Your Guide to UHF RFID Tags

UHF RFID or RAIN RFID is an emerging IoT technology that is gaining adoption across multiple industries because its combination of low-cost tags and several-meter read range. Inventory management, asset tracking and authentication solutions are just some of the use cases being enabled by RFID in retail, logistics, supply chain, healthcare, airline baggage, automotive and manufacturing. Many of these solutions require an optimized tag to enable long read range for tagging a specific material and form factor; this leads to a large variety of tags from tiny near-field button tags to large far-field dipoles. Here is a sample of different tag types and inlays available on the market today.

This article will not cover BAP or Active tags; please contact RFID4U for further information about those tags.

There are some general-purpose passive label-style tags, but most are specifically designed for targeted applications. Label-style tags are also designed for short lifecycles like barcode labels on consumables but there are also durable tags designed for industrial applications that have very long lifecycles. In recent years, label-style consumable tags have seen hyper-growth (>12 billion consumed in 2018) due to adoption in retail apparel for inventory management. Industry experts expect continual growth in retail and supply chain applications which will undoubtably drive further innovation in tag technology. Here’s an example construction of a label-style tag. 

Durable tags generally have RFID inlays encapsulated in hard plastic with an industrial adhesive backing. In some cases, the internal inlay is replaced with a PCB for more durability. These tags can withstand significantly higher temperature ranges, impact resistance and some are designed to work on metal. These durable tags are used to track high valued assets in industrial and warehouse environments.

Be sure to checkout the pre-printed, pre-encoded TagMatiks RFID tags to help speed up your RFID project.

How do tags work?

1. Reader sends RF energy (CW) to tag (black waves)
2. Tag harvests RF energy and wakes up digital controller (-20dBm is enough for the latest tags)
3. Reader continues to send RF energy (CW) plus query (hello) command
4. Tag recognizes query command and backscatters information (red waves). Backscatter energy is simply reflected waves off the tag while the tag is modulating data (0s or 1s) by changing its impedance to the tag antenna.
5. Reader then receives the backscatter waves and demodulates the tag information. 

Key factors to consider when choosing an RFID tag

• Performance - tag read distance is usually the primary performance parameter. Tag chip sensitivity (-20dBm in 2019) and antenna design are the key factors determining performance.
• Size – larger tags generally provide more read distance but often performance is compromised to meet size constraints.
• Type of item – materials have different dielectric properties which require different antenna designs; example: hard plastic vs. cardboard can have very different dielectric properties which influences the antenna performance. Metal and liquid filled containers also require different tag designs.
• Memory usage – Tag IC products offer different memory footprints. EPC length and User memory are often deciding factors.
• Environment – temperature and humidity can both play a role in performance and reliability. This is where tag durable tags can offer more value.
• Lifecycle – RFID technology has a similar lifecycle compared to other electronics, but tag construction can be a limiting factor. 

Understanding the basic components of an RFID inlay

Tag chips are the brains within the RFID tag; they include basic radio blocks, a state machine controller and non-volatile memory. Tag chips are usually 300-400um per side which is about the size of a grain of salt. The chips have bumps or pads for electrical contact with the antenna. High precision pick and place machines are used to place and glue the tag chip in place. Historically, Impinj and NXP have updated their tag chips every 18 months; updates usually include better tag sensitivity, lower costs and additional features like Autotune which provides internal RF tuning capabilities to compensate external changes that can detune performance.

Tag chip blocks

Inductive loop is part of the antenna design. Its main purpose is the resonate with the chip’s capacitance for matching at the desired frequencies (around 900MHz). It's also optimized to deliver the maximum power coupled from the dipole antenna section to the chip.

Dipole antennas are usually half-wavelength (about 15cm at 900MHz) which is too long for current applications; designers tend to meander the dipole transmission line to increase the effective electrical length while maintaining a shorter physical length. The width of the dipole transmission line and end load are designed to match to the optimal source impedance of the tag chip while maximizing antenna radiation performance. The dipole performance will vary depending on the dielectric properties of the material the tag is placed on; for example, tag designed for cardboard (light dielectric) will not be optimal for denim (heavier dielectric). The latest tag chips from Impinj and NXP have autotune technologies that can compensate for some material variance. Autotune is simply a built-in variable capacitor that optimizes the tag chip’s input matching every time it’s powered up. This terminology is foreign to most engineers because it falls into RF or microwave engineering. Antenna design at 900MHz for current RFID chips is not an easy task; please consult with the experts at RFID4U if you need to dive into the details.

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