RFID Journal Staff asked 10 years ago Can passive radio frequency identification technology be utilized for this application? And what format should I use?
—Name withheld
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The vast majority of toll-collection systems currently in use employ active radio frequency identification technology, because active tags broadcast a signal that can be picked up by a reader antenna better than 99.9 percent of the time. As passive RFID systems have improved, however, some toll-collection agencies have begun introducing them (see Argentine Drivers Use Passive Tags to Pay Bridge Tolls and RFID Puts Salt Lake City Drivers in the Fast Lane), since the transponders are cheaper and need not be replaced due to a dead battery (passive tags, of course, have no battery). Passive RFID toll-collection systems almost always use passive ultrahigh-frequency (UHF) technology, because it provides a read distance of 10 feet or more, which is required in order to capture data from tags via readers installed above car lanes.
When you ask about antennas, I'm not sure if you are asking about reader antennas or tag antennas, or if you mean "antenna" as short-hand for the entire reader or tag. But I will try to answer your question as best I can.
You can use a variety of tag antenna formats. There are a few things to understand, however. One is that if you use tags that drivers will place on windshields, you will need tags specially designed for use on windshields. Glass can have a dielectric constant of 14.5, which means it conducts electricity (I'll explain dielectric constants at the end of this post, for those who might be interested). Anything that conducts electricity can detune a tag and reduce performance. Moreover, some windshields have a metallic film that can detune a tag, so a special windshield tag is required—or you could create a plastic holder that would be affixed to the inside of the windshield, but would keep the tag far enough from the glass to avoid performance issues.
Generally speaking, you can use a dipole antenna on a tag and a linear-polarized antenna on a reader, so you can ensure that the waves' orientation from the reader is aligned with the tag antenna's orientation. A linear-polarized antenna will get more energy to the tag, and thereby improve tag readability. (A circular-polarized antenna is preferred for situations in which you cannot control a tag's orientation to a reader antenna). William Frick & Co. and other companies make windshield tags, and there are many firms that offer tags in plastic cases that can be branded and affixed to a windshield.
Dielectric constant is, in laymen's terms, a measure of a material's relative ability to conduct electricity. Air has a dielectric constant of 1, which is why RF energy travels through air freely. Water has a dielectric constant of 80, which is why RF energy doesn't travel it through it very well. A high dielectric constant means that material will detune an RFID antenna and reduce performance, so special tags are needed for use in the presence of these materials.
—Mark Roberti, Founder and Editor, RFID Journal
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