Tags (Transponders)
The AI1422E Reader System uses TransCore’s half-frame and full-frame read-only tags. When selecting a
tag for an application that requires the tag to be placed on the road bed, several design elements need to
be kept in mind.
Beam-powered tags have a shorter footprint than battery or switch-powered tags, and thus the maximum
top speed of the vehicle will be less, all other factors equal.
Battery tags have an inherently larger footprint, but the battery life limitation will require the operating
company to replace the tags every 8 to 10 years. The presence of the battery and its reactive mass reduces
the reliability of the tag, particularly when the tag is placed near high shock or vibration locations such as
switches.
Switch-powered tags have RF characteristics similar to battery tags, but the tags derive power from the
railway switch.
Tag Mounting
All tags used with the AI1422E must be mounted on a flat metal plate. The internal antennas of these tags
are tuned for the backplane of the tag to be in contact with a flat metal surface. This metal surface does
not need to be extended beyond the outer dimensions of the tag, and having a larger metal surface will
not affect the immediate performance of the tag.
Metal located to the sides or above the tag can affect the tag’s performance. Metal surfaces or objects
should not be placed closer than 1 inch (2.5 cm) to the side edges of the tag to ensure that the tag’s
antenna tuning remains within design criteria. Metal placed above the tag can cause shadowing of the RF
beam, both in the incident and return directions, and should be avoided. The only exception to this rule
would be metal placed for the purpose of affecting the read range of the tag or footprint of the system, and
the placement of such metal should be done with careful planning and testing to ensure proper system
performance. With battery-powered tags, the arrangement of intentional metallic obstructions may be the
best way to limit the broad footprint of these tags, as well as improve the repeatability of the TLS signal
with respect to its absolute position relative to the tag. These tag mounting details are determined by the
customer on the basis of each customer’s overall system design and requirements. Keep in mind that metal
placed too close in proximity to the tag will affect the tag’s antenna tuning, and may unintentionally affect
principal design parameters such as VSWR or the impedance match of single paths internal to the tag, to
name a few examples.
Antenna-to-Tag Centerline Alignment
For best performance, mount tags so that the centerline of the tags and the centerline of the reader
antennas are within ±2.9 inches (±7.5 cm) of each other (
Chapter 1 System Overview
TransCore Proprietary
1–3