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Jul 23, 2008

How to Build a Low Cost WiFi Antenna

             Want to connect to distant WiFi access points (AP)? Build a directional WiFi antenna using off-the-shelf parts, no new software, and without opening your computer case. Major signal boost for about $US30

Get a USB Wireless LAN adapter "dongle". This is a small device, about the size of your thumb, that provides WiFi capability to computers without a wireless card. You will need this even if your computer has integrated wireless. For best compatibility, get one that includes the 802.11b and 802.11g standard. Check www.froogle.com or www.pricewatch.com for good prices- simple ones (quite effective close in) cost around US$15-20. Note that the shape is important - you want a small thumb-shaped device. However larger "squashed mouse" models ( ~ US$50-60)are often more sensitive & powerful,& although they may be harder to mount,usually work much better in more demanding setups.
Get a USB passive extension cable ($5). You want a Type A (male)to Type A (female)cable (~US$3-5).

This will connect the dongle to your computer's USB port. Because the antenna is directional, you'll need to position it (perhaps near a window) where it can have a pretty good line of sight view of the distant AP. So buy enough cable to get it there. You can link multiple extension cables together to a limit of 5m(15'),but active USB extenders (~US$10) allow further cable runs-which may even allow elevated outdoor antenna placement.
Get a mesh-covered dish. The easiest to use is an Asian "scoop" cooking utensil (shaped like a wok, but mesh) used to fry things - it's the perfect shape and conveniently comes with a long wooden handle! Other options include sieves,steamers,pot lids and lamp shades -as long as they are dish-shaped and metal. Any parabolic piece of metal mesh will work - bigger means a better signal, but harder to carry around. Larger options include discarded DirectTV dishes or mesh covered umbrellas, and although these should give more signal boost, mounting difficulties & wind resistance tend to make about 300mm (12") diameter the most practical.Flexible stalk desk lamps allow these to be neatly mounted & positioned.
Assemble the system by putting the WiFi dongle & cable in the dish with twist-ties,tape or hot melt glue. You want the dongle at the focal point "hot spot" apex of the dish - radio signals come in and bounce toward the center, a few fingers above the surface of the dish.

The best dongle location spot can be found by simple experiment,with aluminium foil reflection of sunshine particularly straightforward. You may need a short support stick to get the dongle off the surface of the dish into this position. Alternative support methods use string tied across the face of the dish like a spider web,hollowed out plastic garden hose fittings or even chop sticks!
Plug the WiFi dongle into your computer with the USB cable, and set that as your WiFi card using your network settings.
Aim your dish at the remote WiFi antenna you want to communicate with. Your WiFi is now quite directional, so getting the aim right is important- normally point the dish towards the remote antenna, although stray reflections from buildings etc may sometimes give good signals from unexpected directions.
Once you are connected, tune your dish by adjusting the position of the dongle while watching the signal meter on your computer. A program like NetStumbler can greatly help by giving you graphical readouts of signal strengths. Compared with inbuilt WiFi adapters,which are usually down at desk level (& screened by metal walls,partitions,vegetation or your body), even a simple elevated "wokfi" setup like this can boost signals and extend ranges enormously!

There are many popular ways to enhance WiFi reception. Most methods involve collecting the microwave radio frequency (RF) signal and getting that into the computer WLAN card. Since RF is very weak, this may be fraught with complications involving tiny wires & accurate measurements plus costly lossy coax cable & connectors. Our USB based approach puts the powered RF receiver (the dongle) smack into the dish "sweet spot",& thus avoids that whole costly mess! A wide mouthed plastic baby's bottle can make a convenient weather proof container for outdoor setups, but keep it out of the hot sun or your dongle may get "cooked".
This approach is suitable for other microwave radio technologies with a dongle adapter - Bluetooth & ZigBee especially- but won't work for infrared (or memory sticks!)

Many countries do not have WiFi antenna restrictions, but in the USA the FCC and other regulatory bodies may frown on building your own radio devices.

        Thus, this advice is intended only for science fairs, and perhaps not actual use.

* USB Wifi Dongle
* USB extension cable (A to A, Male to Female).
* Dish-shaped metal cookware- experiment!
* Twist-ties,tape or hot melt glue etc.
* A short stick support,or plastic garden hose fitting.
* Signal auditing program on your PC (NetStumbler ?)


Anonymous said...

One suggestion: Separate your paragraphs.

And tell the princes I said hello to them!

July 23, 2008 at 6:36 PM