Wireless Hacks

Slotted Waveguides

Make a high-gain, horizontally polarized omni or unidirectional antenna. And it looks cool too!

Unlike wideband antennas like the BiQuad [Hack #75], slotted waveguides are resonant antennas, and have a relatively narrow operating frequency range. The designs described in this Hack have an adequate bandwidth for any WLAN, but they have been carefully designed and must be equally carefully constructed.

The major attraction of a slotted waveguide design is its simplicity. Once you have built the first one, it is very simple to build many more. The gain varies little across the 802.11b spectrum, dropping a little bit at the extreme ends. A finished 8-element directional is shown in Figure 5-17.

Figure 5-17. An 8-element slotted waveguide.

How a Waveguide Antenna Works

A waveguide is a very low-loss transmission line. It allows us to propagate signals to a number of smaller antennas (slots). The signal is coupled into the waveguide with a simple coaxial probe; as it travels along the guide it traverses the slots. Each of these slots allows a little of the energy to radiate. The slots are in a linear array pattern, and the total of all the radiated signals adds up to a very significant power gain over a small range of angles close to the horizon. In other words, the waveguide antenna transmits almost all of its energy at the horizon, usually exactly where we want it ...