If you are thinking about installing an RFID system, the choices may seem daunting. There are many factors to consider such as Location, Coverage Range, and Potential Interference. RFID applications can be as diverse as environmentally sensitive products for Cold Chain Management to large-volume active tracking products for Homeland/Port Security to impact-resistant products for Distribution & Warehousing settings with heavy equipment and obstructed range.
Because Mobile Marks expertise lies in the area of RF communications and antennas, this paper will address the antenna requirements for effective RFID systems. Efficient RFID antenna designs will provide optimal coverage, quicker registration of the RFID tag and greater hit-rates for tag reading. The right antenna will save the RFID users time and money by providing more reliable and more accurate reporting.
Frequency: RFID antenna designs operate on a variety of frequencies depending on the application and the location. Current RFID systems include: HF at 13.56 MHz, UHF (or UltraHighFID) at 915 MHz (or 868 MHz in Europe) and RTLS (Real Time Location Systems) at 2.4 GHz. HF applications have been around longer and include applications such as the tagging and tracking of library books. Most UHF applications are newer than HF applications, so there are more unanswered questions as we move through the learning curve. For the sake of focus, this paper will cover RFID Antenna Designs for UHF frequencies.
Orientation of the tags: Polarization is important for optimal signal transfer. Typical wireless network antennas that communicate with each other are installed so that they have the same linear polarization, for example both would be vertically aligned or horizontally aligned. Because it is difficult to know the orientation of the small RFID tags placed on many items, RFID antenna designs that are circularly polarized, rather than linearly polarized, typically provide higher read probability for those important high traffic settings; it easier to get reliable read capture rates with a circularly polarized RFID antenna design when the tags are coming at the reader with all different orientations.
Polarization direction: RFID antenna designs are typically circularly polarized in a particular direction. In the US, the standard configuration is right-hand circularly polarized and in Europe it is left-hand circularly polarized. Manufacturers will generally mark the RFID antenna design as either RHCP or LHCP to indicate the type of polarization. In most applications the actual Circular Polarization direction doesnt matter since the tags are linear; either RHCP or LHCP will work the same. The exception would be the new CP (Circularly Polarized) tags which do require matching polarization. Check with your RFID Manufacturer to confirm if your reader and tags are consistent with your RFID antenna design.
Gain: Antennas for the RFID Readers are used to direct and shape the RFID signal from the Reader. The antennas are available in different gains, which allow the network installer to optimize the coverage area. RFID antenna designs with higher gain throw the signal further but with a narrower beam; lower gain antennas will provide a rounder coverage pattern. Outdoor installations, such as in a car lot, may require a higher gain antenna but close-in applications, such as tracking items on a conveyer belt, will call for a lower gain antenna. Selecting the right level of gain for the application is important; for example, an RFID reader installed to read packages coming in one dock door should not be reading tags on packages coming in through a neighboring dock door. Because each application is a bit different, RFID manufacturers often certify a variety of antennas for their RFID Readers so that the installer can select the RFID antenna design that best suits the particular application. Typical mid-range applications call for a 6-8 dBi gain antenna but close in applications, such as on a smart shelf might require only unity or up to 3dBi gain. Larger applications, such as in a car lot, might call for an RFID antenna design of 10-12 dBi gain.
Impact on RF performance: RF signals do not penetrate metal and will be reflected when they hit metal. At a minimum, RFID manufacturers will tell you that item level Tags need to be placed on items in such a way that they are not shielded by metal. Other materials, such as water, can also pose an RF problem; it is difficult to read an RFID tag through liquid.
Impact on durability of antenna: Networks that are set up outdoors will need to be designed to handle a range of weather conditions. This may include large swings in temperature, or resisting moisture damage. Most RFID manufacturers will provide detailed installation instructions with pointers such as dont cover drain holes. These are important guideline; understanding how the antenna should be installed will go a long way towards eliminating problems down the road.
Unusual settings: Every installation is unique but some settings pose more challenges than others. The Fork Lift setting, for example, provides ample challenges for RFID antenna design; the all-metal construction blocks efficient signal transfer while not providing enough backing for a solid ground plane. Look for an RFID antenna design with sufficient shielding and with mounting solution that allows the antenna to be mounted to the load back rest of a forklift truck for palletized scanning.
Another challenging setting involves tracking items that are removed from and replaced in fixed locations such as a retail display or production conveyor belt. The RFID antenna design referenced as a Near Field Loop offers maximum power density in close proximity to the antenna. The read range for this style antenna is tag dependent but is typically up to 3 feet. The high forward directivity of the Near Field Loop Antenna combined with extreme isolation adjacent to the antenna results in fewer instances of misread tags.
Sensor: Some RFID antenna designs feature an antenna present sensing circuit, detecting a 10K ohm resistance or DC Short. Some Antenna Manufacturers offer this as a standard feature on all of their RFID Reader antenna designs.
Connections: It is important to know which connector is used as the RFID reader termination. Check with the manufacturer, or the spec sheet, to see which connector is built into the RFID reader. The connector on the Reader will need to mate with the connector on the Antenna. Some RFID Antenna designs will build the mating connector directly onto the antenna; others antenna designs will put the connector onto a cable pigtail exiting from the RFID Reader.
Useful accessories: An RFID antenna design that features a built-in connector will typically require a jumper cable assembly for final installation. These cable assemblies and other useful accessories are often provided by the RFID manufacturers or by many antenna manufacturers. Jumper cables allow the antenna to be positioned further from the RFID reader which increases the variety of installations possible.
About Mobile Mark, Inc.:
Mobile Mark, Inc. designs and manufactures site, mobile and device antenna for 600 MHz 9 GHz. Applications include GPS Tracking & Fleet Management, Cellular GSM/CDMA, LTE, WiMAX, WiFi, RFID, Public Safety, Military and Machine-to-Machine (M2M). Engineering and custom design services available. Mobile Marks global headquarters, which include research facilities and manufacturing plant, are located near Chicago, IL. An additional manufacturing and sales facility is located near Birmingham, UK.