Reference and acknowledgement is made to biquad antenna construction article and that by Trevor Marshall who has a biquad webpage detailing a similar antenna. It is recommended that the builder review both these sites before constructing the WiFi client below.
Antenna Construction.
Materials
25mm 5/16 brass tube (K&S 133)
35mm RG-8 coaxial cable
Piece of printed circuit board 123mm square
244mm copper wire 1.2mm
60mm thin (RG-316) coaxial cable
USB WiFi adapter (DSE XH8227)
Method.
The 244mm wire is bent in the middle at a right angle. Each side is then bent similarly but in the opposite direction. Each or the four lengths is again bent so that each side is approximately 30.5mm long. The most critical part is the length of each quad which is 122mm.
There has been some discussion as to the total length but I have found the dimensions above to provide the best overall performance. Each corner of the quads does not need to be a sharp bend and in some cases, biquads have been made with each quad approximating a circle.
The 123mm square PCB has a hole drilled in the centre to take the brass tubing.
The brass tube has one side at one end filed away approximately 2mm so that the centre connection of the antenna element will clear.
A piece of RG-8 coaxial cable is stripped back so that only the centre conductor and the core insulation remain. Each end has the insulation cut back so the the remaining insulation is some 2-3mm longer than brass tubing.
The brass tube is soldered into the hole in the PCB so that the tube projects a distance of 16mm. Note that the previously filed section must be parallel with one side of the PCB.
The stripped coax section is slid inside the brass tube with the insulation reaching 1mm below the top of the tube.
Now the biquad element may be attached to the coax and brass tube with the centre of the element attached to the coax conductor and the ends to the brass tube. A spacer 16mm high was made out of layers of cardboard to facilitate keeping the element parallel to the PCB backplane at the required 16mm distance. Some articles show an increased spacing but my experience has shown the closer 16mm spacing to provide an optimum result.
Some careful soldering with with a hot soldering iron, taking care not to melt too much of the centre insulation completes the antenna. Note that the centre of the element does not touch the brass tube.
USB adapter.
A small 802.11b/g USB adapter was purchased from a local (New Zealand) supplier, Dick Smith Electronics, item # XH8227. This adapter is made by Prime Electronics & Satellites, Inc, Taiwan model WU210g. This adapter has a small ceramic antenna that folds up on one side. The feed to this antenna is via a short length of very fine coaxial cable.
The adapter was readily taken apart by removing the small screws holding the two parts of the case together.
The coax feed to the flip up antenna is soldered at one end of the internal circuit board.
This is carefully unsoldered. The soldering iron had a piece of 1.2mm wire wound around the bit with a small length protruding. This made an excellent fine tip for the necessary work.
A short length, about 60mm of fine coax type unknown but taken from a mobile phone car antenna feed, had each end stripped back with the braid twisted together.
One end was carefully soldered to the lands on the adapter where the previous fine coax had been fitted. Again, the fine point of the modified soldering iron was invaluable.
The case was then reassembled with the coax passing out the hole where previously the flip up antenna was mounted.
The adapter is then mounted on the rear of the new antenna with some velcro so it can be removed easily at a later date and the coax cable is soldered to the protruding stub of the antenna feed.
The completed unit was attached again with velco to some supports inside a 2 litre ice cream container.
The USB cable was passed through a slit in the side and the lid fitted providing a good reasonably weatherproof unit. The completed unit can then be mounted on a pole outside with the lid facing the direction of the AP.
While the above methods may not follow completely what may be considered completely correct, the proof has been shown as an easy and cheap method of establishing a client WiFi point around a local neighborhood. Apart from the PCB, brass tube and the adapter, all other items were scrounged. The total cost of a client point was about $NZ60 including the adapter. No doubt, other adapters could equally be used but these particular ones with the seperate antennas originally and the screw together construction with their small size, have been ideal.
Typical outside mounts of completed units.
Ancilliary notes: Almost all the clients installed have USB 1.1 interfaces which only permit a short USB cable. In all cases, a 5 metre USB extension cable was successfully used between the computer and the adapter without problems. One computer (a well known big brand) proved very problematic and was fitted with a VIA USB chipset that had the usual VIA latency problems under Windows XP. This particular computer also had problems in being able to supply enough current to external USB devices. Various driver fixes are usually available but generally it is far better to install a seperate USB 2 card and disable the onboard VIA USB completely. I doubt if a new driver would have increased current drive capability!
Field tests with this client adapter and antenna working back to an AP fitted with a 15dB omni gave reliable connection at 6 Mbs at a distance over a mile with some intervening timber buildings and odd tree in the path. Even reduction of the AP power by 6 dB did not degrade the link.