Parts (what we could scrounge)
55 cm length of 40mm PVC tubing
Oblong zippy box with short side of about 40mm
3mm thick aluminium square metal backplate
2mm solid core coated copper wire
Copper pipe wall connector hammered flat and cut to act as impedance converter!
male N connector
Left handed spiral wire wrap printout (see Jason's site)
Equipment
Drill
Soldering Iron, Sucker & Solder
Glue Gun & Glue Sticks
Screwdriver
Hacksaw
Bench Vice
Construction Method
Once again, full credit to Jason's excellent construction notes. Rather than just cut & paste from Jason's site or rephrasing them here, check out his helical directional antenna page (I might ask permission to mirror his notes here). We'll just make notes where we varied.
Backplate Assembly
Firstly we prepared the backplate by carefully measuring then drilling 4 holes in it. The first hole (dead centre in the plate) was for the bolt than will affix the zippy box to the backplate. The next 3 holes were for the N connector - one for the connector itself to pass through (measured 40mm out from the centre of the plate, and two further holes for 2 mounting bolts to fix it firmly.
Winding Copper Helix around PVC Tube
The next step is winding the copper wire around the PVC tubing. We used the left-handed southpaw spiral printout. Because we were using thicker (2mm) wire, it took two people to do this very carefully. After marking the path of where the wire must lie on the tube, we nicked one end of the tubing with a stanley knife so we could hold the wire steady as we wrapped. we put dowling in a bench vice, put the tube over the dowling, set the wire in the nick of one end and began. Raylab turned to tube and guided the wire while I followed with the glue gun, fixing the wire in place. Messy, but it seemed to work very well. Did I mention it was messy?
Impedance Matching
Being relative RF lightweights, we discussed and tried to nut out just exactly what the triangular piece of copper achieved in the antenna, and how we should check this, given our modifications to the design (specifically we were concerned that we used thicker 2mm copper wire). Words describing antennas such as signal 'resonators' and analogies for RF collection by an antenna were made in terms of pipes and flow. The best way we could think of it was that this strip of copper acted as a gradual 'flow regulator' - a bit like a pipe that slowly changes in diameter to minimise disruption, rather than a sudden change which may cause 'backwash', etc.
The copper pipe wall attachment was beaten flat and cut into the triangular shape as specified. The thin end of triangle was soldered to the end of the copper wire, and the other end was soldered to the tip of the N connector. As you clearly could see below - that is if it wasn't so messy.
Final Touches (More Glue Madness)
Finally, in our case, the tube is attached only by wedging it within the zippy box. To give extra strength, much much more glue was added, both inside the tube, and outside between the zippy box and the tube.
More mess!
Final Prototype
Antenna 'Testing'
Indoor Test
Informal indoor testing was performed the next day, allowing all that messy glue to dry. Our informal test consisted of measuring signal strength as given by the Orinocco Client manager both with a pigtail connector and the helical antenna and without. An 18-21dB difference was observed at different distances within the room. In terms of dB, it seemed to perform slightly better than a (far mor expensive and far less messy) Lucent yagi.
'Stick it out a window jump in a car and drive as far as you can' Test
Gaffer tape the antenna to be tested to a long wooden pole. Push pole out 3rd floor window as far as possible. Anchor in position so antenna is pointing down street with good line of sight. Jump in car with laptop and Lucent yagi (to go that extra distance). Drive.
Good reception for about a kilometer where the road bends, and the tall dense houses start to obscure line of sight. Longest distance achieved aligns with loss of line of sight at about 1.2 kilometers.
'Go upstairs, knock on neighbour's door, climb the roof and look for groundzero.free2air.net' Test
Encouraged by detecting very short glimpses of associations to groundzero.free2air.net indoors (after about one hour of fine positioning), an outdoor rooftop test was required.
groundzero is almost two kilometers away, and we were hopeful, but unfortunately no signal at all was detected. There are some gasworks near the line of sight to raylab and that may explain the brief detection indoors one day, and absolutely no detection outdoors on another.
So now the search is on for an appropriate relay point ...
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