The Geekcorps BottleNet Antenna is a low-cost antenna that can be used for television and WiFi. The antenna uses materials that are easily available in Mali, including plastic water bottles, used motorcycle parts, window screen cuttings and coaxial cables. This approach minimizes the technical skills needed to build the antennas and significantly reduces costs.

The first tests - performed using prototypes assembled from materials costing about $1 per antenna - concluded that the antennas provided clear signal and a reliable Internet connection. To further reduce costs, the radio receivers were powered by cell phone chargers and mounted with the antenna directly on the antenna mast, eliminating the need to purchase expensive cabling to connect the antenna to the receiver.

BottleNet Antenna Based on Waveguide Theory
This homemade antenna design is one based on waveguide theory. A metal pipe is used to guide radio waves to an active element; one end of the pipe is open to the air, and it is this open end that is pointed to the source or destination. Typical designs are the cantenna designs, where an appropriately sized commercial can is pressed into service as an antenna.
Unfortunately, the goods available in Mali do not include anything in a tin the right size to be effective at wireless networking frequencies. However, the plastic bottles used for mineral water, specifically the 1.5 liter Diago brand, are a good choice to form the basis of an antenna.

Waveguide theory
Antennas transmit or receive radio energy through a driven element. The length of the element determines the frequency. The energy can be directed in a certain way by other elements of the antenna; the improvement of a particular direction is referred to as the gain of the antenna. Note that there is no amplification of the signal — the signal is just concentrated in a particular direction, to the exclusion of others.

Waveguide antennas have the driven element in a metal tube, with one end closed and one open. The simplest designs have a cylindrical tube, with one end covered with a flat metal reflector and the other left open. The placement of the driven element is critical, as is its orientation (vertical or horizontal) after the antenna is mounted.

Does size matter?
The diameter of the cylinder determines what frequencies the antenna will pick up or transmit. The length of the cylinder needs to be mathematically related to the diameter; this means that a random cylinder diameter/length combination is not likely to work, or will work at undesired frequencies. The attached OpenOffice spreadsheet that will help you determine the correct combinations.

Waveguide calculator
As mentioned above, there are no appropiately sized cans on market shelves in Mali. There are, however, perfectly sized plastic bottles. The Diago brand in particular lends itself to bottle-making. Not only is the bottle the correct diameter, and long enough to cut back to achieve a specific wavelength; it also has plastic moldings in exactly the right positions for inserting the probe, and cutting for a single wavelength.

A test in a microwave oven found that the plastic this bottle is made of is virtually microwave transparent. The only problem is that the bottle will not reflect microwaves, the fundamental duty of a waveguide antenna. Enter flyscreen, which is readily available and inexpensive in Mali. Its mesh is a 1-millimeter weave; this is small enough, when compared to the wavelength of wireless networking frequencies, that it appears at those frequencies to be a solid reflective metal surface.

In summary, BottleNet is a design that uses a bottle like Diago’s for the shape and rigidity, and flyscreen mesh as the metallic reflective surface.

>> Learn How to Make a BottleNet Antenna - Includes Ingredient List & Assembly Instructions(PDF)

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