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Want To Give Your In-Store Cell Reception A Huge, Cheap Boost? Use The AC Ducts

Written by Evan Schuman
September 15th, 2010

As retailers get more creative about ways to lure mobile-phone-lugging shoppers into their stores, they have to deal with the fact that the buildings they use were never designed to allow outside signals to mix with the customers and the display cases. A research team at North Carolina State University has come up with an unorthodox tactic that may help retailers pull in outside cellular tower signals for shoppers, boost Wi-Fi signals and extend the range for RFID: air-conditioning ducts.

It seems that those ducts—officially dubbed heating, ventilating and air-conditioning (HVAC) ducts—snaking through these old multi-level escalator-using buildings are natural signal boosters. “The HVAC ductwork is an excellent conduit for the radio transmissions because the ducts typically consist of hollow metal pipes,” the university’s report said. “Those pipes can be used to guide the radio waves, keeping the waves from dispersing, and helping to maintain a strong signal over a greater distance.”

The study was originally conducted to study RFID signals and the ducts can certainly be used for that purpose, but many retailers today consider the cell tower signal problem more challenging. Sam’s Club, for example, had to offer its customers Wi-Fi partially because they often couldn’t use mobile devices any other way, which would have put a crimp in the mobile plans of the Wal-Mart-owned warehouse club. Other chains have struggled with metal roofs that never considered cell signals during construction.

“In some buildings, this could be a very elegant solution,” said Dan Stancil, co-author of the study paper, professor and head of NC State’s Department of Electrical and Computer Engineering.

Although the stores would still need a microcell repeater to grab and strengthen the signal, the ducts don’t merely serve as a convenient, hidden place to house equipment. “You’re using the duct itself as a distributed antennae,” Stancil said.

When used for Wi-Fi or RFID location-tracking (regardless of whether it’s pallets, store associates, item-level products or customer mobile devices being tracked), Stancil stressed that engineers and installers will have to significantly recalculate where transmitters are placed. That’s because the ducts create lines of sight that are very different than a straight line so “the signal delays will not be as expected. This wouldn’t be the shortest path,” he said, but it may be the path with the smallest amount of signal loss.

“The ducts will increase the range in a very different way, meaning that you can cover Wi-Fi with far fewer access points,” Stancil said.


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One Comment | Read Want To Give Your In-Store Cell Reception A Huge, Cheap Boost? Use The AC Ducts

  1. Terry Hare Says:

    HVAC ductwork sound like a great oportunity, but don’t jump in just yet.
    NATIONAL FIRE CODE requires plenum ratings for equipment installed in the building air plenum.
    Active equipment would have to be seperated from the plenum by a rated enclosure and the antenna or waveguide port would have to be plenum rated.
    That means no plastic or fiberglass antennas. Consider a waveguide port connection at the end of the duct or an all metal antenna.
    This trick will only work in buildings that have all metal ducts.
    Also note that the system should be shut down for HVAC maintenance to protect personnel from RF.
    I would consider all the risks of trying something like this versus the cost of traditional repeaters that are already listed and give more even coverage.
    -TCPPRO DOTNET

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