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Hack 70 Deep Dish Cylindrical Parabolic Reflector

This simple design provides high gain without
pigtails or modifying your AP.

I needed a parabolic reflector to
eliminate off-property coverage. This design can reduce signal from
some areas while enhancing signal in other areas. I designed this
reflector to be installed in outdoor enclosures with
WAP-11
access points, but it is becoming quite popular with people building
indoor LANs, as well as with people building very short
point-to-point links. This design offers very high performance and
easy availability (scissors, tape, cardboard, tin foil, and 20
minutes, and you are in business). See it in action in Figure 5-1.

This antenna is so easy to make, tune, and install, and it performs
so well, that it is foolish not to try one before electing to
purchase a commercial antenna, if for no other reason than you can
check to see whether you are purchasing enough commercial antenna
gain to make the link you want to make.

Advantages over other antennas:[Hack #66] required

No modification to AP (no voiding of warranty)

No matching (SWR) problems

No purchased parts

Trivially easy construction

Very low probability of error

As good as or better performance than the Pringles can antenna [Hack #72]

Superior front-to-back/front-to-rear ratio

Improves wireless LAN privacy

Reduces interference

This design can easily complete links up to one kilometer by sitting
two WAP-11s in windows at each end of a link with clean line of
sight. The 6-inch version of the antenna gives you about 10 to 12 dB
of gain over the stock antenna. With a WAP11, this equates to
approximately 27 to 33 dB of Effective Isotropically
Radiated Power
(EIRP).
This means you wind up with an apparent power in the favored
direction between 500 mw and 2 Watts.

Of course, that gain has to come from somewhere. It
comes from the back side of the reflector, so power that is normally
transmitted in that direction is
"bounced" forward. That feature of
this antenna can be used to enhance the privacy of your wireless
network, which was my reason for designing it in the first place. The
rest is just gravy (but it is very real and rather tasty gravy). The
approximate radiation pattern of a 9-inch reflector is shown in Figure 5-2.

To build this reflector, you can use the
sample template in the Appendix, or download the original reflector
template from http://www.freeantennas.com/projects/template/parabolic.pdf.
The drawing can be scaled using a copy machine to make a dish of any
reasonable size. The gain computations for various sizes of the dish
are also provided on the web site, as well as rough graphs showing
beam width and gain/frequency. This reflector is frequency
independent, meaning it works with any wireless gear, on any band.

There is a square drawn upon the diagram. It will help you to ensure
that your scaling does not corrupt the aspect ratio of the template.
In other words, if the square is still square after you enlarge or
reduce the template, you still have a good template.

Focal length varies proportionally with the size of the dish, so the
focal point is also shown in the drawings. Positioning of the
feed point
(focal point) is the most critical aspect of a deep dish parabolic.
Errors of 1/4" or more are unacceptable at these frequencies. It may
help to "fiddle" with the
positioning as small irregularities (~1/4" or greater) will move the
focal point slightly. If the dipole is not in the focal point, you
will lose gain. Parabolic reflectors also lose gain if your finished
reflector varies much from the correct curve.

The reflector is designed to be fed by a dipole, which is why it is not
circular. A dipole is long and cylindrical, while the focal point on
a circular dish is circular. The focal point on this design is a
cylinder. Many access points (such as the WAP-11)
use one or more dipoles as their antenna. This reflector is the
optimal shape for such an antenna. Some units, such as the WET-11, do
not use dipoles as their antenna. You can
download a modified template for the WET-11 at the address listed at
the end of this hack.

The reflector should be made from a piece of square material to shape
the curve. If you need to reduce height for packaging reasons, a
shorter antenna will work but will lose roughly 3 dB for each halving
of reflector height. It is also important to try to get the dipole
lined up in the center of the reflector.

Front-to-back ratio is a
measurement of how well a directional antenna rejects interference
from directions other than the desired direction. The front-to-back
ratio with this antenna depends upon the size of the wire mesh you
use to make the antenna. Finer mesh yields only slightly better gain
but yields much better front-to-back ratio. Modeling shows the F/B
ratio to be better than ~25 dB if you use 1/4" or smaller mesh. My
calculated gain figures presume the reflector is 55 percent
efficient. If you use a solid sheet of aluminum or copper as your
reflector, your gain figures may be a little bit higher than these.
The radiation pattern is narrower in the vertical plane than the
horizontal plane.

People have made good reflectors from Pringles cans, large tin cans,
wire screen, aluminum sheet, and tin roofing material. Any flat metal
surface or screen, such as tinfoil taped to cardboard, will work. You
can build one of these in less than a half an hour using an old shoe
box and a roll of tin foil.

The original article that this Hack is based on is available online
at http://www.freeantennas.com/projects/template/indexl.

Michael Erskine