Technical Note - Part 1

Why the Triple-Axis Walkthrough Antenna Design Overcomes the Problems with Single Loop Walkthrough Antennas

Tags can only be read with single axis (single loop) antennas when they are within a certain area, or field, adjacent to the antenna. The shape of the field depends on the orientation of the tag. The speed of the tag will determine whether the tag remains in the field for the length of time necessary for it to be read. The tag must be in the field for more than one and half cycles, about 150ns for it to be read.

The typical problems experienced with single loop antennas are explained below; the diagrams illustrate how the orientation of a tag affects the shape of the field where it can be read.

The first diagram shows the field pattern of a vertical antenna when viewed from above (remember this is a single dimension view, the antenna links with the tag in three dimensions all behaving in the same manner). The axis of the tag is on the same axis as the antenna. The specified read range for a particular tag is generally obtained with this orientation. A read range measured in this manner can be very misleading. No allowances are made for the changes with tag orientation or field shape.
The second diagram shows the change in the field shape when the axis of the tag is rotated to an angle of 45 degrees to that of the antenna. Note the reduction in maximum read range and the large areas where the tag cannot be read.
The third diagram shows the shape of the fields when the axis of the tag is at 90 degrees to the axis of the antenna. The read range is only about 65% of the maximum; note the large area near the centre of the antenna where there is no field. A tag cannot be read here and it is possible to pass a tag straight through the antenna without it being read.

The Triple-Axis Walkthrough Antenna

A detailed investigation into the performance of single and multi-loop walkthrough antennas was carried out by ATL in the late 1990's and the analysis of the results provided a clear insight into the problems which cause poor antenna performance.

The conclusion made was that the only foolproof solution which could ensure that a tag was read in all orientations was a combination of three antennas mounted on the three Cartesian axis, x,y and z. This arrangement provides an uninterrupted field for the length required to read a tag in any orientation at the speeds necessary for accurate identification of cattle in markets and abattoirs.

Various construction techniques were considered, protection of the cables and the ability of the structure to withstand the passage of cattle being the main priorities; steel offered the best solution.

The outcome was this investigation was the Triple-Axis Walkthrough Antenna.

ATL automatic identification technology is protected by international patents.