RJ Burkholder, Emeritus Research Professor of Electromagnetics and RF at The Ohio State University
Probing for Tag Readability
In the last blog we looked at the importance of tag deployment in a UHF RFID system. Often this is out of the control of the designer in applications involving a large variety of tagged items, but there are many specialized applications where the tag placement is unchanging and needs to be optimized. In this blog we investigate how a given environment may be easily probed, or pre-tested, to determine the readability of a tag at any point in the coverage zone.
To review, we explored the basic characteristics of an antenna: gain, Effective Isotropic Radiated Power (EIRP) and polarization. Assuming the reader antennas are deployed for good illumination of the volume containing the tags, polarization is the most important consideration in the deployment of a tag. As we learned previously, polarization defines the predominant direction of the electric field radiated or received by an antenna. For a conventional antenna like the patch antenna, the polarization is always transverse to the direction of radiation outside the near-field region of the antenna. Therefore, the polarization of the receiving antenna (RFID tag) needs to be at least partially aligned with the polarization of the transmitting antenna, as illustrated below.
Alignment of the receiving antennas (RFID tags) relative to the polarization of the reader antenna (red arrow) and radiation direction (blue arrow).
Using more than one antenna certainly helps the situation as shown below. As discussed in previous blogs, this introduces antenna diversity and polarization diversity. RFID readers typically have more than one antenna port for this reason. They cycle through the antennas to maximize the total number of reads in a given coverage area. If one antenna polarization is not aligned with all of the tags, it is likely that one of the other antennas is aligned.
Two antennas provide polarization diversity for reading an RFID tag that a single antenna is not able to read due to the tag orientation.
Many of the above issues associated with conventional patch antennas are overcome by the specially designed Wave® antenna as described previously and shown below. The multiple overlapping beams of the Wave® antenna naturally provide spatial and polarization diversity. For this reason, it is the obvious choice for item-level RFID systems. The Wave® antenna virtually eliminates read errors caused by misaligned tags.
The Wave® antenna has distributed radiation from multiple points along its length. Tags in virtually any orientation are readable.
Received Signal Strength Indicator (RSSI)
To optimize the number and deployment of the reader antennas, it is essential to know precisely how well a given zone is covered. This can be achieved by probing the RF field at a sufficient number of points in the zone using a small dipole receiving antenna. The polarization at each point is measured by rotating the dipole in three orthogonal directions, such as the (x,y,z) directions. However, a passive RFID tag already does this using only the reader data. The Received Signal Strength Indicator(RSSI) is directly related to the electric field component aligned with the dipole tag as shown below.
Passive UHF RFID tags can be used to probe the electric field radiated by the reader antenna. A dipole tag measures the component of the electric field aligned with the dipole.
As the figure illustrates, the z-oriented tag measures the z-component of the electric field. Another x-oriented tag is placed near the z-oriented tag to measure the orthogonal polarization. A third y-oriented tag could also be used for a complete 3-component measurement.
There is an important reason why the RSSI is proportional to the electric field at the tag. It is often assumed that the RSSI is the magnitude of the reflected signal from the tag. This is not true for passive tags. The RSSI is the power required to activate the tag chip. Passive tags do not have an onboard power supply, so they must harvest power from the RF field radiated by the reader. The RF power is received through the tag antenna and rectified to power up the chip. Therefore, the RSSI is directly related to the one-way signal propagation path, not the two-way reflected path.
RFID Tag Panels
RFID tags provide a cheap and efficient means of probing a large coverage volume. Ordinarily, to probe RF fields with a receiving antenna, the antenna must be moved from point-to-point and the data collected at a very large number of points. This can be very time consuming. On the other hand, a large number of RFID tags can simply be placed at the desired points and measured all at once.
A very practical means of achieving volumetric probe data is to mount the tags on a sheet of RF-transparent material such as Styrofoam as shown below. The tags are spaced about ½ wavelength (6 inches) and oriented in two orthogonal directions. One measurement provides the tangential field components over a cross-section of the volume of interest. The panel is moved in increments of 6 inches in the y-direction to cover the entire volume. To measure the 3rd component normal to the panel (y-direction), the panel may be rotated 90° and moved in the x-direction.
RFID tag panel used for probing the fields of volumetric regions. Tags are spaced about ½ wavelength (6 inches) and oriented in two orthogonal directions.
A very advanced panel is being used by NeWave RFID to probe the fields of portals as shown below.2 Besides the tags mounted vertically and horizontally on the panel to probe the x- and z-components, it has tags protruding out from the panel to probe the y-component. This large 7’ x 8’ panel measures all three components simultaneously as it is moved through the portal.
Advanced 7’ x 8’ NeWave panel can probe all three components of the electric field over a large area. The panel is used to probe RFID portals and other large spaces.
Pg. 5 of 6Sample RSSI results for the NeWave panel in a portal are shown below. The (x,y,z) components are plotted on a dB color scale for three Wave™ antennas installed on the left side of the portal. The fourth antenna port is turned off. The Max RSSI is the maximum of all three antennas. It shows very good coverage of the entire area even though the antennas are on the left side of the portal.
Probing the fields radiated by the reader antennas is a good idea after an RFID system is implemented. It provides a direct performance evaluation of the system for all points within the coverage zone and will expose regions of poor coverage. Of course, when there are tagged objects placed in the zone, the field distribution will change due to blockage and multipath. The probing of the zone should be performed with no objects present to provide a baseline metric. It also highlights areas that need better coverage by adjusting the reader antenna deployment or adding more antennas.
It is worth the time and effort to perform this type of field probe analysis for any fixed RFID system. Efficiency will be optimized by determining the minimum number of required readers and reader antennas, and the read rate will be optimized by insuring that there are no coverage holes in the zone.