What are radar reflectors?
Radar reflectors are simple devices that are used to make something more visible to radars. For example, kayakers in foggy areas use radar reflectors to avoid being run over by bigger boats (with the radars); weather balloons have reflectors that allow them to appear in Air Traffic Control radars, and so on.
How do they work?
They are typically built to have reflective surfaces (eg of metal) in 3D at 90-degrees at each other. Incoming radar waves bounce off one, two, three surfaces and head back to the source direction. The principle behind them is similar to the one that makes you always see your 'eye' in the point where two mirrors cross each other at 90 degrees.
Why did I build my own?
I needed a radar reflector that would be extremely light, to use in some experiments involving balloons and imagery. Typical DIY reflectors use cookie sheets. Commercial ones are available for $30 and above, but are quite heavy. There are options to build them e.g. by wireframes and mylar sheets; which could be lighter than this construction, but are also more complex to build.
How is this one built?
This radar reflector is built of 3 music/data CDs, assembled by cutting notches into each other. I built it in under 1 hour.
Having made other instructables for light radar reflectors (example) I thought this option should be available too.
Each CD tends to weigh 15..16g, so this one weighs around 45..48g (Less than 1.6 oz)
Step 1: Get CDs
Get 3 blank CDs.
They don't need to be blank, they could just be burnt with bad music or useless old backups, but unburnt CDs have higher metal coverage in their surface.
Try to use CDs that have shiny (non-painted, not matte) 'top'. See the pics for the ones I chose.
Step 2: Cut and Notch CDs
Label the three CDs A, B and C
Cut a vertical gap in CD A from the center to one side
Cut a similar gap in B
In B, make two additional gaps 1/2 the radius of the CD at 90º, from the outside
Take CD C,
cut it in half, and cut from the inside two gaps.
Use a saw or some other cutting device that takes away material, essentially you want there to be a gap left just as wide as a CD is thick, so it fits just so. (Not too tight otherwise it will deform the reflector)
Step 3: Assemble
See the diagram for assembly.
1) Take CD B, facing you, and slide CD A so both long gaps couple with each other.
2) Take each half of C and assemble it onto the side notches of B.
Step 4: Enjoy!
You are ready! You can use from a paraglider or balloon knowing it takes minimum weight and has much higher reflectivity than a random bunch of tinfoil or one or two CDs hanging from a stream. Plus, it looks cooler!