Subject: [NEXTEL-1] Canny Antenna Design

From: [email-masked]
Date: Tue, 17 Jun 2003 21:27:26 -0400

Canny Antenna Design
June 13, 2003

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." 

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!"

http://www.80211-planet.com/columns/print.php/2221601

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From: "Paul J. Zawada" <[email-masked]>
Date: Wed, 18 Jun 2003 09:23:18 -0500

Am I the only one on this list that thinks of the Heathkit dummy load 
when he hears the name "Cantenna?"  It was a  50-ohm load mounted in 
a paint can full of mineral oil...

--zawada

-- 
Paul J. Zawada, PE       | Principal Network Research Engineer
[email-masked]          | National Center for Supercomputing Applications
+1 217 649 2233          | University of Illinois at Urbana-Champaign

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From: Stephen Verbil <[email-masked]>
Date: Wed, 18 Jun 2003 10:31:29 -0400

> a paint can full of mineral oil...

Nope, you're not the only one.  I get the feling that the folks making the 
2.4 gHz antenna never bought a "real" cantenna.  Mine's sitting over in the 
corner, still filled with mineral oil...

---Steve