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21st Century
Cat’s Whiskers
Perhaps you wonder what cats and electronics have in common?
It transpires that both pet food and the pet food container
show pleasing microwave responses.
If you’d like to get into the world of Wi-Fi, treat your cat –
purchase a large (shallow) tin of sardines and read on!
by Stan Swan*
66 Silicon Chip
www.siliconchip.com.au
I
n an age when abbreviations and
acronyms abound (not bad alliteration, eh?), it’s perhaps very
unfortunate that recent computer
networking breakthroughs have been
christened “Wi-Fi”.
Naturally this is often confused with
“Hi-Fi” (music) or “Firewire” (high
speed data over cable), with attendant
frustration! Further mentioning “Bluetooth” may result in mutterings about
the electronic age having gone techno
babble mad. But mad or not, this new
technology shows much the same
mainstream potential as the emerging
Internet did in the mid 1990s. Back
then, few people knew of “www” – and
even less cared!
Wi-Fi, an abbreviation of Wireless
Fidelity, refers to low power short
range wireless computer data communications, formally specified as
IEEE802.11, developed by the Institute
of Electrical and Electronic Engineers
(IEEE).
Signals are at microwave frequencies (around 2.4GHz) in a globally-licence-free ISM (industrial, scientific
and medical) part of the radio spectrum. A large part of the appeal of
Wi-Fi seems to relate to its innovative
democratic spectrum sharing.
Although the band is already cluttered by intentional or non-intentional
signals from such things as microwave
ovens, video/TV extenders and cordless phones signals, Wi-Fi nimbly
extracts wanted signals from the noise
– in the manner of a cyclist weaving
through heavy city traffic.
Like such flea power cyclists (compared to other traffic), Wi-Fi signals are
typically just 30mW. This is similar to
the energy needs of a LED and is about
one-tenth that of a cell phone!
Wi-Fi uptake has been particularly
dramatic since the September 11th
2001 events, partly fueled by a surge in
notebook PC adoption with their flexible computing benefits but also due
to a recognition of cabled networking
installation costs and layout problems.
Behind a typical desktop PC is normally a snake’s pit of cables of course,
with users often petrified to touch
anything around the back. A deskbound PC is perhaps akin to mobile
phones only being used when tethered
to chargers, or digital cameras when
docked at the computer!
And although the standard is only
a few years old, important enhancements have already occurred, with
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the b,a and g versions having crucial
differences:
IEE802.11 – The original 2Mbps at
2.4Ghz. Began about 1998 but now
obsolete.
IEEE802.11b – Today’s 11Mbps,
2.4GHz. Looks the most durable form.
IEEE802.11a – Quieter 5GHz spectrum
and 54Mbps, but showing less range.
IEEE802.11g – 54Mbps but at 2.4GHz
again. Likely release 2003.
The new amateur radio?
I was raised in rural New Zealand
(Nelson) and in my teens took to ham
radio with relish, partly as a communication aid in an era when even
toll calls were a novelty. Much of my
hands-on experience was with the
3.5MHz (80m) band, where signals
had wavelengths in tens of metres
(compared to Wi-Fi’s millimetres).
Dimensions of aerials then were
some 1000 times greater, with nerve
wracking ascents of towering pine
trees a consequence.
Wi-Fi antennas, in contrast, can
be rustled up on a table top with just
simple tools and a tape measure.
A key radio fact needed for DIY
antennas is the relationship between
signal frequency and antenna dimensions. In fact, all waves follow such
a formula:
Propagation speed (metres/second)
= Frequency (Hertz) x Wavelength
(metres).
Radio waves slow down slightly
when in conductors but for most purposes their speed can be assumed to
be that of light: 300,000km per second
(3 x 108 m/sec).
This means 2.4GHz (Giga means x
109) Wi-Fi signals have a wavelength
of approximately:
3 x108/2. 4 x 109 = 125mm.
The symbol for wavelength is the
Greek alphabetical character “λ”
(Lambda).
The ISM spectrum in fact offers 11
Wi-Fi channels between 2.4 – 2.48
GHz, so the actual dimensions relate
to the channel frequency used.
When talking about designing antennas, you’ll often find expressions
involving fractions of wavelengths,
especially quarter wave (¼ λ) and half
wave (½ λ). At 2.4GHz ¼ x 125mm =
about 31.25mm, the reason why this
length is often noted in these articles.
Phew – that’s almost all the maths!
Now – what about those sardines?
Patience!
Microwave behaviour
Microwaves travel best “line of
sight” – that is, short range and they
don’t bend to follow either the Earth’s
curvature or geographic features such
as mountains.
They are easily absorbed by concrete, steel, hills and even (full leaf)
vegetation. (That’s why you rarely, if
A similar type of “bow tie” antenna to that on the opposite page, operating from
a notebook computer running “Netstumbler” software. But this one is built onto
a stock-standard CD (taking advantage of the metallised layer under the plastic).
Building these antennas is easy – we show you how later in this article.
November 2002 67
Nice view – but that’s not why we’re showing it. Most businesses would be horrified to find that you can listen in to them using Wi-Fi – because few have robust
security built into their wireless LAN systems. Here the antenna is aimed at the
Wellington (NZ) CBD, about 3km away – and the signals abounded!
the de facto standard. Numerous other
makers (Apple, HP-Compaq and Dell,
etc) rebadge this card and it’s the best
supported for monitoring software
such as Netstumbler.
The one drawback: the Orinoco
PCMCIA antenna connection is very
small and needs a costly “pigtail”
connector to externally link it.
USB has significant appeal, since
not only can the units easily swap
between PCs but the radio signal decoding is done within, with energising
power also USB supplied.
Cheap USB extension leads and
connectors can be used (respecting
the USB 5-metre cable limit), as only
digital signals (rather than microwave)
run along the lines. A further bonus
means the whole unit can be easily
shifted around, or hung above head
height on the wall, for a signal “sweet
spot”.
However, due to the decoding
overhead there will be a signal speed
reduction.
My experiences show that indoor
Wi-Fi typically has such a maze of
reflected signals (from metalwork,
wiring, people, etc) that even shifting
the Wi-Fi hardware a mere handspan
may hugely alter signal strength.
Such shifts are of course NOT easy
to do with a desktop PC, and even a
notebook may need sliding around a
desktop for best connections.
ever, see a satellite TV reception dish
with a tree in front of it. Satellite TV
is another service which uses microwave bands.
In contrast to the satellite TV signal
paths which are many thousands of
kilometres, Wi-Fi coverage around
a home or office will usually only
amount to around 50 metres or so, with
timber walls, floors and partitions absorbing signals significantly. But with
a clear view (perhaps innocently via
a window), signals can travel many
kilometres!
Elevated directional antennas at
each end of the link can push this up
to tens of kilometres. Data rates may reduce over such distances but even if as
“low” as 1Mbps, they are still some 20
times a normal dial-up modem speed
and capable of handling simultaneous
data, Internet sharing, audio, phone
calls and even video (MS NetMeeting
is especially effective).
The present Wi-Fi world record
distance, some 35km across water, was
attained in Chile using small parabolic
dishes, with the curvature of the Earth
eventually a factor.
Ahh, Chile – is this where the
sardines come in? Almost – but first
we’ll look at hardware needs, with a
South American river (the Orinoco)
to the fore.
were available in 2002 but these included PCMCIA, PCI and USB types.
A major limiting factor of many PCMCIA and inbuilt PCI cards relates
to their lack of an external antenna
connection.
Not only will the performance therefore be at the mercy of the card’s small
inbuilt aerial (and perhaps shielded by
PC metal work but will also be very
close to computer “noise”.
For more flexibility an external
antenna is usual crucial and for this
the Hermes chip set Lucent/Agere/
Avaya “Orinoco” PC card is virtually
Wi-Fi cards
Another view across Wellington harbour, this time looking towards Petone,
some 10km away (marked by the red ‘X’). A solid Wi-Fi signal was detected on
the notebook computer.
Only a modest selection of cards
68 Silicon Chip
Software
Most cards have installation software for Windows (especially XP)
Apple Mac and even Linux. Configu-
X
www.siliconchip.com.au
Several sites noted here in Wellington (NZ) had default passwords and
signals easily monitored from nearby
line-of-sight hilltops and parks.
The owner of a simple USB home
Internet sharing wireless LAN, detected near my workplace, was astounded
to know we were able to receive his
signals streets away when he had difficulty in his house! Fortunately, he
had at least enable Wired Equivalent
Privacy (WEP) security, although even
that may now be broken (over time) by
determined snoopers using AirSnort
under Linux.
External Antenna
To avoid the cost of the espensive pigtail, for initial DIY antenna trials you could
just carefully expose the 3mm coax braid and central wires to make a simple
push-on connection to the card socket. This can be held in place with a piece of
tape. Do this at your own risk, though – insertion losses may be significant!
ration occurs in either Ad Hoc (peer
to peer) or Access Point modes and
good signal strength and auditing
features usually apply. However the
standout monitoring software, not yet
able to be run on all cards, is Netstumbler.
Initially developed for detecting
the presence of nearby wireless LANs
(WLAN) with a view to perhaps accessing them, Netstumbler also offers
ready antenna tuning applications.
It’s perhaps worth borrowing an
Orinoco card and a notebook PC running Netstumbler, just to fine-tune and
audit your Wi-Fi setup and coverage.
Incidentally, even with a simple
antenna, Netstumbler usually reveals
dozens of WLANs (many of them
insecure) during a drive around most
cities now!
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Classic antenna theory says that the
best transmitting antenna also makes
the best receiving one (reciprocity) but
this needs modifying when, in spite of
good signal strengths, local noise may
need nulling out.
Some antenna types may suit minimising signals from a nearby “noisy”
microwave oven or cordless phone.
Blue-tooth, the very short range (under
10 metres) wireless technology just
finding use with some mobile phones
and PDAs, seems especially noisy to
Wi-Fi.
Numerous Internet websites and
usergroups now offer designs ranging
from 5-minute specials (using much
over-rated Pringle cans, etc) to converted satellite TV parabolic dishes.
Aside from cost and assembly skills,
further factors to consider are coaxial
cable runs, specialised connectors,
weather proofing and eventual intended use.
Parabolic dishes offer very high gain
Here’s all the dimensions for building your own sardine-tin special! The
31.25mm quarter-wavelength has been rounded up to 32mm to take into account
the radius of the bend. Original drawings of this antenna first appeared on this
website: www.wlan.org.uk/simple double quad.gif
November 2002 69
3m (say 3dB loss) of coax loss may
overall be tolerable if a 20dB (that’s
100 times) improvement results from
a better signal take off.
Cost savings may be considerable,
especially when specialised crimping
tools are not needed.
“Sardine can” design (at last)
The quarter-wave omnidirectional (ie, radiates in all directions). Performance
might not be as good as the bow-tie but it’s a much simpler antenna. And even
this can give a good account of itself in Wi-Fi hotspots (good signal strengths).
but need accurate alignment and robust installation and hence are hardly
tempting to slip in with your notebook
PC for use at the library!
If omnidirectional coverage is needed, the inconvenience of adjusting a
directional antenna may be a factor
too, especially when mounted outdoors.
In my experience, perhaps the best
high gain, easily constructed type, is
the “plumbers special” helical – but
this may rather intimidate your neighbours or workmates!
such as N-connectors, have loss. The
better the connector, the lower the loss
– but for better you can also use the
word dearer. They also usually require
special tools for assembly.
With this in mind, it may be better
to standardise on cheaper connectors
(such as BNC) and coax if only to
ensure a sweet spot for one’s signal.
Several BNC connectors (at maybe
1dB insertion loss each) and maybe
For simplicity, a bi-quad “Bow-Tie”
design offers good gain and front-toback ratio plus directivity and compactness. Ideally the side arms should
be λ/4 (= 31.25mm) each, as should
the wall height, with best matching
at 15mm (= λ/8) spacing from the can
reflector bottom.
But you don’t have to be this accurate: diverse variations I’ve made have
not shown things to be too critical.
Even the reflector seems non-critical,
with an old compact disc (exploiting
its metallised layer) being pushed into
good service!
In the spirit of “make it do, use
it up, wear it out”, oval sardine tins
have shown sprightly (should that
be “spratly”?) performance, certainly much better than bulkier tin-can
waveguides.
The overall size neatly fits into a padded pencil case incidentally, yielding
a satisfying integrated design when
placed near one’s notebook PC.
In “downunder” spirit I was determined to push Milo cans into antenna service but have so far had little
success (Milo was first introduced
Sorry, more maths
High school maths log theory time!
Antenna gain is measured in decibels
(dB), with a 3dB gain being equivalent
to doubling power (recall log102 =
0.3010?).
Each 6dB gain will double the theoretical range, so a 12dB gain antenna
(about as good as homebuilt gets)
should yield four times the range. With
one of these at each end, eight times
the link range should result, meaning
perhaps 4km rather than 500 metres.
For short-range work, especially
in built-up areas, such large changes
result from diverse reflections so that
it’s difficult to be exact on gains and
losses.
Each time you make a connection,
there is signal loss. Even purpose-designed types for microwave levels,
70 Silicon Chip
Instead of going to the office PC, let the office LAN come to you. Networking out
of thin air . . .
www.siliconchip.com.au
References and URLs:
For convenience these are also available hotlinked at:
http://manuka.orconhosting.net.nz
Warchalking!
www.arrl.org/catalog/?item=8047 “ ARRL Antenna Book” 19th Ed.
(The American Radio Relay League amateur radio enthusiast’s standby)
www.antennas3.com “Antennas for all Applications” 3rd Ed. Kraus and Marhefka
(The classic professional’s text).
www.netstumbler.com “Netstumbler” and PDA version “Ministumbler” download site
alt.internet.wireless (Usegroup: access via Google Groups ) for helpful advice ,
experiences and opinion
www.oreillynet.com/cs/weblog/view/wlg/448 Pringle can antenna enhancement
www.wlan.org.uk/tincan.gif Tin can microwave antenna details
www.wlan.org.uk/simple_double_quad.gif Pictures and details of the basic “bow tie”
http://trevormarshall.com/biquad.htm Details of bow tie used with parabolic reflector
www.wireless.org.au/~jhecker/helix/helical.html Jason Heckers high gain helix
http://helix.remco.tk Helical cookbook
www.saunalahti.fi/~elepal/antenna1.html Cake tin homebrew short backfire antenna
www.seattlewireless.net/index.cgi/PigTail Sources for Orinoco Pigtail connectors
www.seattlewireless.net/index.cgi/MicroTVAerial Very small 2.4GHz Yagi
www.hyperlinktech.com/web/connectors.html Microwave connectors listings and
pictures
www.warchalking.org Warchalking background and news.
1933 Sydney Royal Show). The wide
Milo cans have great signal capture
potential but show trivial gain beside
the Bow-Ties.
“Dog tucker” omnidirectional
By now your cat’s probably happy
with the sardines but your dog may
feel left out. So feed him and build
another antenna!
For sites with good signal strength,
simple quarter-wave tin lid designs
have shown to be very effective. Select
a lid or bottom on a tin can – as wide
as possible since this becomes the radiating ground plane – and then drill
out to take an N (or BNC ?) connector.
Now remove the lid with a can opener
and maybe use the plastic cover to
protect yourself from sharp edges.
Solder a sturdy wire of 31mm length
(λ/4) to the central post.
Because all the metal could bleed
away the heat from a single soldering
iron, consider enlisting a mate to hold
a second iron. For really fine tuning a
thin brass tube (perhaps from a hobby
shop) can be used instead, with a small
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self-tapping screw in the top adjusted
for best radiant length. This set up
may be neatly attached to the top of
a notebook PC's screen with Velcro
(careful of the LCD screen!), or even
magnetically attached to a car roof
when mobile.
At its present explosive rate of uptake, Wi-Fi looks set as an essential
communication tool and is likely to
be as rapidly adopted as USB has
been. Already significant productivity
gains have been noted, especially with
meeting attendees being able to access
up-to-date information via their Wi-Fi
notebooks.
Business travellers find airport
“hotspots” allow email access while
awaiting flights and perhaps most
promising of all, neighbourhood area
networks (NANs) may cheaply offer
fast datacomms to schools and communities presently near-strangled with
slow dial-up links. Although security
is still an issue, Wi-Fi overall looks
mainstream bound.
*s.t.swan<at>massey.ac.nz
We cannot complete this brief look at
W-Fi without also looking at the very new
– and somewhat controversial – subject
of Warchalking.
What is Warchalking? It’s a very unfortunate term indeed, since it implies
warmongering and scheming.
The 1930s US depression saw subtle
“hobo markings” scratched on fences
by tramps, informing others of a dry
barn, kind housewife or angry sheriff. Warchalking follows this signage
technique – but with more high-tech
outcomes in mind.
“Chalking “ arose in London only
in late June this year, but has already
become globally commonplace, thanks
of course to the Internet.
The term dates from the 1983 movie
“WarGames”, where a teenager modem-dialled random phone numbers
and inadvertently linked to a defence
computer. Such “wardialing” came to
mean attempts (often automated) to
access modems at unpublished phone
numbers, perhaps with eventual hacking
in mind. From an Internet perspective,
such mischief now seems almost quaint!
Fast-forward 20 years. The Wi-Fi
age has titled wireless drive-by WLAN
snooping as “wardriving”. With co-operation and productivity more the intent,
chalk symbols arose mid-2002 as a symbolic alert to the presence and nature of
nearby wireless LANs, especially those
that are open for Internet browsing by
passing handheld Wi-Fi devices. Some
firms, particularly hotels and coffee
shops, already now proudly display
the symbols in an attempt to cultivate
custom and goodwill. Others react with
alarm to the concept. If you find such
chalk symbols near your home or workplace, make sure it’s not your bandwidth
that’s being mined unannounced. SC
November 2002 71
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