RFID antennas are always connected to an interrogator. This allows for the transmission of signals to and from the tag.
Depending on the design, antennas can be either mono-static or bi-static:
- Mono-static antennas are based on a principle by which a single antenna transmits a signal coming from the interrogator to the area as well as receives a signal coming from tags and these functions are switched in fractions of seconds. This requires use of a circulator in a reader that multiplexes the receive and transmit signals through a single port. There is some loss and phase distortion due to the use of a circulator.
- Bi-static antennas include two antennas, where one antenna is dedicated to transmitting, and the other antenna is dedicated to receiving. Both dedicated antennas can be but do not have to be in the same casing. In bi-static antenna, a circulator is not required, which improves the performance and sensitivity of the antenna.
Antenna polarity is very important because it affects the quality of communication between the interrogator and tag.
The interrogator's antenna and the tag's antenna should have the same polarization. If polarization is not realized, a severe loss in signal, along with a drastic decrease in a read range, which results in unsuccessful communication with a tag, can be experienced.
Polarization can be either circular or linear. Linear polarization is relative to the surface of the earth. Linear polarization can also be either horizontal or vertical:
- Horizontally polarized signals propagate parallel to the earth.
- Vertically polarized signals propagate perpendicular to the earth.
Antennas with circular polarization can receive signals from both the vertical and horizontal planes by injecting the signal at two points on the antenna radiated slightly out of phase creating a rotating effect on the field. However, there is a slight loss of signal strength, due to the constructive and deconstructive effect of the field being slightly out of phase.
Antenna Installation Considerations
Because RFID tags are subject to RF anomalies and orientation issues, it is often desirable to use several antennas grouped together and controlled by a common interrogator.
In supply chain applications, portals and tunnels are the commonly used configurations. Energizing and reading RFID tags are exercises in probability, in which the probability has to be maximized to increase the likelihood that a given RFID tag will be in the field of an interrogator's signal long enough that it can be read.
The best way to understand how an RFID portal works is through its typical application—for instance, in a warehouse where a forklift is moving inventory through a dock door. As the fork truck removes a pallet of goods from a truck at a receiving dock, the antennas on the portal are positioned such that the RFID tags on the pallet pass through the signal from the interrogator. The antennas of the portal may be connected to a single RFID interrogator, or each antenna can connect to its own individual interrogator. They are application dependent. An example of an antenna portal is shown in Figure 3.2.
Figure 3.2 RFID antenna portal.
In Figure 3.3, notice the four antennas. There are gaps in the coverage. These gaps may be acceptable if you are positive of the tag's placement relative to the antennas of the portal—for example, if you are certain that the tags on all the products that go through the portal are at the same height and location every time they go through the portal.
Figure 3.3 Dock door RFID antenna portal.
When RFID antennas are installed, the installation efforts should aim to create a sweet spot. A sweet spot is a volume of space where the likelihood of communication with RFID tags is maximized. Ensure that you create a portal sweet spot when designing your portal.
As discussed earlier, an RFID tag, once energized, requires a certain amount of time to power up and respond. It is important that the RF energy that is energizing the tag remain at a level sufficient to sustain the tag's functionality until it has successfully retrieved data from or to store data in to its memory. Because the tag can be powered only when it is in the beam of the RFID antenna(s), the tag must remain in the beam long enough for the required operations to occur. This is known as dwell time or time in beam. If the dwell time is too short, the tag may power down prematurely, and the read or write operation will not be completed. One way to maximize the tag's time in beam is to position the antennas so that the sweet spot is as large as is practical.
RFID tunnels are a variation on the portal theme and are typically used with conveyor systems.
Tunnels are often enclosed in RF-absorptive material, such as anechoic material. This is called a Faraday cage. Enclosing the tunnel this way helps contain the RF signal that is concentrating the RF energy.
A tunnel reduces the power output requirement of the interrogator. However, it can accidentally energize a tag on an item on a different conveyor by mistake. Therefore, care must be taken when planning and constructing a tunnel.
As with portals, multiple antennas increase the read probability in a tunnel. The enclosure helps to contain and concentrate the RF energy. Properly constructed enclosures also help keep the interrogators on one conveyor from interfering with the interrogators on another conveyor.
If you choose not to use a tunnel, proper antenna placement and attenuation in conjunction with a conveyor can produce similar results.
Imperfections in an Antenna's Coverage
Holes can be caused by a number of factors:
- Reflections of RF energy interfering with the radiated wave field (multipath)
- Imperfections in the antenna's reflector
- Interference from external sources
By using several overlapping wave fields, you can counter interference with any one wave, which is called multi-path interference. This can cause null points (holes) as well as points with very strong RF signal or noise.
Moving the tagged item through the wave field ensures that the tag does not stay in a hole for very long.