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 cardboard squares, covered in sticky aluminum tape used for weather insulation, and assembled into the 3D intersecting squares shown. I built it in under 1 hour.
You may see smaller but simpler versions done with CDs here: http://Instructable on how to make a reflector using CDs CDs are easier but their smaller surface makes them marginally OK for the purpose. You can use the one here for situations where you know you want a larger one.

Step 1: Acquire Materials

Get the following materials:
- Cardboard (get the smooth, light and rigid kind)
- Aluminum foil tape (Home Depot)

I found the right kind of cardboard the cheapest in discount 'portfolio' folders. Plus, I felt some empathy with the Tazmanian devil jumping off a plane...and who doesn't like puppies?

The foil tape I got at Home Depot.

Step 2: Cut Squares

Cut the cardboard into 3 squares. Each square needs only one layer of cardboard. I thought structural rigidity was going to be an issue so I left the 'thicker' margins, but in retrospect it was unnecessary.

The sizes of the squares determine obviously the size of the whole thing. Usually 7 to 12 inches is a good size. Depending on what you need (portability, reliability, etc) you may want to adjust these. I made each square side ~9 inches.

Step 3: Cut Assembly Notches Into Squares

To be able to assemble the reflector, you need to either cut all the pieces and glue them back together or assemble them in some ingenious way. The latter seemed better as it was more likely it would retain the shape it needs with as close as sharp 90º internal angles as you can make it.

I labeled the 3 squares A, B and C.
A and B assemble into each other quite intuitively with vertical half-length notches.
B is a bit more tricky as it requires 2 half-length notches on either side.

Here are the schematics for each square. They were done with a Livescribe pen.
When assembling, start with A and B assembled vertically and then slide (with a bit of wrestling) C in to have its inner notches match into the outer notches of B

Make the notches wide enough, plus a bit, for the cardboard + tape to be inserted in. Don't worry about making it too loose; it is better than tight as too tight will add pressure and tensions that will deform the reflector from the desired 90º shape. 1/16th of an inch+ a bit seems ok.

Step 4: Cover in Foil

Start covering each piece in Foil Tape. It is VERY sticky, I recommend working on a better surface than the coffee table. You don;t want it going crumply as this will reduce the radar reflectiveness..

Fortunately it has a backing that makes things easier. I went strip by strip.
Make it whole, smooth pieces.

Step 5: Smoothen Foil

Here are all the pieces. Make sure the notches are cut through the metal tape as well!

Step 6: Assemble Reflector

Assemble the parts. It may seem a bit crazy to assemble it but you won't need any glue.

Step 7: Final Adjustments

I used the corner of the table to set the reflector to 90º angles in 3 Dimensions.

Step 8: You Are Done!

You are ready to go!

Depending on the application you may want to hang it from the tip of a square so it hangs at 45º or from a joint in which case it will hang horizontally. The 45º is great for horizontal reflection (eg on a kayak). The square hanging may be better where you want to maximize reflection downwards towards the angles (eg for a balloon)