Wireless Hacks

Cut Cable Omni Antenna

Make a high-gain omni out of bits of feed cable.

Most of the designs on the Web for a 2.4 GHz omni antenna seem to involve brass tubing and LMR-400 cable, none of which are readily available to me. I then found a coax-only design for 444 MHz that was based on the same idea. The only reasonable cable I could get my hands on was RG-213 from Maplin (http://www.maplin.co.uk/). I thought I’d give it a try by scaling the 444 MHz design up to 2.4 GHz and using RG-213. In order to get about a 6 db gain from the antenna, it needs 8 sectors, with a 1/4-wave section at the top and a fly lead with an N-connector at the bottom.

It should take about two to three hours to build an antenna using this design, but don’t worry if it takes longer. You will get quicker, especially as you need to make the jig only once. Figure 5-11 shows the completed antenna.

Figure 5-11. The completed omni antenna.

Each sector of the antenna needs to be 1/2 a wavelength long, multiplied by the velocity factor of the cable. The velocity factor of RG-213 is 0.66. If you decide to use a different cable (such as LMR-400), then you need to get the velocity factor of that cable (which will be different), and recalculate all the dimensions.

 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 = ...