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RFID packaging: Sometimes more important than circuits

Electronic engineers often spend a lot of time on circuitry and components, as they should; after all, that’s their job. Still, there are cases where that aspect of the project is relatively straightforward compared to the overall product packaging. Radio-frequency identification, better known as RFID, is a good example of this.

Take a look at the recently introduced Omni Marker II RF marker tag series from  Tempo Communications which are designed to be  buried with, then used to locate underground pipes, utility lines, and more, Figure 1.

Figure 1. The Omni Marker II RF marker tags for buried pipes and cables from Tempo Communications are passive, sealed, and color-coded to their frequency and intended application.
Figure 2. The corresponding EML-100 Electronic Marker Locator can energize and sense all the frequencies supported by the RFID tags, and indicate which one, to allow differentiation when different-frequency tags are located in proximity to each other (a common occurrence). Click image to enlarge.

The tags use the classic passive RF technology, with an inductive coil acting as an antenna and capacitance forming an LC circuit that resonates at the working frequency with a dipole magnetic field; the marker derives its energy from the magnetic field of the detector. These tags operate between about 50 kHz and 150 kHz with specific frequencies assigned to the various applications, Table.

They are housed in high density polyethylene, have a detection range of about 1.5 meters (5 feet), a diameter of 100 mm (about 4 inches), and weigh 154 grams (about 5½ oz); their frequency tolerance is specified at ±0.35%. To find the tags regardless of frequency, the company offers the EML-100 Electronic Marker Locator which looks like (and is likely similar to) a metal detector, Figure 2.

Given the frequency and their passive nature, the circuit itself must be fairly simple (I couldn’t find a schematic, but it’s a good bet). While many RFID tag circuits are packaged in thin plastic sandwiches, often with adhesive on the back, these tags have a more sophisticated gimbaled package, Figure 3.

Since the orientation of the tag when placed in the ground is random, it makes sense to have a gimbaled coil mount so the tag to optimize its “up/down” radiation pattern (it’s supposed to be attached to the pipe firm in a fixed position using cable ties, but stuff happens), Figure 4.

Figure 3. The Omni Marker II RF marker tags have an internal gimbal mount for the coil to ensure optimum orientation towards the surface.
Figure 4. The received signal strength is strongest when right over the tag, since the antenna is self-gimbled to point straight up.

Interestingly, the company also offers an older RFID tag called the Omnimarker which has three orthogonal coils, Figure 5; their US Patent 5,699,048 (“Omnidirectional Passive Electrical Marker for Underground Use”) shows the arrangement of the coils and the resultant RF-field pattern while US Patent 6,097,293 (“Passive Electrical Marker for Underground Use and Method of making Thereof”) details the construction and assembly. I wonder if the gimbaled tag is now offered as improvement since makes better use of RF stimulus energy, or provides a better response signal due to its inherent orientation towards the surface, or both.

Figure 5. A previous tag, the Omnimarker, uses a trio of orthogonal coils and thus needed no gimbal arrangement.

Regardless of electronic-design objectives, both the three-coil version and the gimbaled-coil version of these passive RFID markers show that packaging can be a major consideration of product design. Have you ever been involved with a simple circuit where the packaging effort and production issues were as much or more involved than the electronics alone?

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