V * C 0.66 * 299792458
1/2 wavelength = —— = —————- = 0.0405m = 40.5mm
2 * F 2 * 2441000000

V = Velocity Factor of RG213 = 0.66
C = Speed of light = 299792458
F = Frequency of Signal = 2441000000 (mid point of 2.4Ghz range)
The 1/4 wave element is not adjusted by the Velocity factor, as it is in the open, so it works out at just 31 mm long, giving a total antenna length of 355 mm + fly lead.

All of the parts needed to make this antenna are available cheaply from either Maplin or any DIY shop. You need the following:

1 meter RG-213U cable (available by the meter from Maplin). This is enough for two antennas. Buy more for whatever fly lead length you want.

1 N connector. Depending on what you want to connect the antenna to, use either male or female connectors, and inline or bulkhead. Remember that inline connectors need to fit 10 mm diameter RG-213 cable.

20 mm PVC conduit (available from any DIY store) It should have a 20 mm inside diameter, and 22 mm outside.

22 mm pipe clips (depending on how you want to mount the antenna). Pipe clips make mounting the antenna easy.

You also need the following standard tools.

Millimeter ruler for measuring

Junior hacksaw

Stanley blade knife

Pliers

Standard soldering iron (you don’t need a heavy duty one) and solder

Scraps of wood to make a jig to aid soldering

Bench or vice to hold the cable while you cut it