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Tips for Setting Up Portal Interrogators

By Andrew Price
Tags: Retail
Jan 03, 2007—By Mark Roberti

Configure interrogators:
Most UHF interrogators have general, portal and conveyor settings. Interrogators are usually set for general use before being shipped, so you need to change the settings. Doing this before the interrogators are shipped to the facility where they will be installed reduces disruption within the facility and speeds the installation process.

Protect interrogators:
If interrogators and antennas are in danger of being hit by a forklift truck or otherwise damaged, they should be protected either with hardened cases or bollards. Simon Langford, director of RFID and transportation for Wal-Mart stores, also recommends securing the power supply so people within a facility can't unplug an interrogator to plug in some other device. This prevents the loss of data that would occur if an interrogator were unplugged. And make sure there are no visible "On/Off" switches that people could hit, either accidentally or deliberately.

Set up bistatic antennas properly:
With an interrogator that uses bistatic antennas—one emitting RF energy and the other receiving it—never point the transmit and receive antennas toward one another, says Susan Flake, RFID systems engineering manager for Symbol Technologies. Otherwise the energy from the transmit antenna might drown out the weaker signal from the tag, preventing you from reading tags.

To allow maximum energy to reach the tags, which are likely to be in the center of the portal, Flake recommends alternating the transmit antennas and the receive antennas from top to bottom. One exception is when employees use skate racks—accordion-like metal conveyors on wheels. In that case, place the transmit antenna in line with and at the same height as the tagged product and the receive antenna directly above it to maximize your ability to energize the tag.

Use software to identify portals:
With UHF systems, you might read tags on products coming through adjacent portals, which means you might not be sure which dock door a pallet arrived at or was being shipped through. This is a problem if you are using RFID to confirm the right pallet is being put on a truck. Some companies have installed elaborate Faraday cages, or shielding, to ensure that the interrogator at dock door No. 1 reads tags only on those products being shipped through dock door No. 1, but it is cheaper, faster and easier to do it with software.

Toby Rush, president of Rush Tracking Systems, says some middleware products can be configured to deduce which door a tagged item came through. Let's say a tag is read for the first time by an antenna at dock door No. 1, and that same antenna reads the tag another 30 times as it moves through the portal. Another antenna at dock door No. 2 reads the same tag six times, after the interrogator at dock door No. 1 picked it up the first time. Software can be used to infer that the tag was on a case coming through dock door No. 1 because it was read there first and most frequently.

If products are moving quickly and action is required, such as on a conveyor diverting products based on tag reads, there might not be time for middleware to analyze reads and make a determination about which interrogator read a specific tag. In such cases, you will need to carefully control the edge of the read field (see step 5 in the main story).

Map out cables:
Coaxial cable is considered "lossy," which means running it beyond 20 feet between the interrogator and the antenna will result in a loss of signal to that antenna. Make sure the installation plan doesn't require antennas to be farther than 20 feet from an interrogator. And make sure you use the proper type of cable for the interrogator you purchased and the application for which it is being used.

As for where to place the interrogators and antennas, the pros say each installation is different because each facility is different. The important thing to remember is you are trying to restrict RF energy to the interrogation zone in front of the portal, without interfering with the operations of the facility.
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