By Jeffrey Polly, Vice President of Technical Services
RFID antennas perform two very important functions. First, they transmit power to the RFID tags by activating them, and second, they receive data back from the activated tags. A single antenna can activate and receive data from multiple tags simultaneously. The antennas are connected to the RFID reader using a coaxial cable. The antennas have no intelligence and are completely controlled by the RFID reader.
When operating the antenna, it propagates a cloud of radio frequency (RF) energy. Any RFID tags present in this cloud will be activated and read. This cloud often referred to as the tag read. The field varies in size and shape depending on the antenna type and design.
There are two common types of antennas. There are the linear and circular polarized antennas. Linear antennas generally propagate a narrow but longer tag read field. Circular antennas propagate a much wider but shorter tag read field. Circular antennas are more commonly used for most applications whereas linear antennas are used to read tags in specific targeted areas.
There are several important criteria to consider when selecting the right antenna. First, you need to know the type of tags to be read. Are they single dipole or dual dipole? A single dipole RFID tag has one antenna similar to a linear RFID antenna while a dual dipole RFID tag has two antenna elements similar to a circular polarized RFID antenna. Here are the basic rules for the relationship between RFID tags and RFID antennas:
- Circular RFID antennas perform well with both single and dual dipole RFID tags but have a somewhat limited read range due to signal loss
- Linear RFID antennas perform well with dual dipole RFID tags and have a longer read range than the circular antenna due to less signal loss
- Using single dipole RFID tags and linear RFID antennas require careful planning that insure the RFID tag maintains proper physical orientation in the tag read field or the tag may not be read.
RFID antennas propagate both a horizontal (azimuth) and vertical (elevation) beam that forms the tag read field. The width of these beams is a function of each antennas individual design. The width and height of these beams are expressed in degrees (0-360) and measured from the center of the antenna.
Once you have a clear understanding of the physical area where RFID tags need to be read, you can then reference different antenna beam width and height specifications, referred to a beam patterns and lastly, to find the antenna that provides the physical coverage you need. These beam pattern specifications are readily available online from all the RFID antenna manufacturers. Make sure your site is ready for RFID with our readiness assessment.
Another important antenna specification is antenna gain. Gain is expressed in decibels (dB) and describes either an increase beam strength (gain) or a decrease in signal strength (loss). When you attach a reader to an antenna, use a coaxial cable. Depending on the quality and length of the cable, you will lose a certain amount of power between the reader and the antenna. This is known as line loss.
RFID antennas typically provide gain to make up for the line loss. A general rule of thumb is that for every 3 dB in gain your beam power doubles and conversely for every 3 dB in loss your beam power is cut in half. So you can conclude that varying the gain of an antenna varies the beam power which in turn varies the physical size of the tag read field. You must pay attention to your antennas gain specification to ensure you produce the desired RFID range while staying within FCC rules.
With this basic understanding of the relationship of an RFID tag to an RFID antenna and RFID antenna properties, you are now better equipped to create successful RFID tag read zones.
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