 A
popular 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.
Manufacture/Ingredients
You will need:
- A 1.5 litre bottle of Diago mineral water
- A piece of metal woven flyscreen 300 mm x 220 mm (for the cylinder)
- A piece of metal woven flyscreen 100 mm x 100 mm (for the end reflector)
- A 31 mm length of 14 or 16AWG wire (for the driven element)
- An appropriate connector (N-type female bulkheads are great but not so easy to find)
Handy tools:
- Pliers
- A drill bit or sharp pointy object for probe insertion through plastic bottle
- A soldering iron and solder
- Scissors or wirecutters for cutting mesh
- Leather gloves for handling sharp edges of flyscreen, and/or Band-Aids for failing to take this advice
Method
- Drink the water from the Diago bottle. Remove the label and allow the bottle to dry thoroughly.
- Prepare the end reflector: Ensure the 100mm x 100mm flyscreen mesh
will not fray. For at least three edges, roll the mesh over itself,
ensuring that there is a flat surface in the center. Put aside for
later.
- Prepare the 300mm x 220mm flyscreen mesh: Ensure one 300mm edge
will not fray. Preferably the cut of mesh will have the manufacturing
weave edge as this edge; if not, roll the edge over itself.
- Using the 300mm x 220mm flyscreen mesh, wrap the outside of the
bottle. Use the non-fraying 300mm edge toward the top of the bottle.
Bring the two 220mm edges together, and roll them down over themselves
until they are mechanically joined. Ensure the final roll leaves as
flat a surface on the inside of the cylinder as possible. NB: it is
very important to wrap this cleanly, so that the same horizontal wire
meets itself on the other side of the join. This will greatly assist
the reflector positioning later.
- Mark a length of as close to 208mm as you can down the cylinder,
with an overhang at the base of the bottle. Aligning this mark with the
actual bottom edge of the bottle should leave the top reaching just to
the very top of the plastic moulding of the Diago logo.
- Fray the mesh from the 208mm mark to the end (about 12mm) by
removing horizontal wires (ie, ones going around the bottle) until the
vertical wires stand upright. It is worth putting in time at this point
to make sure all the wires are straight and parallel — in particular,
that they do not cross each other. Spending time doing this now will
save a huge amount of frustration in the next step.
- Affix the end reflector prepared earlier. Feed the frayed wires
from the cylinder into the reflector, keeping it as circular as
possible. Push the reflector down as far as it will go (i.e., so it is
as close as possible to the 208mm mark) ensuring it is square to the
line of the cylinder.
- Adjust the position of the bottle in the mesh cage so that the
bottom rests lightly on the reflector, without distoring the plane of
the reflector.
- Prepare the driven element: solder the 31mm length of wire into the
center core connector hole in the Appropriate Connector. If soldering
is not available, this needs to be as fitted as possible with a good
electrical connection and unlikely to come loose.[NB: If it is made
using 16AWG gauge wire, the probe will fit very tightly into the centre
core connector hole. As such, solder may not be necessary. It may be
enough to place the probe in the hole, then either place a single drop
of Superglue on the core hole or just lightly crimp the metal around
the hole onto the probe. *Ensure the probe wire is as straight as
possible, and pointing directly away from the appropriate Connector.
*Measure the probe length again. This length is the most critical one
in the entire design, and performance can greatly vary with tiny
variances.
- Drill a hole for the driven element. The hole needs to be as close
as possible to a distance of 52mm from the reflector, which is the
first point of decorative moulding in the Diago bottle.
- Attach the probe to the bottle. The outside needs to make good
electrical contact with the outside of the cylinder. Affix in place
with tape or wire circling the entire bottle.
- To come: information about mounting.
- Connect antenna between a radio source and a receiver.
- Align and enjoy!
Serves 2.4Ghz! Source: http://mali.geekcorps.org/2005/11/07/how-to-make-a-bottlenet-antenna/
| 