We wanted to measure two things: how well the interrogator can read the tag from various distances, and how the orientation of the tag impacts read distance. To obtain that data, we placed the tag at varying distances from the interrogator's transmit antenna—from 1 to 5 feet, in 1-foot intervals. Then, to understand the impact of orientation, we rotated the tag along two angles at 5-degree increments or less. Next, at each distance and angle, we performed 100 read attempts and recorded the number of successful reads. All told, we performed at least 72,000 read attempts for each tag.

For any object in space, there are three angles of rotation. The X and Y axes form the E-plane (E stands for "electric field"), and rotations in that plane are defined by the Greek letter f. The X and Z axes form the H-plane (H stands for "magnetic intensity"), and rotations in that plane are defined by the Greek letter q. These two angles and planes represent the different ways that we rotated the tag at different distances.

Planes of rotation: To understand how the orientation of the tag impacts read distance, we rotated the tag along two angles.

The third plane, defined by the Y and Z axes, is the polarization plane. If the interrogator antennas are circularly polarized, the orientation in the Y-Z plane should not matter. But we found that the Mercury4 antennas were not completely circularly polarized. The Mercury4 was better able to read the tag when the long dimension of the tag and interrogator antennas were perpendicular rather than parallel with each other. Therefore, all our measurements were taken with the tags and antennas perpendicular. What this means is that the quality and polarization of interrogator antennas can have a substantial impact on tag performance.

What We Found
We plotted our test results using a color-coded polar plot that we call a radiation pattern. The distance between the tag and interrogator transmit antenna is represented by concentric circles; circles farther away from the middle represent larger distances between the tag and interrogator. The angle in the plot represents how we rotated the tag.

The colors in "Color Code for Response Rate" correspond to the percent of responses at a distance and orientation. Between data points, we used a simple linear interpolation. So, for example, the Rafsec tag was readable about 100 percent at a distance 2 feet away and rotated at an angle of 60 degrees in the E-plane. The tag becomes unreadable when rotated to 90 degrees, or rotated 60 degrees, and placed 3 feet away.