I made a 5 turn helical antenna a while back that has been fairly popular. The original antenna is found here; http://www.thingiverse.com/thing:858112
Since then I have gotten several requests for a longer 7 turned version, so I thought I would take a stab at it. The older antenna was a remix of another design, but the new one is made from scratch.

I have wondered why the base reflector of helical antennas are one wavelength in diameter. Would a wider base make any difference? A while back I found Serbian university research papers that described practical tests conducted with different reflector designs for helical antennas. It turned out that a bowl reflector with a 2.5 wavelength diameter is vastly superior to the standard 1 wavelength disc. I have designed such a reflector. If you decide to use it, you could attach aluminium or copper foil to the surface, or use the design as a template for a reflector in copper or brass mesh.
There are further papers that suggests use of a cylindrical reflector. It seems promising, since it would be easy to realize in many materials, but I have not made any design for it:
http://home.etf.rs/~milanilic/publications/papers/AWPL-2006_01624449.pdf
http://home.etf.rs/~milanilic/publications/papers/Olcan-Eucap06.pdf

The design criteria are the same as for the 5-turn version: I wanted an antenna that offered full protection for the wire, was compact and easy to print and did not require using glue for assembly. My design consists of two items that are held together with a single screw. It can simply not get any simpler.

There are both left and right hand polarized versions of the former and the wire holder. Make sure to print the version you need.

Build Instructions

• The reflectors should be seen mainly as templates. Print one and use it to produce a reflector out of PCB material or copper. If you use a PCB, the copper surface must be turned forward, i.e. be the side that the wire holder is mounted to.
• Insert the antenna connector in the out-most hole of the reflector and cut the wire 5-6 mm above the surface.
• Fan out the outer isolation of the antenna wire and solder it to the reflector. There is a picture of this step in the process to make it easy to see what I mean.
• Paint or lacquer the reflector to make it fingerprint proof. This step is purely aesthetic and may be skipped.
• Print the holder and the former.
• Use the wire former to create a wire spiral.
• Thread the wire on the holder and cut it to length in place.
• Use an M3 screw and nut to fasten the holder to the reflector.
• The last step is soldering the main wire and the antenna center wire together. To achieve the best results possible the connection to the antenna wire is very important. The main wire should be in parallel with the reflector board for the last 4 mm of it´s length. This section of the wire should be located 2 mm from the surface of the reflector (thanks to Dave855 for the information)

How I Designed This

Designed in FreeCAD, the competent and free although somewhat hard to master 3D design program.