF tags can achieve up to 30+ meters of read range in ideal conditions. This frequency is typically used with race timing, IT asset tracking, file tracking, and laundry management as all these applications typically need more than a meter of read range.
As a general rule, higher frequencies will have shorter, higher-energy wavelengths and, in turn, longer read ranges. Moreover, the higher the frequency, generally speaking, the more issues an RFID system will have around non-RFID-friendly materials like water and metal.
Pros of Passive RFID:
Much cheaper tags
Thinner/more flexible tags
Higher range of tag options
Tags can last a lifetime without a battery (depending on the wear and tear)