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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 www.siliconchip.com.au 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! www.siliconchip.com.au 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 www.siliconchip.com.au 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