Better Pringles Can Antenna Construction
The O'Reilly & Associates
book 'Building Community Networks' has popularised the Pringles Can
antenna. The pringles can is aluminium lined can 230mm long, with an
internal diameter of about 70mm. This is not ideal for a 2.4 GHz
antenna, and is better suited for 3.1 GHz under the design
from the ARRL.
Designs for the Pringles Can require a dipole just shorter than the centre
of the can to be inserted 86mm from the closed end of the can.
λ = 3.0 x 108 x (1 / 2.462) x 10-9
λ = (3.0 / 2.462) x 10-1
λ = 0.122 meters
1 / 4 * λ = 30.5mm
The published design calls for five 1" washers to be placed on a continuous
screw inside the antenna, with each washer being kept apart by some fine
aluminium tube. This tube is so fine that it will deform easally
if you tighten the nuts too tight. Cutting the stuff is not
easy either, and it is not cheap. Apart from cables and connectors it
is the most expensive part of the whole design. So get rid of it. Do not
use it!
There are better things to keep the washers in place. They are
called nuts. And they were designed for the purpose
A small nut on each side of the washer will keep it
in place and will make adjustments easier. The washers need to
be placed about 30.5mm apart. This should be measured from one end, so
that measurement errors are not multiplied. Nuts are also much cheaper
than the Aluminium tube.
As far as I can tell the washers just set up a series of
cavities where the microwave signal can resonate. Again I do not think
that the washers are the correct size. I would imagine that they should be
about 30mm in size too. About the only thing that is correct about
this design is the seperation of the elements. The spacing lines up
to about every 1 / 4 * λ
The issue then becomes tuning the antenna for the best results. The reason for
this is that minor changes in the injection point will move the tuning. Tuning
is non-orthoginal, meaning that moving one washer will affect all other washer
positions. Get it good and dont touch it again. Dont even think about adding
one washer at a time - it does not work that way.
