Friday, 8 July 2011

Chicken mesh and plywood radio/wifi reflector dish.

One of the first problems I had to solve staying in the valley in New Zealand was how to get internet into the place. There was dialup, but only ran at about quarter speed of dialup, and even then dropped out half the time. There was also supposed to be a pretty high tech microwave repeater link going in over the hill, but when I arrived that was still in the planning phase after a year, so I elected not to pin my hopes on it.
My father had the idea of some kind of reflector with a Telecom (unfortunately the only mobile provider in the area, my god do they suck, don't even get me started) 3G mobile USB modem dongle thing. To test the principle we did a lap of the community with a borrowed dongle and a frying pan, and managed to get a basic connection within ten minutes of hunting.

So, confident that it would work with a larger reflector my wwoofers and me set about making this:

and, because no one had told us about the spare jigsaw we could've used, all the curves of which were cut with this:

A process which nearly killed all four of us.
Use a jigsaw.

But the dish works beautifully. We didn't even bother aiming or optimising it, just chucked it out, pointed at a tree, with the dongle kind of in there somewhere, peaked out at over 100 kilobytes per second. Nearest cell phone tower was I think about twelve kilometres away, down a rather bushy river valley.

Cost about NZ$20 in materials (plywood, mesh, cable ties, screws), took a couple days to make, but really only because of the lack of proper power tools.

And in the interest of open source, here's the schematic: (click for large view and or download the jpeg)

Construction is pretty simple.

First, mark out the templates on plywood. You'll need 4 of the red one, 4 of the blue one, and 2 of the green.This is if you want to make the 1.5 square meter dish, you'll need to scale the numbers for different sizes.

To draw the parabolas the easiest way is to get yourself some light chain. When a chain hangs it makes a catenery curve, which is reasonably close to a parabola.
1. Mark off a length of chain as described on the template (135 cm for the blue, 160 for green).
2. Attach that length to the ply at the points marked, making sure they are the right distance apart.
3. Spraypaint over the chain to mark it's curve.

Cut out the templates with a jigsaw or similar.

Slot them all together, like so:

The easiest way to firmly attach everything is just drill some holes in apprpriate places and cable tie it all togrther.

Then attach your chicken mesh. If you're using this to receive 3G mobile/internet signal you'll want to have the mesh size no larger than 40 mm diameter. We figured that out using wikipedia and maths.

1. Drill 4 or 5 holes in the ply per gap, near the curve where the mesh will be attached.
2. Get yourself a bunch of cable ties.
3. Cut off a rectangle of mesh to cover half the dish.
4. Press the mesh into the cradle so that the point halfway along one side sits right in the middle.
5. Attach this point with a cable tie.
6. Work your way out in an expanding circle, pressing the mesh into the cradle and attaching with ties until the whole thing is locked down, including the circle perimeter. Don't worry about getting this perfect, it's not a mirror, and is pretty forgiving.
7. Repeat for the other half of the dish, overlaps are fine, but you can trim them off if you like.

Now attach two lengths of string tight from the sides of the circle, so you know where your centre is, and can attach your dongle/antenna/mobile there.

You're done!

Chuck it somewhere appropriate.

You can download the 3d model and schematics here:
Rhino3D 3dm file 203 kB
Google Sketchup 3DS file 39 kB

If this was helpful in some way, please let me know.

Saturday, 2 July 2011

Not sure if it'll be of use, but if any of you use CAD, here is the full 3d model of the most recent solarflower prototype.
3dm - Rhino3d file, what it was built in so most detailed.

iges - nurbs surfaces only, so missing 12 threaded bolt objects.

obj - low resolution polygon version. Quick to view but lowest quality version.

 3ds - same poly version as above, smaller file, and I'm pretty sure you can load this directly into Google Sketchup (free and easy 3d package/viewer)