Canny Antenna Design

Canny Antenna Design

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Written By Eric Sandler

By Gerry Blackwell

June 13, 2003

It’s a step up from the days of enthusiast orders, now WISPs and hotspot operators are starting to use Wireless Garden’s low-cost Pringles-can-inspired antennas.

It’s hard not to love the spirit of ingenuity and entrepreneurship behind the Super Cantenna, a $19.95 range-increasing Wi-Fi antenna from Wireless Garden of Carlsbad, Calif. — or the vision of its founder and president, Jason Brook.

Brook’s idea — he calls it “cans across America” — is that his company’s inexpensive antennas could be used to create ubiquitous Wi-Fi coverage by linking and extending the range of existing and soon-to-be-deployed public and freenet access points.

The Cantenna’s design, of course, is inspired by and based on the famously homespun Pringles can antennas used by Wi-Fi war drivers and freenet operators. Only it’s much better, says Brook — good enough in fact to use in commercial networks.

The idea of a home-made tin can radio frequency (RF) antenna isn’t original to Wi-Fi enthusiasts, he points out. Ham radio hobbyists were there first, decades ago, with coffee can antennas.

“When the Wi-Fi enthusiasts started pulling out their antenna handbooks, they recognized that many traditional antenna designs, though they didn’t necessarily anticipate Wi-Fi, were nevertheless applicable,” Brook says.

One of those traditional designs — which is also used in much more expensive commercial products — is the waveguide or feed antenna, a directional antenna design. It captures an RF signal in a metal tube and focuses it on a sensor or probe inserted into the tube. The probe is connected by a cable to the RF transceiver, which in this case would be an 802.11 access point.

The diameter and length of the tube and the placement of the probe are critical factors in antenna design. That is why the Cantenna is so much better than a home-made Pringles cantenna, Brook explains.

“A Pringles can is really not the best for 2.4 GHz. The diameter is too small to capture the complete spectrum. It’s only about 2-1/4 or 2-1/2 inches wide.” A Pringles can as a result has less range and will only deliver some of the channels available in the spectrum, he says.

Brook and his team optimized every aspect of the Super Cantenna design for Wi-Fi, making the can wider and longer (12 inches) than a Pringles can and carefully engineering the placement of the probe.

The result is a directional antenna that delivers 12 dBi of gain, compared to ratings of as little as 2 dBi for the antennas built into Wi-Fi access points. That translates into greater range. Brook hedges a little on just how much greater, and how much greater compared to a Pringles can.

“It varies,” he says, “based on the power of the radio, atmospheric and other propagation conditions — trees, hills — the length of cable. We hear of [the range on Wi-Fi access points] going from 500 ft up to several miles with the Super Cantenna.”

A mile would be a better estimate of the typical range extension, he says. The Super Cantenna is not omni-directional, however. It has a beam only about 30 degrees wide.

The other thing about a Pringles can, Brook points out, is that it’s not metal so it’s not much good outdoors, and that’s where more and more customers, including wireless ISPs and hotspot operators want to put them.

In the beginning, Wireless Garden was selling the product mainly to Wi-Fi enthusiasts. People use them for illegal and quasi legal activities, Brook admits, such as war driving — driving around a city, scanning the air waves for unsecured Wi-Fi signals.

They also use them to share high-speed Internet connections in a neighborhood. This may seem no more ethically problematic than using a Wi-Fi LAN to distribute a high-speed cable or DSL connection throughout a house, a practice many service providers turn a blind to. Brook points out, however, that sharing a high-speed Internet connection with a neighbor almost certainly violates the terms of the contract the subscriber signed to get the service. He predicts that “cable and DSL providers will eventually come down hard on them.”

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There are legitimate variations. Some homeowners are using Cantennas to extend high-speed Internet and local area network services to guest cottages or remote patios and decks on their properties.

Some businesses are using them for temporary bridging between an old office and the new one they’re moving into. The company can leave the Internet connection in place in the old office and start moving people into the new space. The Cantenna link extends the office LAN so employees in the new space get the same services, including Internet access.

Another application Brook likes is gamers using the Super Cantenna to link their game units or networked PCs so they can play mano a mano over the direct connection at very high speed. “That’s cool,” he says.

Word has spread about the Cantenna, though, and Wireless Garden’s market is slowly changing now. Professional network security auditors are using the Cantenna to test the outer boundaries of their companies’ or clients’ wireless LANs, for example.

More important, wireless ISPs and hotspot operators all over the world are starting to beat a path to Brook’s door, virtually speaking. The day we talked to him, a Dutch WISP had placed an order for 300 Super Cantennas.

“We’re just now starting to get significant volume orders from WISPs and hotspot operators,” he says. “We’re even private labeling — if you’re Joe’s WISP, we can put your art work on the Cantenna.”

The case for WISPs using Super Cantennas is strong, Brook contends. The typical situation is a service provider with a transmitter on a hill providing coverage of a town in the valley below. Cantennas mounted on the roofs of subscribers’ homes or businesses can extend the service providers’ range to reach customers that would otherwise not be covered. Commercial antennas from traditional vendors would do the same thing, of course, but they’re much more expensive.

Even where WISPs don’t have problems with out-of-range customers, they can reduce the potential for interference by turning down the power on their tower-mounted radios, using more directional antennas and Cantennas on subscribers’ roofs to make up the lost range.

Interference will be a problem for Wi-Fi, Brook believes. “More and more people are purchasing residential Wi-Fi networks. With all the 2.4 GHz already out there, from cordless phones to baby monitors to microwave ovens, you wonder if we aren’t creating quite a bit of interference.”

The cure is focusing signals to avoid airwave pollution — and low-cost directional antennas like the Super Cantenna are part of the prescription.

Wireless Garden has two new products coming — Wireless Booster antennas — that offer even greater range because they’re longer. The 18-inch XL ($30) delivers 18 dBi of gain, the 24-inch XXL ($50) delivers 24 dBi. Testing was still underway at the time of writing so Brook would say nothing definite about range.

He insists his Cantennas are easy to install. You need a cable — a pigtail — to connect it to the antenna port on the radio. They typically cost about $20. The Wireless Garden Web site offers a pigtail finder — select your access point or router model from a drop-down list and it tells you which pigtail to buy.

Many PC card adapters of course don’t have removable antennas and some access points don’t either. But the Super Cantenna works with most access points. You will have to remove one of the existing antennas to attach the pigtail. The remaining antenna will continue to provide omni-directional coverage.

Aiming the Cantenna is simple, Brook says. “It really is just plug in, point and shoot — like pointing a rifle.”

As community mesh networks made up of volunteer Wi-Fi relay stations begin to spread, he says, the Cantenna could be the glue that holds potentially huge ad hoc networks together.

“We could have access points networked from San Diego to New York City with Cantennas acting as repeaters,” Brook enthuses. “Cans across America!”

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