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WiFry: Cooking up 2.4GHz Antennas By the inscrutable Stan Swan As well as their celebrated discoveries of gunpowder and the compass, did the ancient Chinese stumble on microwaves? Did Ming Dynasty wizards then disguise the antenna as cookware to prevent such signalling secrets falling into barbarian hands? Does this also explain classical Asian expertise with “semiconductor” ceramics ? T here has recently been a flood of cheap USB WiFi “dongle” adaptors that have greatly stimulated 2.4GHz WiFi applications and experimentation. We obtained some from Dick Smith Electronics here in NZ – alas, DSE Australia appear to have decided not to stock these little beauties but they (or items very similar) are available from a variety of suppliers. (For those not fortunate enough to reside on the eastern islands [aka North Island or South Island], DSE NZ does do mail orders back to Australia [see contacts at the end of this article].) 14  Silicon Chip Given the line-of-sight (LOS) nature of microwave signals, obstructions from head high objects, partitions and vegetation may be at least partially overcome if the antenna can be mounted even a couple of metres or so higher, or at least clear of obstructions, away from noisy and shielding PCs into signal “sweet spots”. But extending WiFi antennas themselves (if at all possible) has normally involved an alphabet soup of costly cables and proprietary connectors (SMA, F, N, TNC, etc), with often significant microwave signal losses. So why not go the other way: put the USB WiFi adaptor (complete with its antenna) in a sweet spot and extend the USB cable? That way, all the 2.4GHz signal decoding to slower 12Mbps digital remains in the unit itself and although such “mast head massaging” will reduce final data rates, regular cheap USB fittings and cables then deliver Mbps rate signals to the PC. Experience indicates you may be able to add passive extensions up to the USB cable limit of 5m. Beyond this, an active USB extender should theoretically be used (but we have found it possible to go beyond 5m with quality cable and connectors). A beauty of the USB wireless approach is that it’s modular in one’s siliconchip.com.au Take one parabolic skimmer, a few hand tools and a USB cable . . . . . . determine the focal point (by maths or by trial and error) . . . . . . and mount the USB socket at the focal point, ready for the dongle . . . choice of device, not locking you into a particular standard. Unplug the 802.11b unit and just replace it with a faster USB 802.11g, 802.11a (heaven forbid), or Bluetooth adaptor. You could even swap out a memory stick, web cam, hub or IrDA adaptor of course ! Inspection of several USB adaptors reveals most have a short (~9cm) ceramic antenna soldered to the PC board with near-omnidirectional radiation patterns. Naturally modifying such a precision component involves con- siderable skill, so, given that design engineers had probably optimised performance anyway, it was decided to leave well alone and just explore gathering weak microwave signals at the antenna’s location. efficiently in one direction only. Homemade WiFi antenna designs now abound, many as variations on the infamous “Pringles” can, but construction techniques usually demand considerable attention to detail and fine metal working skills. Remember we’re trying to make things easier for those of you with two left hands! That brings us to the parabola. It can be regarded as a very efficient reflector. Were you alert during that High School parabola class? Well (just in case you did doze off . . .) parabolic reflectors A Parabolic Wifi antenna Most antennas consist of a “driven element” and one or more reflectors or directors which concentrate received signals onto that element. They do this at the expense of direction: high gain antennas invariably operate most . . .and here’s a prototype (not yet secured with the hose joiner) – a high-performance WiFi antenna that we’ve measured up to 15dB gain, giving an effective increase in range of several kilometres! A WiFi AP, 5km away on Somes Island in Wellingon Harbour (just visible behind the left side of the antenna) was readily accessed. siliconchip.com.au September 2004  15 shows this sweet spot – but take care – even under weak sunshine, such a small parabola can easily burn a finger or ignite paper. Don’t cook your WiFi adaptor in such a solar BBQ! (Don’t laugh: years ago Dick Smith sold such a device. And it worked!) Poor man’s WiFi? Don’t try this at home, kids: aiming a shiny parabola at the sun and putting your finger at the focal point is more than likely to result in a burnt finger! This method is one way of determining the focal point (just don’t aim it at the sun for too long!) have the unique property of a fixed focal point, unrelated to frequency, so that distant signals are surface reflected and delivered to a point, just as satellite dishes work Mmm – here’s an idea. If the parabola collects signals and focuses them on this point, what happens if we place a USB dongle (with its own “driven element”) in this position? Shouldn’t it benefit from concentrated WiFi signals, especially on reception (since some transmissions may not “illuminate” the parabola and be lost). Recall the maths relating focal point to dimensions of the parabola? No? (Aha! You were asleep!).You can find the focal point of a parabola with this simple formula: FP = D2 16 x c where FP = focal point distance from centre, D=dish diameter, and c= depth from rim to centre (all in mm) For a 300mm diameter dish, this yields a focal point at 94mm out from the centre. You can also determine the focal point of a parabola with the “visible light” technique. If you press some shiny aluminium cooking foil to the parabola’s shape and note where the reflections are brightest, you’ve found the focal point. Bringing the sun to a focus readily After assorted DIY parabolas were developed, many web-inspired as conductive foil glued to curved cardboard, a chance visit to a Chinese emporium revealed diverse Asian cookware of seductive profile. Other customers gave bemused glances as assorted woks, lids and pans were measured up & focal points calculated! Although tempting, normal woks are of course very solid and may have serious wind resistance – a major factor in exposed outdoor sites such as here in coastal NZ. But behold – a range of parabolic cooking vat scoops, complete with user-friendly bamboo handles, that are cheap, sturdy, of fine galvanised open mesh, that mate with a USB dongle beautifully. Chinese students here inform me that such classic cookware items are known as “Spider Skimmers” and come in imperial sizes: 6”, 8” and 9”, with the largest 12” (300mm) as we eventually used. Now you know why we called this article “Wi Fry!” The mesh gaps approximate 5mm, and as 2.4GHz signals have a wavelength of some 125mm, these spaces are well inside the 12.5mm limit that RF theory relates to 1/10th of a wavelength. Extensive field testing by IT students from Massey Netstumbler or WiFiFoFum,(the PDA equivalent) are University (Wellington, NZ) have proved just how effective invaluable programs for WiFi antenna tweaking. 2.4GHz this little WiFi antenna is. Many have built their own! signals often wander 5dB in strength or reflect randomly. 16  Silicon Chip siliconchip.com.au Here’s an alternative: mount a bracket on the skimmer handle which places the dongle at the parabola focus (also known as the “sweet spot.” Saves butchering your scoop! Mating of such ancient (food) and modern (electronics) items is yet to be Feng Shui assessed but subsequent performance of the hybrid has been most promising ! Finishing off involved prowling hardware stores for antenna dongle holders. A garden hose mender was found to make a perfect support for the USB socket, with only modest mesh trimming needed if rear mounted. A hack saw angled slice cut in the hose mender allows USB cable placement which the screw-on rings later secure. The plug-in USB adaptor even fits in the recess at just on the desired focal point too, when internal joiner plastic trimmed or drilled off to suit. Performance In spite of its simple homemade design, this antenna certainly cuts the mustard (soy?) and extensive testing and feedback verifies a 15dB gain is readily achieved with a 12” scoop. Most tweaking has been done with the latest release ( Ver 0.4) of NetStumbler, which thankfully now supports most WiFi adaptors, as its graphing features have proved invaluable. Since every 6dB gain equates to doubling the line of sight coverage, 15dB siliconchip.com.au The technique is not limited to wifi: mobile phones, wireless PDAs, in fact a variety of “receiving” equipment can benefit from having a parabolic dish focus signals. gain over a bare adaptor means some 6-8 times range extension is feasible. Most bare dongles only stretch to about 500m LOS, so some 3km (6 x 500m) can be expected with a scoop. With one at each end 5–8km point to point links should be possible at good data rates, making feasible broad band Internet sharing, streaming video, multiple voice traffic channels or even impromptu/emergency conferences. Enthusiasm for this USB approach has been immense, testified to by some 150,000 site visits in a few weeks, and the design seems to have struck a democratic chord globally. For experimenters on tight budgets, this “Poor Man’s WiFi” USB approach looks a winner. Other apps Earlier on we mentioned that the parabolic dish was independent of frequency (remember the “Sooper Snooper” audio parabolic microphone published in SILICON CHIP in September 2001). So we figured mobile phones, wireless PDAs, and so on would also benefit from similar treatment. Our research proves that they do: some mobile phone signals that wouldn’t even register on the LCD suddenly became usable! Just one tip: don’t use a polished dish and cook your mobile phone! SC References and URLs: 1. www.usbwifi.orcon.net.nz    Project web site, with many pictures, lab. notes, case studies, links and more technical references. 2. www.dse.co.nz Dick Smith Electronics (NZ) – stockists of the ZyDAS ZD1201 based XH6822 and XH6859 adaptors. Dick Smith Electronics Australia (www.dse.com.au) stock a similar XH4268 “Spirit” for ~ $AU50 that’s been reported to work just as well. 3. www.netstumbler.com WiFi signal auditing software, Netstumbler Ver 0.4 4. www.wififofum.org WiFiFoFum PocketPC 2003 WiFi scanner 5. www.component.tdk. com/2.4GHAnntena.pdf Ceramic antenna technical details September 2004  17